AEROLOGY

A E R O L O G Y,

THE doctrine or science of AIR, its nature and different species, with their ingredients, properties, phenomena, and uses.

Air, in a general sense, is that invisible fluid everywhere surrounding this globe; on which depends not only animal but vegetable life; and which seems, in short, to be one of the great agents employed by nature in carrying on her operations throughout the world.

Though the attention of philosophers has in all ages been engaged in some measure by inquiries concerning the nature of the atmosphere, yet till within these last 30 years, little more than the mere mechanical action of this fluid was discovered, with the existence of some anomalous and permanently elastic vapours, whose properties and relation to the air we breathe were almost entirely unknown. Within the above-mentioned period, however, the discoveries concerning the constituent parts of the atmosphere itself, as well as the nature of the different permanently elastic fluids which go under the general name of air, have been so numerous and rapid, that they have at once raised this subject to the dignity of a Science, and now form a very considerable, as well as important, part of the modern system of natural philosophy.

Those discoveries, indeed, have not been more interesting to philosophers, than useful to science and beneficial to society. Many perplexing processes in chemistry have been explained in consequence of them, several have been facilitated, and a number of new and useful ones have been introduced. The phenomena attending metallic calcinations and reductions have been greatly elucidated. The knowledge of the use of the air in respiration; the method of ascertaining its purity and fitness for that function; the investigation of dephlogisticated air; the method of impregnating water with fixed air; are all calculated to answer purposes of the highest utility. The medicinal properties of fixed air have been in a great measure ascertained, and its antiseptic qualities in other respects promise to be of considerable advantage. The method of ascertaining the purity of the air of a place, and the manner of ventilating an apartment, are of

great use for those concerned in public buildings. In short, there is perhaps no station in life where some knowledge of this subject may not be of use.

SECT. I. Of the general Constitution, Mechanical Properties, and Operations of the Air.

§ 1. The general Constitution of the Air we breathe.—

For many ages this fluid was supposed to be simple and homogeneous; its common operations to depend on its heat, cold, moisture, or dryness; and any effects which could not be explained by these (such as the appearance of pestilential diseases), were reckoned to be entirely supernatural, and the immediate effects of Divine power. But, however simple and homogeneous this fluid may have been thought in former times, it is so far from possessing the simplicity of an element, that it is the receptacle of all kinds of effluvia produced from terrestrial substances either naturally or artificially. Hence, whatever may be the nature of the aerial fluid when absolutely pure, that which we breathe, and commonly goes under the name of air, must be considered as an exceedingly heterogeneous mixture, various at various times, and which it is by no means possible to analyse with accuracy.

Though, in this view, air seems to be a kind of sink or common sewer, where all the poisonous effluvia arising from putrid and corrupted matters are deposited; yet it has a wonderful facility of purifying itself, and one way or other of depositing those vapours contained in it; so that it never becomes noxious except in particular places, and for a short time; the general mass remaining upon all occasions pretty much the same. The way in which this purification is effected is different, according to the nature of the vapour with which the air is loaded. That which most universally prevails is water; and from experiments it appears, that the quantity of aqueous vapour contained in the atmosphere is immense. Dr Halley, from an experiment on the evaporation from a fluid surface heated to the same degree with that given by our meridian sun, has calculated, that the evaporation from the Mediterranean sea alone is sufficient to yield all the water of

the rivers which run in to it. Dr Watson, in his Chemical Essays, has given an account of some experiments made with a view to determine the quantity of the water raised from the earth itself in time of drought. He informs us, that, when there had been no rain for above a month, and the grass was become quite brown and parched, the evaporation from an acre was not less than 1600 gallons in 24 hours. Making afterwards two experiments, when the ground had been wetted by a thunder-shower the day before, the one gave 1973, the other 1905, gallons in 12 hours. From this the air is every moment purified by the ascent of the vapour, which flying off into the clouds, thus leaves room for the exhalation of fresh quantities; so that as the vapour is considerably lighter than the common atmosphere, and of consequence ascends with great velocity, the air during all this time is said to be dry, notwithstanding the vast quantity of aqueous fluid that passes through it.

Nor is it only from the aqueous vapour that the air is purified at this time. Much of that vapour arising from decayed and putrid animal and vegetable substances, and which by some modern philosophers is called phlogiston, attaches itself to the aqueous vapour, and ascends along with it. Another part is absorbed by vegetables; for the phlogistic vapour, as is shown under AGRICULTURE, n^o 5, is probably the food of plants. The phlogistic vapours which ascend along with the water, probably continue there and descend along with the rain; whence the fertilizing qualities of rain-water above those of any other. Thus we may see why a dry air, whether cold or hot, must always be wholesome; but as the atmosphere cannot always receive vapours, it is obvious, that when great rains come on, especially if attended with heat, the lower regions of the air must be overloaded with vapours both of the aqueous and phlogistic kind, and of consequence be very unwholesome.

But besides the aqueous and phlogistic vapours, both of which are specifically lighter than common air, there are others, which, being specifically heavier, cannot be carried off in this manner. Hence these gross vapours contaminate certain places of the atmosphere, rendering them not only unhealthy, but absolutely poisonous. Of these are, 1. Sulphureous, acid, and metalline exhalations. These are produced principally by volcanoes; and as they descend, in consequence of their specific gravity, they suffocate and spread destruction all around them, poisoning not only animals, but vegetables also. 2. The vapours arising from houses where lead and other metals are smelted, have the same pernicious qualities; inasmuch that the men who breathe them, the cattle who eat the grass, and the fishes who inhabit the waters on which they fall, are poisoned by them if taken into the body in a certain proportion. 3. Of the same kind are the mo-fites, or emanations of fixed air, which sometimes proceed from old lavas, or perhaps from some other places even of the surface. From all these the air seems not capable of purifying itself, otherwise than either by dispersing them by winds, or by letting them subside by their superior gravity, till they are absorbed either by the earth or water, according as it is their nature to unite with one or other of these elements. 4. Of this kind also seem to be the vapours which are called

properly pestilential. The contagion of the plague itself seems to be of an heavy sluggish nature, incapable of arising in the air, but attaching itself to the walls of houses, bed-cloths, and wearing apparel. Hence scarce any constitution of the atmosphere can dispel these noxious effluvia; nor does it seem probable that pestilential distempers ever cease until the contagion has operated so long, and been so frequently communicated from one to another, that, like a ferment much exposed to the atmosphere, it becomes vapid, communicates a milder infection, and at last loses its strength altogether.

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62. Mechanical Properties of the AIR.—In common specific with water, the air we breathe possesses gravity, and gravity of the air. consequently will perform every thing in that way which water can do, making allowance for the great difference between the specific gravity of water and of air. This difference indeed is exceedingly great, and has been variously calculated. Ricciolus estimates the gravity of air to be that of water as 1 to 1000; Mersennus, as 1 to 1300, or 1 to 1356; Lana, as 1 to 640; and Galileo, only as 1 to 400. Mr Boyle, by more accurate experiments, makes the air at London to be to water as 1 to 938; and thinks, that, all things considered, the proportion of 1 to 1000 may be taken as a medium. But by three experiments made since that time before the Royal Society, the specific gravity of the air was determined to be that of water as 1 to 840, 852, and 860. By a very accurate experiment, Mr Hauksbee fixed the proportion as 1 to 885. But as all these experiments were made when the barometer was at 29$\backslash frac\{1\}\{2\}$ inches, Dr Jurin supposes, that, at a medium between heat and cold, when the barometer is 30 inches high, the proportion between the two fluids may be taken as 1 to 800; and this agrees with the observations of the Hon. Mr Cavendish, made when the barometer was at 29$\backslash frac\{1\}\{2\}$ inches, and the thermometer at 50.

By means of its gravity, the air presses with great effects of force upon all bodies, according to the extent of their gravity surface. M. Pascal has computed the quantity of this of the air. pressure to be no less than 2232 pounds upon every square foot of surface, or upwards of 15 pounds on every square inch. According to some experiments made by M. Amontons and de la Hire, a column of air on the surface of the earth, and 36 fathoms high, is equal in weight to three lines depth of mercury. From the barometer, however, we know that the whole pressure of the atmosphere is very different; sometimes being equal only to a column of 28 inches, and varying from thence to 31 inches. The whole quantity of pressure must thus be immense, and has been computed equal to a globe of lead 60 miles in diameter.

By means of its gravity, the atmosphere accomplishes many useful purposes in nature. It prevents the arterial vessels of animals and the sap-vessels of plants from being too much distended by the expansive power (whatever it is), which has a perpetual tendency to swell them out. Thus we see, that, in the operation of cupping, where the pressure of the air is taken off from a particular part, the expansive force instantly acts, and swells out the vessels to a great degree. Hence also, when animals are put into an air-pump, their whole bodies swell.

By its gravity, the air promotes the union of fluid bodies, which would instantly cease in vacuo. Thus oils and salts, which remain united in air, separate as soon as that fluid is extracted. Hence also, when hot water is put under an exhausted receiver, it boils violently; because the pressure of the air being now taken off, the particles of steam, which existed invisibly among the water, and which the gravity of the atmosphere prevented from flying off so soon, are now hurried up with great velocity, by means of the excessive comparative gravity of the aqueous fluid.

On the gravity of the air depend the ascent of water in pumps, syphons, &c. and likewise all the phenomena of the barometer.

Besides its gravity, which the air has in common with water and other fluids, there is another which it has only in common with steam or vapour. This is called its elasticity; by which, like a spring, it allows itself to be compressed into a smaller bulk, and then returns again to its original size upon removing the pressure.

The elasticity of the air was first ascertained by some experiments of lord Bacon, who, upon this principle, constructed the first thermometer, which he called his vitrum calendarum. Of this power we have numerous proofs. Thus, a blown bladder being squeezed in the hand, we find the included air sensibly resist; so that, upon ceasing to compress, the cavities or impressions made in its surface are readily expanded again and filled up.

The structure and office of the Air-Pump depend on this elastic property. Every particle of air always exerts a nius or endeavour to expand, and thus strives against an equal endeavour of the ambient particles; whose resistance happening by any means to be weakened, it immediately diffuses itself into an immense extent. Hence it is that thin glass bubbles, or bladders filled with air, and exactly closed, being included in the exhausted receiver of an air-pump, burst by the force of the air they contain; and a bladder almost quite flaccid, swells in the receiver and appears full. The same effect also takes place, though in a smaller degree, on carrying the flaccid bladder to the top of an high mountain.

It has been questioned among philosophers, whether this elastic power of the air is capable of being destroyed or diminished. Mr Boyle made several experiments with a view to discover how long air would retain its spring after having assumed the greatest degree of expansion his air-pump would give it; but he was never able to observe any sensible diminution. Desaguliers found, that air, after having been inclosed for half a year in a wind-gun, had lost none of its elasticity; and Roberval, after preserving it in the same manner for 16 years, observed, that its expansive projectile force was the same as if it had been recently condensed. Nevertheless, Mr Hauksbee concludes, from a later experiment, that the spring of the air may be disturbed by a violent pressure, in such a manner as to require some time to return to its natural tone. Dr Hales inferred, from a number of experiments, that the elasticity of the air is capable of being impaired and diminished by a variety of causes.

The weight or pressure of the air has no dependence on its elasticity; but would be the same whether it had

such a property or not. The air, however, being elastic, is necessarily affected by the pressure, which reduces it into such a space, that the elasticity, which resists against the compressing weight, is equal to that weight. In effect, the law of this elasticity is, that it increases as the density of the air increases; and the density increases as the force increases by which it is pressed. Now there must necessarily be a balance between the action and re-action; i. e. the gravity of the air which tends to compress it, and the elasticity by which it endeavours to expand, must be equal. Hence the elasticity increasing, or diminishing universally, as the density increases or diminishes, it is no matter whether the air be compressed and retained in such a space by the weight of the atmosphere, or by any other means; it must endeavour in either case to expand with the same force. And hence, if air near the earth be pent up in a vessel, and all communication with the external fluid cut off, the pressure of the inclosed air will be equal to the weight of the atmosphere at the time the quantity was confined. Accordingly, we find mercury sustained to the same height, by the elastic force of air inclosed in a glass vessel, as by the whole atmospheric pressure. On the same principle air may be artificially condensed; and hence the structure of the Air-Gun.

The utmost limits to which air, of the density which it possesses at the surface of the earth, is capable of being compressed, have not been ascertained. Mr Boyle made it 13 times more dense; Dr Halley says that he has seen it compressed so as to be 60 times denser than in its natural state, which is farther confirmed by M. Papin and M. Huygens. Dr Hales, by means of a press, condensed it 38 times; and by forcing water in an iron ball or globe, into 1551 times less space than it naturally occupies. However, Dr Halley has asserted, in the Philosophical Transactions, Abr. vol. ii. p. 17. that from the experiments made at London, and by the academy del Cimento at Florence, it might be safely concluded, that no force whatever is able to reduce air into 800 times less space than that which it naturally possesses on the surface of our earth. In answer to this, M. Amontons, in the Memoirs of the French Academy, maintains, that there is no fixing any bounds to its condensation; that greater and greater weights will still reduce it into less and less compass; that it is only elastic in virtue of the fire which it contains; and that as it is impossible ever to drive all the fire out of it, it is impossible ever to make the utmost condensation.

The dilatation of the air, by virtue of its elastic force, is found to be very surprising; and yet Dr Wallis suggests, that we are far from knowing the utmost of which it is capable. In several experiments made by Mr Boyle, it dilated first into nine times its former space; then into 31 times; then into 60; then into 150. Afterwards it was brought to dilate into 8000 times its space, then into 10,000, and even at last into 13,679 times its space; and this altogether by its own expansive force, without the help of fire. On this depend the structure and use of the MANOMETER.

Hence it appears, that the air we breathe near the surface of the earth is compressed by its own weight into at least the 13,679^th part of the space it would possess in vacuo. But if the same air be condensed by

Of Air in general. art, the space it will take up when most dilated, to that it possesses when condensed, will be, according to the same author's experiments, as 550,000 to 1.

II. Expansion of the air by heat. M. Amontons, and others, we have already observed, attribute the rarefaction of the air wholly to the fire contained in it; and therefore, by increasing the degree of heat, the degree of rarefaction may be carried still farther than its spontaneous dilatation. Air is expanded one-third of its bulk by boiling water.

Dr Hales found, that the air in a retort, when the bottom of the vessel was just beginning to be red-hot, was expanded through twice its former space; and in a white, or almost melting heat, it occupied thrice its former space; but Mr Robins found it was expanded by the heat of iron, just beginning to be white, to four times its former bulk. On this principle depend the structure and office of the THERMOMETER.

M. Amontons first discovered that air will expand in proportion to its density with the same degree of heat. On this foundation the ingenious author has a discourse, to prove "that the spring and weight of the air, with a moderate degree of warmth, may enable it to produce even earthquakes, and other of the most vehement commotions of nature." See the article EARTHQUAKE.

12. General ef. air's elasticity. The elastic power of the air, then, is the second great feds of the source of the effects of this important fluid. Thus it insinuates into the pores of bodies; and, by possessing this prodigious faculty of expanding, which is so easily excited, it must necessarily put the particles of bodies into which it insinuates itself into perpetual oscillations. Indeed, the degree of heat, and the air's gravity and density, and consequently its elasticity and expansion, never remaining the same for the least space of time, there must be an incessant vibration or dilatation and contraction in all bodies.

We observe this reciprocation in several instances, particularly in plants, the air-vessels of which do the office of lungs; for the contained air alternately expanding and contracting, according to the increase or diminution of the heat, alternately presses the vessels and eases them again, thus keeping up a perpetual motion in their juices.

Hence we find, that no vegetation or germination will proceed in vacuo. Indeed, beans have been observed to grow a little tumid therein; and this has led some to attribute that to vegetation which was really owing to no other cause than the dilatation of the air within them. The air is very instrumental in the production and growth of vegetables, not only by invigorating their several juices while in an elastic active state, but also by greatly contributing in a fixed state to the union and firm connection of their several constituent parts.

From the same cause it is, that the air contained in bubbles of ice, by its continual action bursts the ice. Thus also, entire columns of marble sometimes cleave in the winter time, from the increased elasticity of some little bubble of air contained in them. From the same principle arise all putrefaction and fermentation; neither of which will proceed, even in the best disposed subjects, in vacuo.

Since we find such great quantities of elastic air generated in the solution of animal and vegetable substances, a good deal must constantly arise from the dis-

solution of these aliments in the stomach and bowels, which is much promoted by it; and, in reality, all natural corruption and alteration seem to depend on air.

§ 3. Effects of the different Ingredients of Air.— This fluid acts not only by its common properties of gravity and elasticity, but produces numerous other effects arising from the peculiar ingredients of which it consists.

Thus, 1. It not only dissolves and attenuates bodies by its pressure and attrition, but as a chaos containing all kinds of menstrua, and consequently possessing powers for dissolving all bodies. It is known that iron and copper readily dissolve and become rusty in air, unless well defended with oil. Boerhaave assures us, that he has seen pillars of iron so reduced by air, that they might be crumbled to dust between the fingers; and as for copper, it is converted by the air into a substance much like the verdigris produced by vinegar.

Mr Boyle relates, that in the southern English colonies the great guns rust so fast, that after lying in the air for a few years, large cakes of crocus martis may be separated from them. Acostra adds, that in Peru the air dissolves lead, and considerably increases its weight. Yet gold is generally esteemed indissoluble by air, being never found to contract rust, though exposed to it ever so long. In the laboratories of chemists, however, where aqua regia is prepared, the air becoming impregnated with a quantity of the vapour of this menstruum, gold contracts a rust like other bodies.

Stones also undergo the changes incident to metals. Thus Parbeck stone, of which Salisbury cathedral consists, is observed gradually to become softer, and to moulder away in the air; and Mr Boyle gives the same account of Blackington stone. He adds, that air may have a considerable operation on vitriol, even when a strong fire could act no farther upon it. And he has found, that the fumes of a corrosive liquor work more suddenly and manifestly on a certain metal when sustained in the air, than the menstruum itself did, which emitted fumes on those parts of the metal which it covered; referring to the effects of the effluvia of vinegar on copper.

The dissolving power of air is increased by heat, and by other causes. It combines with water; and by access of cold, deposits part of the matter which was kept dissolved in it by a greater degree of heat. Hence the water, by being deposited and condensed upon any cold body, such as glass, &c. in windows, forms fogs, and becomes visible.

In the various operations of chemistry, air is a very necessary and important agent; the result of particular processes depending on its presence or absence, on its being open or inclosed. Thus, the parts of animals and vegetables can only be calcined in open air; in close vessels they never become any other than black coals. And these operations are affected by the changes to which the air is liable. Many instances might be adduced to this purpose. Let it suffice to observe, that it is very difficult to procure oil of sulphur, per campanam, in a clear dry atmosphere; but in a thick moist air it may be obtained with greater ease, and in larger quantities. So, pure well-fermented wine, if it be carried to a place where the air is replenished with

the fumes of new wine then fermenting, will begin to ferment as fresh.

The changes in the air arise from various causes, and are observable, not only in its mechanical properties, such as gravity, density, &c. but in the ingredients that compose it. Thus, at Fashlun in Sweden, noted for copper-mines, the mineral exhalations affect the air in such a manner as to discolour the silver coin in purses; and the same effluvia change the colour of brass. In Carniola, Campania, &c. where are mines of sulphur, the air sometimes becomes very unwholesome, which occasions frequent epidemic diseases, &c.

The effluvia of animals also have their effect in varying the air; as is evident in contagious diseases, plagues, murrains, and other mortalities, which are spread by an infected air.

For the vivifying principle of air, see the article BLOOD.

SECT. II. Historical Account of the principal Discoveries concerning the Composition of Atmospheric Air and other Aerial Fluids.

WHILE the preceding discoveries were making concerning the mechanical and other properties of the air, little notice seems to have been taken of the elementary parts of the air itself, or the different kinds of fluid which go under that name. It was known, indeed, that air was separable from terrestrial bodies by means of fire, fermentation, &c. but this was commonly reckoned to be the same with what we breathe. Van Helmont, a disciple of Paracelsus, was the first who undertook to make inquiries concerning this species of air. He gave it the name of gas sylvestre, from the Dutch word ghoast, signifying spirit; and observes, that some bodies resolve themselves almost entirely into it. "Not (says he) that it had been actually contained in that form in the bodies from which it was separated; but it was contained under a concrete form, as if fixed, or coagulated." According to this author, the gas sylvestre is the same with what is separated from all substances by fermentation; from vegetables by the action of fire; from gun-powder when it explodes; and from charcoal when burning. On this occasion he asserts, that 62 pounds of charcoal contain 61 pounds of gas and only one pound of earth. To the effluvium of gas he also attributes the fatal effects of the grotto del Cani in Italy, and the suffocation of workmen in mines. He asserts, that it is to the corruption of the aliment, and the gas discharged from it, that we are to attribute wind, and the discharges of it from the bowels. Upon the same principles he accounts for the swelling of dead bodies which have remained for a time under water, and for the tumours which arise on some parts of the body in certain diseases. He also determines, that this gas is different from the air we breathe; that it has a greater affinity with water: and he imagined it might consist of water reduced to vapours, or a very subtil acid combined with volatile alkali.

Mr Boyle repeated all Van Helmont's experiments to more advantage than he himself had performed them; but seems not to have proceeded further in his discoveries than Van Helmont did: only he found some bodies, such as sulphur, amber, camphor, &c. diminish the volume of air in which they burn.

Dr Hales first attempted to determine the quantity of air produced from different bodies; for which purpose he made experiments on almost every known substance in nature, examining them by distillation, fermentation, combustion, combinations, &c. He also first suspected, that the briskness and sparkling of the vapours of waters, called acidulous, were owing to the air they contained. But notwithstanding all his discoveries concerning the quantity of elastic fluid obtained from different bodies, he did not imagine there was any essential difference between this fluid and the air we breathe; only that the former was loaded with noxious vapours, foreign to its nature. His suspicion concerning this impregnation was confirmed by M. Venel, professor of chemistry at Montpellier, in a memoir read before the Royal Academy of Sciences in 1750. This gentleman was able to disengage the air from the Seltzer waters, and to measure its quantity; which he constantly found to amount to about one-fifth of its bulk. The water thus deprived of its air became flat, and ceased to sparkle; the only difference then betwixt it and common water was, that the former contained a small quantity of sea-salt. Upon these principles he attempted to recompose Seltzer water, by dissolving in a pint of common water two drachms of fossile alkali, and then adding an equal quantity of marine acid. The quantity of sea-salt produced by the union of these two, he knew would prove equal to that contained in a pint of Seltzer water; and the effervescence produced by the action of the acid and alkali upon each other, he imagined, would produce air sufficient for the impregnation of the water. In this he was not deceived; the water thus produced was not only analogous to Seltzer, but much more strongly impregnated with air.

Dr Black first discovered, that chalk, and the other earths reducible to quicklime by calcination, consist of an alkaline earth, by itself soluble in water, but which, combined with a large quantity of fixed air, becomes insoluble; losing the properties of quicklime, and assuming the natural appearance we observe those earths to have when not reduced into lime. The same thing he discovered in magnesia alba, and in alkalis both fixed and volatile. On the fixed air contained in these bodies, he found not only their property of effervescing with acids to depend, but likewise their miscibility; both the alkalis and calcareous earth being highly caustic when deprived of their fixed air. He also found, that this fluid, which he called fixed air, had different degrees of affinity with different substances; that it was stronger with calcareous earth than with fixed alkali; with fixed alkali, than magnesia; and with magnesia, than volatile alkali. He also suspected, that the fixed air of alkaline salts unites itself with the precipitates of metals, when thrown down from acids; and that the increase of weight observable in these precipitates was owing to this cause. But he was of opinion, that the fluid which he called fixed air was very different from the common air we breathe; and therefore adopted the name of air, merely as one already established, whatever impropriety there might be in the term.

It was not long before the discovery of this species of air suggested new theories in physiology and natural philosophy. Mr Haller had inferred, from Dr Hales's experiments,

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By Dr Hales
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20
Confirmed
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Discoveries by Dr Black.

²¹ Of Air in general. experiments, that air is the real cement of bodies; which, fixing itself in the solids and fluids, unites them to each other, and serves as a bond by which they are kept from dissolution. In 1764, Dr Macbride of Dublin published a number of experiments in support of this doctrine. From his work it appears, that fixed air is separated, not only from all substances in fermentation, but also from all animal substances as they begin to putrefy; and that this air is capable of uniting itself to all calcareous earths, as well as alkalis both fixed and volatile, and restoring to them the property of effervescing with acids when they have by any means been deprived of it. But though these opinions have since been found erroneous, the conclusions drawn by him from his numerous experiments still hold good, viz. that fixed air is an elastic fluid, very different from the common air we breathe: that it is possessed of a strong antiseptic quality, and may be introduced with safety into the intestinal canal, and other parts of the animal economy, where common air would have fatal effects; but is mortal if breathed into the lungs, &c.

²² Quantity of fixed air contained in alkaline salts determined by Mr Cavendish. In 1766 and 1767, Mr Cavendish communicated some new experiments to the Royal Society at London, wherein he determines the quantity of air contained in fixed alkali, when fully saturated with it, to be five-twelfths of its weight, and seven-twelfths in volatile alkali: that water is capable of absorbing more than its own bulk of this air; that it has then an agreeable, spirituous, and acidulous taste; and that it has the property of dissolving calcareous earths and magnesia, as well as almost all the metals, especially iron and zinc: that the vapour of burning charcoal occasions a remarkable diminution of common air, at the same time that a considerable quantity of fixed air is produced in the operation. He also found, that solution of copper in spirit of salt, instead of producing inflammable air, like that of iron or zinc, afforded a species of air which lost its elasticity as soon as it came into contact with water.

²³ Contests concerning the doctrine of fixed air. The discoveries of Dr Black concerning fixed air had not been long published, when they were violently attacked by some foreign chemists, while his cause was as eagerly espoused by others. The principal opponents were Mr Meyer apothecary at Osnabruck, Mr Cravis physician to his Russian Majesty, and Mr de Smeth at Utrecht. Their arguments, however, were effectually answered at the time by Mr Jacquin, botanical professor at Vienna; and the numerous discoveries made since that time have given such additional confirmation to his doctrine, that it is now universally adopted by chemists both in Britain and other countries. It was reserved, however, for Dr Priestley to

²⁴ Composition of the atmosphere discovered. make the great discovery concerning the nature of our atmosphere; and to inform the world, that it is composed of two fluids; the one absolutely noxious, and incapable of supporting animal life for a moment; the other extremely salutary, and capable of preserving animals alive and healthy for a much longer time than the purest air we can meet with. This may be considered as the ultimate period of our history: for since that time the discoveries of philosophers still living, in many different countries, have been so rapid, that it is difficult to ascertain the dates of them by any authentic documents; especially as, by reason of such numerous experiments, the same things have not unfrequently

been discovered by different persons unknown to each other. We shall therefore proceed to give an account of the different kinds of aerial fluids, beginning with those which are known, or supposed, to constitute a part of our atmosphere.

²⁵ 1. Discovery and Methods of procuring this Kind of Air.—Dephlogisticated air was first obtained by Dr Priestley on the 18th of August 1774. The circumstances which led him to the discovery, were his having always procured inflammable air from spirit of salt, by adding to it spirit of wine, oil of olives, oil of turpentine, charcoal, phosphorus, bees wax, and even sulphur. Hence he suspected, that the common air we breathe might be composed of some kind of acid united with phlogiston. On this supposition he extracted air from ²⁵ mercurius calcinatus per se, by exposing it to the focus of a burning-glass 12 inches in diameter; and, having repeated the experiment with red precipitate and minimum, he found, that though a quantity of fixed air was always produced, yet after that was separated, the remainder supported flame much more vigorously than common air; for a candle burned in it with a flame very much enlarged, and with a crackling noise, at the same time that it appeared fully as much diminished by the test of nitrous air. Whence he concluded, that it was respirable; and, on making the experiment, found that it actually was so, for a mouse lived a full half hour in a quantity of this fluid; which, had it been common air, would only have kept it alive half that time. Nor did the animal seem to be otherwise injured than by the cold; as it presently revived on bringing it near the fire, and the remainder of the air still appeared better than that of the atmosphere, when the test of nitrous air was applied to it.

²⁶ This pure kind of air being discovered, the Doctor next proceeded to name it dephlogisticated, from his ²⁶ opinion that common air, in the act of burning, ²⁶ absorbed phlogiston; of consequence, he supposed, that which absorbed the most, or which most vigorously and for the greatest length of time supported flame, was supposed to contain the smallest quantity of this substance. In the course of his inquiries why this kind of air comes to be so much dephlogisticated, he fell upon a method of extracting it from a great variety of substances; viz. by moistening them with spirit of nitre, and then distilling them with a strong heat. Thus he obtained it from flowers of zinc, chalk, quicklime, slacked lime, tobacco-pipe clay, flint, Muscovy talcs, and even glass. He then found, that by simply dissolving any metal in the nitrous acid, and then distilling the solution, he could obtain very pure air: and Mr Warltire found even the trouble of distillation unnecessary; nothing more being requisite than to moisten red lead with the spirit of nitre, and then pour upon it the oil of vitriol, which instantly disengaged the dephlogisticated air without applying any more heat than what was generated by the mixture.

²⁷ While discoveries of this kind engaged Dr Priestley in England, Mr Scheele was employed in a similar manner in Sweden; and had actually obtained the same kind of air, without knowing any thing of what Dr Priestley had done. The latter had the merit of the prior

prior discovery: but Mr Scheele's method was more simple, consisting only in the distillation of nitre with a strong heat; by which means it is now found that dephlogisticated air may be obtained in very considerable quantity, and in as great purity, as by the more expensive processes. The pure air from nitre had indeed partly been obtained by Dr Hales long before this time; since he informs us, that half a cubic inch of nitre yielded 90 cubic inches of air, which was undoubtedly the fluid we speak of; but as he neglected to prosecute the discovery, nothing farther was known at that time.

As the nitrous acid was universally concerned in the first processes for obtaining this kind of air, it was for some time generally believed to be a peculiar property of that acid alone to produce it; but the indefatigable genius of Dr Priestley soon found, that it might not only be procured where no nitrous acid was employed, but where the substances were treated with vitriolic acid. It was indeed evident, from the very first experiment, that nitrous acid was not essentially necessary; since pure air was procured from precipitate per se, in the preparation of which no nitrous acid is employed. The Abbé Fontana found, that 192 grains of this substance yielded 26½ cubic inches of dephlogisticated air, at the same time that the weight of it was reduced to 178½ grains, which is nearly the weight of that quantity of air. It had formerly been observed, that the weight of mercury is augmented during its conversion into precipitate per se, as that of lead is by its conversion into minium. The experiments just now mentioned, therefore, show, that during this process the air is decomposed; the pure dephlogisticated part of it being absorbed by the metal, and appearing again on the application of heat; and the same appears to be the case with red lead, from the experiment of Mr Walsire already mentioned. With regard to this last substance, however, a very great singularity is observed; viz. that when newly prepared it yields none at all, and even for some time after the produce is much smaller than when it has been long kept. The reason of this seems to be, that the minium still contains a considerable quantity of phlogiston, which flies off into the atmosphere by long keeping, a larger quantity of the dephlogisticated part of the atmosphere being imbibed at the same time. The mode of applying heat has also a very considerable effect on the quantity of air produced. Thus, Dr Priestley remarks*, that "from equal quantities of red lead, without any mixture of spirit of nitre, and using the same apparatus for distilling it, he obtained, by means of heat applied suddenly, more air than when slowly applied, in the proportion of ten to six. The proportion of fixed air was the same in both cases, and the remainder equally dephlogisticated."

By heat alone, the Doctor found, that sedative salt, manganese, lapis calaminaris, and the mineral called lapis ponderosus, wolfram, or tungsten, would yield dephlogisticated air; the first indeed in very small quantity, and sometimes even of a quality very little superior to common air. In these experiments, he made use of small-bellied retorts of green glass, which can stand the fire best, containing about an ounce of water, and having narrow necks 18 or 20 inches long. The substance to be examined was put into a retort of this

kind, and then exposed to a red heat, either in sand or dephlogisticated over a naked fire, while the neck of the vessel was plunged in water or mercury.

Having dissolved six pennyweights of very clean iron in oil of vitriol, and then distilled the solution to dryness in a long-necked retort, he received the common air a little phlogisticated, some fixed air, much vitriolic acid air, and lastly 18 ounce measures of dephlogisticated air. The iron that remained undissolved weighed 23 grains, so that the air was yielded by five pennyweights one grain of iron. The ochre weighed seven pennyweights thirteen grains: so that, says he, there probably remained a quantity of oil of vitriol in it; and consequently, had the heat been greater, more air would have been obtained.

In his experiments with the nitrous acid, as it had constantly been found, that by pouring on more nitrous acid on the residuum, and repeating the operation, more dephlogisticated air might be obtained, the Doctor determined to try whether the same would not hold good with vitriolic acid also. For this purpose, he added more oil of vitriol to the residuum of the last-mentioned experiment. When in a red heat with a glass retort, it yielded a quantity of vitriolic acid air, no fixed air, but about 24 ounce measures of dephlogisticated air; when, the retort being melted, a good deal of the air was necessarily lost; but, on resuming the process in a gun-barrel, he procured as much air as had been got before.—Pursuing these experiments, he obtained with common crust of iron and oil of vitriol, dephlogisticated air at the first distillation, and a great deal more from the residuum, by pouring fresh oil of vitriol upon it. The same product he obtained from blue vitriol, solution of copper in the vitriolic acid, and from a solution of mercury in that acid. On this substance he remarks, that "either by means of oil of vitriol or spirit of nitre, it yields a great quantity of dephlogisticated air: but with this difference, that in the process with spirit of nitre, almost the whole of the mercury is revived (not more than a twentieth part being lost, if the process be conducted with care); but in that with vitriolic acid, almost the whole is lost." From the later experiments of Mr Lavoisier, however, it appears that the Doctor's process had not been conducted with sufficient care; as from two ounces of the dry salt formed by a combination of vitriolic acid with mercury, the former obtained 6 drachms 12 grains of running mercury, besides 3 drachms 58 grains of mercurial sublimate of two different colours. Dephlogisticated air was likewise obtained from pure calx of tin, or putty, mixed with oil of vitriol; but none in any trial with the marine acid, excepting when it was mixed with minium; in which case the air obtained was probably that which the minium would have yielded without any addition.

The result of all these, and innumerable other experiments made by philosophers in different countries, was, that dephlogisticated air may be obtained from a vast variety of mineral and metallic substances by means of the vitriolic and nitrous acids. It now remained only to discover in what manner this fluid, so essentially necessary to the support of animal life, is naturally produced in quantities sufficient for the great expense of it throughout the whole world, by the breathing of animals, the support of fires, &c. This discovery, indeed,

Dephlogisticated Air. deed, had been made before even the existence of dephlogisticated air itself was known. Dr Priestley, after having tried various methods of purifying contaminated air unsuccessfully, found at last, that some kinds of vegetables answered this purpose very effectually; for which discovery he received the thanks of the Royal Society. Among the vegetables employed on this occasion, he found mint answer the purpose very effectually.

* Exper. and Observ. vol. I. p. 1. sect. 4. "When air," says he*, "has been freshly and strongly tainted with putrefaction, so as to smell through the water, sprigs of mint have presently died upon being put into it, their leaves turning black; but if they do not die presently, they thrive in a most surprising manner. In no other circumstances have I seen vegetation so vigorous as in this kind of air, which is immediately fatal to animal life. Though these plants have been crowded in jars filled with this kind of air, every leaf has been full of life; fresh shoots have branched out in various directions, and grown much faster than other similar plants growing in the same exposure in common air."—Having in consequence of this observation rendered a quantity of air thoroughly noxious, by mice breathing and dying in it, he divided it into two receivers inverted in water, introducing a sprig of mint into one of them, and keeping the other receiver unaltered. About eight or nine days after, he found that the air of the receiver into which he had introduced the sprig had become respirable; for a mouse lived very well in this, whereas it died the moment it was put into the other.

33. Noxious air improved by vegetating mint.

34. Experiments seemingly contradictory. From these experiments the Doctor at first concluded, that in all cases the air was meliorated by the vegetation of plants: but even in his first volume he observes, that some experiments of this kind did not answer so well towards the end of the year as they had done in the hot season; and a second course seemed to be almost entirely contrary to the former. Having tried the power of several sorts of vegetables upon air infected by respiration or by the burning of candles, he found that it was generally rendered worse by their vegetation; and the longer the plants were kept in the infected air, the more they phlogisticated it; though in several cases it was undoubtedly meliorated, especially by the shoots of strawberries and some other plants, introduced into the vials containing foul air, and inverted in water, which were placed near them, whilst their roots continued in the earth in the garden. Sometimes the infected air was so far mended by the vegetation of plants, that it was in a great measure turned into dephlogisticated air. "On the whole," says Dr Priestley, "I still think it probable, that the vegetation of healthy plants, growing in situations natural to them, have a salutary effect on the air in which they grow.—For one instance of the melioration of air in these circumstances should weigh against an hundred in which the air is made worse by it, both on account of the disadvantages under which all plants labour, in the circumstances in which these experiments must be made, as well as the great attention and many precautions that are requisite in conducting such a process."

35. Experiments of Dr Ingenhousz, by the absorption of phlogiston from that which had been tainted; but the experiments of Dr Ingenhousz, made in 1779, showed that this was accomplished, not

only by the absorption just mentioned, but by the emission of dephlogisticated air. He observed in general, that plants have a power of correcting bad air, and even of improving common air in a few hours, when exposed to the light of the sun; but, in the night-time, or when they are not influenced by the solar rays, they contaminate the air. This property, however, does not belong in an equal degree to all kinds of plants: nor is it possible to discover by the external properties of a plant, whether it be fit for this purpose or not; as some which have a bad smell, and are entirely unfit for food, show themselves much superior to others whose external appearance would seem preferable. His method of making the experiment was, to fill a vial with air, fouled either by respiration or combustion; after which a sprig of any plant was introduced, by passing it through the water in which the vial was immersed. The vial was then stopped; or it was removed into a small basin full of water, and exposed to the sun, or situated in some other proper place as occasion required. Air phlogisticated by breathing, and in which a candle could not burn, after being exposed to the sun for three hours, with a sprig of peppermint in it, was so far corrected, as to be again capable of supporting flame. The following experiment, however, made with a mustard plant, may be looked upon as decisive: A plant of this kind was put into a glass receiver containing common air, and its stem cut off even with the mouth of the receiver. The vessel was then inverted in an earthen pan, containing so much water to keep the plant alive, and the whole apparatus was set over-night in a room. Next morning the air was found so much contaminated, that it extinguished the flame of a wax taper. On exposing the apparatus to the sun for a quarter of an hour, the air was found to be somewhat corrected; and after an hour and an half it was so far improved, that by the test of nitrous air it appeared considerably better than common air.

36. Before we proceed farther in the account of Dr Ingenhousz's experiments, it will be necessary to relate some observations made by Dr Priestley; from which it appears, that dephlogisticated air, in very considerable quantity, may, in certain circumstances, be procured from water alone. The substance of these is, that water, especially pump-water, when exposed to the light of the sun, emits air slowly: but after some time a green matter appears on the bottom and sides of the glass; after which it emits very pure air in great quantity, and continues to do so for a very long time, even after the green matter has shown some symptoms of decay by becoming yellow. He observed, that the water which naturally contained the greatest quantity of fixed air, yielded also the greatest quantity of that which was dephlogisticated; but that the quantity of the latter much exceeded that of the fixed air contained even in any water. The light of the sun was found to be an essential requisite in the formation of this air, as very little, and that of a much worse quality, was produced in the dark.

As the green matter produced in Dr Priestley's glasses, was by himself, as well as others, considered as belonging to the vegetable kingdom, Dr Ingenhousz improved upon his process, by putting the leaves of plants into water, and exposing them to the sun. All plants were not equally fit for producing dephlogisticated air by

by this method more than by the other. Some poisonous plants, as the hyoscyamus, lauro-cerasus, night-shade, the tobacco-plant, a triplex vulgaris, cicuta aquatica, and fabina, were found very fit for the purpose; but the purest kind of air was extracted from some aquatic vegetables, the turpentine-trees, and especially from the green matter he collected in a stone trough which was kept continually filled with water from a spring near the high-road. The purity of this dephlogisticated air, he says, was equal, if not superior, to that procured by the best chemical processes; as it sometimes required eight times its own quantity of nitrous air to saturate it. All parts of the plants were not found equally proper for the production of dephlogisticated air; the full grown leaves yielded it in greatest quantity and purity, especially from their under surface. It was also procured from the green stalks.—One hundred leaves of Nasturtium Indicum, put into a jar holding a gallon, filled with ordinary pump-water, and exposed to the sun from 10 to 12 o'clock, yielded as much air as filled a cylindrical jar four inches and an half in length, and one and three quarters in breadth. On removing this quantity of air, and exposing them again to the sun till seven o'clock, about half as much was produced, of a quality still superior to the former; and next morning by eleven o'clock, they yielded as much more of an equal quality. The roots of plants, he says, when kept out of ground, generally yield bad air, and at all times contaminate common air, a few only excepted. Flowers and fruits, in general, yield a very small quantity of noxious air, and contaminate a great quantity of common air at all times, especially in the night, and when kept in the dark. Two dozen of young and small French beans, kept in a quart-jar of common air for a single night, contaminated the air to such a degree, that a very lively chicken died by being confined in it less than half a minute.

The observations of Dr Ingenhousz on the whole, says Mr Cavallo, clearly show, "that the vegetation of plants is one of the great means employed by nature to purify the atmosphere, so as to counteract, in great measure, the damage done by animal respiration, combustion, &c. It may only be said, that vegetation does not appear to be sufficient to remedy entirely that damage." The Doctor himself, however, speaks very highly of the powers of vegetables in this respect. He informs us, that their office in yielding dephlogisticated air begins a few hours after the sun has made his appearance in the horizon, or rather after it has passed the meridian, and ceases with the close of day; excepting some plants which continue it a short time after sunset: The quantity of dephlogisticated air, yielded by plants in general, is greater in a clear day than when it is somewhat cloudy. It is also greater when the plants are more exposed to the sun, than when they are situated in shady places. He observes, moreover, that the damage done by plants in the night, is more than counterbalanced by the benefit they afford in the day-time. "By a rough calculation, (says he), I found the poisonous air, yielded by any plant during the whole night, could not amount to one hundredth part of the dephlogisticated air which the same plant yielded in two hours time in a fair day."—It does not appear, however, that plants yield dephlogisticated air by any kind of generation of that fluid, but only by filtering the common

air, which all plants absorb through their pores; the dephlogistic part becoming part of their substance, and probably being the true vegetable food, as is explained more at large under the article AGRICULTURE.—Dry plants have little or no effect upon the air until they are moistened.—On all these experiments, however, it must be observed, that they have sometimes failed in the hands of those whom we cannot but suppose very capable of trying them; as Mr Scheele, Mr Cavallo, and the Abbé Fontana.

After the publication of Dr Ingenhousz's experiments, it became generally believed, that the atmosphere was meliorated by the common process of vegetation, and that plants absorbed the phlogistic part as their food, discharging the pure dephlogisticated air as an excrement; which is just the reverse of what happens to animals, who absorb the pure part in respiration, and reject the phlogistic. In the Philosophical Transactions for 1787, however, we find a number of experiments related by Sir Benjamin Thompson, which seem to render this matter dubious.—One very considerable objection is, that the green matter, already mentioned in Dr Priestley's experiments, when carefully observed by a good microscope, appears not to be of a vegetable, but of an animal nature. The colouring Green matter of the water, says he, is evidently of an animal nature; being nothing more than the assemblage of an infinite number of very small, active, oval-formed animalcules, without any thing resembling tremella, or that kind of green matter or water-moss which forms nature upon the bottom and sides of the vessel when this water is suffered to remain on it for a considerable time, and into which Dr Ingenhousz supposes the animalcules above mentioned to be actually transformed.

This gentleman has also found, that several animal substances, as well as vegetables, have a power of separating dephlogisticated air from water when exposed to the light of the sun, and that for a very great length of time. Not that the same quantity of water will always continue to furnish air; but the same animal substance being taken out, washed, and again put into fresh water, seems to yield dephlogisticated air, without any kind of limitation.

Raw silk possesses a remarkable power of this kind. To determine it, Sir Benjamin introduced 30 grains of this substance, previously washed in water, into a thin glass globe $4\backslash frac\{1\}\{2\}$ inches in diameter, having a cylindrical neck $\backslash frac\{1\}\{2\}$th of an inch wide, and twelve inches long, inverting the globe into a jar filled with the same kind of water, and exposing it to the action of the sun in the window. It had not been ten minutes in this situation, when the silk became covered with an infinite number of air-bubbles, gradually increasing in size, till, at the end of two hours, the silk was buoyed up, by their means, to the top of the water. By degrees they began to separate themselves, and form a collection of air in the upper part of the globe; which, when examined by the test of nitrous air, appeared to be very pure. In three days he had collected $3\backslash frac\{1\}\{4\}$ cubic inches of air; into which a wax-taper being introduced, that had just before been blown out, the wick only remaining red, it instantly took fire, and burned with a bright and enlarged flame. The water in the globe appeared to have lost something of its transparency, and had changed its colour to a very faint greenish cast, having

at the same time acquired the smell of raw silk.—This was several times repeated with fresh water, retaining the same silk, and always with a similar result; but with this difference, that when the sun shone very bright, the quantity of air produced was not only greater, but its quality superior to that yielded when the sun's rays were feeble, or when they were frequently intercepted by flying clouds. "The air, however, (says he), was always not only much better than common air, but even than that produced by the fresh leaves of plants exposed in water to the sun's rays in the experiments of Dr Ingenhoufz; and, under the most favourable circumstances, it was so good, that one measure of it required four of nitrous air to saturate it, and the whole five measures were reduced to 1.35."

An experiment was next made in order to determine the effect of darkness upon the production of air: and in this case only a few inconsiderable bubbles were formed, which remained attached to the silk; nor was the case altered by removing the globe into a German stove. Some single bubbles, indeed, had detached themselves from the silk and ascended to the top, but the air was in too little quantity to be measured or proved.—The medium heat of the globe, when exposed to the sun's rays, was about 90° of Fahrenheit, though sometimes it would rise as high as 96; but air was frequently produced, when the heat did not exceed 65 and 70°.—On reversing this experiment, in order to try the effect of light without heat, it was found, that by plunging the globe into a mixture of ice and water, which brought it to the temperature of about 50° of Fahrenheit, the produce of air was diminished, though it still continued in considerable quantity.

The effect of artificial light, instead of that of the sun, was next tried. For this purpose all the air was removed from the globe; and its place being supplied with a quantity of fresh water, so as to render it quite full, it was again inverted in the jar, and removed into a dark room surrounded with six lamps and reflectors; six wax candles were also placed at different distances from three to six inches from it, and disposed in such a manner as to throw the greatest quantity of light possible upon the silk, taking care at the same time that the water should not acquire a greater heat than 90°. In this situation the silk began to be covered with air-bubbles in about ten minutes; and in six hours as much was collected as could be proved by nitrous air, when it was found to be very pure. A fresh-gathered, healthy leaf of a peach tree, and a stem of the pea-plant with three leaves upon it, furnished air by exposure to the same light, but in smaller quantities than by the action of the solar rays. The air produced in the dark, in whatever manner procured, was always in too small quantity to be measured.

In making these experiments, as it was found somewhat troublesome to invert the globes in water, they were at last only kept in an inclined posture on the table, as represented in Plate VIII. fig. 1. the air collecting itself in the upper part of the belly. Having provided himself with a number of globes of different sizes, he then proceeded in his experiments in the following manner.

Finding that raw silk, exposed to the action of light, produced so great a quantity of air, he was induced to try whether some other substances might not be found out capable of doing the same. Having therefore

provided six globes of 4½ inches in diameter, and filled them with spring water, he introduced into each of them 15 grains of one of the following substances, viz. sheep's wool, eider-down, fur of a Russian hare, cotton wool, lint or the ravelings of linen yarn, and human hair.—The results of these experiments were, 1. The globe containing the sheep's wool began to yield air in three days; but several days of cloudy weather intervening, he did not remove it for some time, when only 1¼ths of an inch of air was collected, which proved very pure when tried with nitrous air; but the wool, even in the most favourable circumstances, never afforded more than one third of the quantity which would have been yielded by silk. 2. The water with the eider-down began to furnish air almost immediately, and continued to do so in quantities little less than had been furnished by the silk, and nearly of the same quality. One cubic inch and three quarters of this air, furnished the eighth day from the beginning of the experiment, with three measures of nitrous air, was reduced to 1.34. 3. The fur of the hare produced more air than the wool, but less than the eider-down. Two cubic inches of air were collected in four days; which made its appearance in a different manner from that of the other substances, the air-bubbles being at considerable distances from one another, and growing to an uncommon size before they detached themselves from the fur. The cotton yielded a considerable quantity of air of a better quality than any of the former. The ravelings of linen were very slow in furnishing air, and produced but a small quantity; only two cubic inches being collected in the space of a fortnight. This substance appeared to be the very reverse of the hare's fur; for the air, instead of attaching and collecting itself about the substance in large bubbles, scarce ever made its appearance in sufficient quantity to raise it to the top of the water. The human hair furnished still less than the linen, and the produce was of inferior quality, though still superior to the common atmosphere.

In order to discover the comparative fineness of air produced from vegetables and from raw silk, a small quantity of air from the stem of a pea-plant, which had four healthy leaves upon it, was proved with nitrous air, and found greatly inferior to that from raw silk and several of the substances already mentioned. An entire plant of housewort, of a moderate size, furnished only ¼ths of a cubic inch of air in seven hours, and that greatly inferior to common air; but the leaves alone afforded a much greater quantity, and of a quality greatly superior.

Having proceeded thus far, it was next determined to ascertain how much air a given quantity of water would yield by exposure to the sun's rays. For this purpose, a globe of fine white, clear, and very thin of these substances, containing 296 inches, being filled with fresh spring water, and 30 grains of raw silk immersed in it, was exposed to the air for three days in the month of May, but for the most part cold and cloudy. During this time only 9½ inches of air were produced; but next day, by exposure to the sun from nine in the morning till five in the afternoon, the weather being very fine, 8.46 inches more were produced. The water had now assumed a light greenish colour. Next day, the product of air was nine cubic inches, of a better quality; and the day following, six inches still

superior, though exposed only for three hours and an ⁴th of an inch of air were produced, and these manifestly inferior to the foregoing. No more air could afterwards be procured, excepting one quarter of a cubic inch; so that from 296 inches of this water, 33.96 of air were obtained.

In this experiment the air produced was every day removed from the globe, and its place supplied with water: the following were made, to determine what alternation would take place on allowing the quantity of air produced to remain from first to last. The globe being therefore filled again, and the silk well washed and replaced in it, the quantity of air produced amounted in four days to 30.1 cubic inches; and would probably have been more considerable, had not the globe been unable to contain it along with the water, and therefore there was a necessity for putting an end to the experiment. The quality was superior to the former.—In this experiment the water had lost its transparency, and acquired a greenish cast; a quantity of yellowish earth was precipitated to the bottom, and attached itself so strongly to the glass, that it could not be removed without great difficulty.

On varying the experiment, by employing unwashed raw silk, it was found, that 17 grains of it in 20 cubic inches of water, produced, for the first four days, air of a worse quality than the atmosphere; but afterwards yielded near two inches of a superior quality. The quantity of this air was superior to that in other experiments, though its quality was somewhat inferior.

In reflecting on the experiments above related, it occurred to Sir Benjamin, that the cotton-like substance produced by the populus nigra, a species of poplar tree, might be a proper substitute for the raw silk; especially as he recollected, that on rendering it very dry for some other purpose, some parcels of it had quitted the plate on which they were laid, and mounted up to the top of the room. An hundred and twenty grains of this substance were therefore put into the large globe containing 296 inches; but after exposure to the sun for some hours, the air produced, in quantity about $\backslash frac\{1\}\{4\}$th of a cubic inch, was found to be little better than phlogisticated air. In three days after, only one cubic inch was formed; and this appeared to be completely phlogisticated. Next day, only a few inconsequential air-bubbles appeared; but, the day following, the water suddenly changed to a greenish colour, and began all at once to give good air, and in great abundance. This day 10.42 cubic inches were produced, and the next 14.34. The same water continued to furnish air for four days longer; the whole quantity amounting to 44$\backslash frac\{1\}\{2\}$ cubic inches, the quality of which was superior to that of the air produced in former experiments.

⁴⁷ Of the cause of this production of air. In speculating on the cause of this production of air, it occurred to our author, that perhaps the quantity of it might be in proportion to the surfaces of both. In order to ascertain this, he viewed an hair of silk, and another of poplar-cotton, through a good microscope, when the former appeared twice the diameter of the latter. The specific gravity of the cotton was found

to be nearly equivalent to that of water; and, by a comparative view of the two through a microscope, the surfaces appeared to be as 1000 to 3468. By proceeding in this calculation, it appeared that the surface of 30 grains of the cotton could not be less than 6600 square inches, while that of a like quantity of the silk amounted to no more than 476. Hence it evidently appeared, that the produce of air from the two substances was neither in proportion to their weights nor their surfaces. It appeared also, that the quality of the air produced at first was considerably inferior to that yielded some time afterwards. In order to ascertain the times at which air of the best quality was produced, &c. the following experiments were made: 1. A

⁴⁸ At what times air of the best quality is produced. globe, containing 46 cubic inches, being filled with water, and 30 grains of raw silk, well washed, and freed from the remains of former experiments, put into it, yielded in a cold and cloudy day only $\backslash frac\{1\}\{4\}$th of a cubic inch of air: the two following days it yielded 3$\backslash frac\{1\}\{2\}$ cubic inches, the quality of which was superior to that of the former in the proportion of 296 to 114 (A). 2. The globe being filled again with water, in two other days when the sunshine was less powerful, the quality was 197, and the quantity 1$\backslash frac\{1\}\{4\}$th; but afterwards, when the weather became fine, the quantity was again 3.8 inches, and quality 342. 3. The globe being again filled with water, and exposed to the sun for two days, yielded 2.2 inches of air, of a quality equal to 233. 4. A similar globe, with poplar-cotton which had been used in former experiments, gave 2.53 inches, of a quality 280. 5. A small globe of 20 inches, with 17 grains of raw silk, gave one cubic inch of air, of the quality 253. 6. A large globe of 296 inches, filled with fresh water, and a small quantity of conserva ricularis, gave $\backslash frac\{1\}\{4\}$ cubic inch, of the quality only of 124. The water was changed to a brown colour. 7. On repeating the experiment with a small handful of the conserva, 13.14 cubic inches of air were produced, of the quality 246. The water was very faintly tinged, towards the end of the experiment, of a greenish cast. 8. The globe of 46 inches, with 30 grains of raw silk used in many former experiments, produced in two days 1.6 cubic inches of air, of the quality 204. 9. A globe of equal capacity, with 15 grains of poplar-cotton, produced in the same time 1.28 inches, of the quality 260. In both these experiments, the water had acquired a faint greenish cast; but the colour of that with the cotton was deeper. On examining this water with a microscope, it was found to contain a great number of animalcules exceedingly small, and nearly of an oval figure; that with the silk contained them likewise, but not in such numbers: however, our author assures us, that in all cases in which the water acquired a greenish hue, he never failed to find them; and thinks, that from their presence alone, the colour of the water in the first instance universally arose.

⁴⁹ As Sir Benjamin was now more than ever embarrassed with respect to the share the silk and other bodies employed in these experiments had in producing the air, he made the following experiment to determine the matter: "Concluding (says he), that if silk and other bodies,

bodies, used in the foregoing experiments, actually did not contribute any thing, considered as chemical substances, in the process of the production of pure air yielded by water; but if, on the contrary, they acted merely as a mechanical aid in its separation from the water, by affording a convenient surface for the air to attach itself to; in this case, any other body having a large surface, and attracting air in water, might be made use of instead of the silk in the experiment, and pure air would be furnished, though the body should be totally incapable of communicating any thing whatever to the water."

With a view to ascertain this, the large globe being made perfectly clean, and filled with spring-water, he introduced into it a quantity of the fine thread of glass commonly called spun-glass, such as is used for making a brush for cleaning jewels, and an artificial feather fold by Jew pedlars. The result of the experiment was, that the globe being exposed in the sun, air-bubbles began almost instantly to make their appearance on the surface, and in four hours 0.77 of a cubic inch of air was procured, which, with nitrous air, showed a quality of 88; after which, not a single globe more was produced, though the globe was exposed for a whole week in fine sunshine weather. Hence it appears, that something more than mere surface was wanted to produce depllogifcated air from water by means of the sun's light.

The following experiments were made with a view to determine the quantity and quality of air produced by means of the heat and light of the sun from water alone. A large jar of clear glass, containing 455 cubic inches, being washed very clean, was filled with fresh spring water, inverted in a glass basin of the same, and exposed to the weather for 28 days. At the same time, another similar jar was filled with water taken from a pond in a garden in which many aquatic plants were growing, and exposed in the same place, and during the same period. The latter began to yield air in pretty large quantities on the third day, and continued to do so till the 14^th; the former yielded little or none till the 14^th, when it began to emit air, and continued to do so till the 22^d. On removing the air produced, that from the spring-water was 14 inches in quantity, and 138 in quality; but from the pond water, 31$\backslash frac\{1\}\{2\}$ in quantity, and 252 in quality. The colour of the waters was not changed; but both of them had deposited a considerable quantity of earth, which was found adhering to the surfaces of the glass basins in which the jars were inverted. As these basins, however, were very thick, and consequently had but little transparency, the sediment of the water was in a great measure deprived of the benefit of the sun's light; the experiment was therefore repeated with the following variation: In a large cylindrical jar of very fine transparent glass, 10 inches in diameter and 12 inches high, filled with spring-water, a conical jar, 9$\backslash frac\{1\}\{2\}$ inches in diameter at the bottom, and containing 344 inches, was inverted, and the whole exposed to the sun for 21 days. Little air was furnished till the 7^th day, when the liquor assumed a greenish cast, and a fine slimy sediment of the same colour, the green matter of Dr Priefley, beginning to be formed on the bottom, air was generated in abundance, and was furnished in pretty large quantities till the 18^th, when it entirely

These are the principal experiments contained in Sir Benjamin Thompson's letter to Sir Joseph Banks. Dr Ingenhousz's theory confirmed. In his postscript he observes, that as he never was thoroughly satisfied with the opinion of Dr Ingenhousz, that the depllogifcated air was elaborated in the vessels of the plant, he found his doubts rather confirmed than diminished by the experiments above related. "That the fresh leaves of certain vegetables (says he), exposed in water to the action of the sun's rays, cause a certain quantity of pure air to be produced, is a fact which has been put beyond all doubt: but it does not appear to me by any means so clearly proved, that this air is 'elaborated' in the plant by the powers of vegetation,—phlogifcated or fixed air being received by the plant as food, and the depllogifcated air rejected as an excrement?" for besides that many other substances, and in which no elaboration or circulation can possibly be supposed to take place, cause the water in which they are exposed to the action of the light to yield depllogifcated air as well as plants, and even in much greater quantities, and of a more eminent quality; the circumstances of the leaves of a vegetable, which, accustomed to grow in air, are separated from its stem and confined in water, are so unnatural, that I cannot conceive that they can perform the same functions in such different situations.

"Among many facts which have been brought in support of the received opinion of the elaboration of air in the vessels of plants, there is one upon which great stress is laid, which, I think, requires further examination. The fresh healthy leaves of vegetables, separated from the plant, and exposed in water to the action of the sun's rays, appear, by all the experiments which have hitherto been made, to furnish air only for a short time. After a day or two, the leaves, changing colour, cease to yield air. This has been conceived to arise from the powers of vegetation being destroyed, or, in other words, the death of the plant; and from hence it has been inferred, with some degree of plausibility, not only that the leaves actually retained their vegetative powers for some time after they were separated from their stock; but that it was in consequence of the exertion of those powers, that the air yielded in the experiment was produced.

"But I have found, that though the leaves, exposed in water to the action of light, actually do cease to furnish air after a certain time, yet that they regain this power after a short interval, when they furnish (or rather cause the water to furnish) more and better air than at first; which can hardly be accounted for upon the supposition that the air is elaborated in the vessels of the plant."

In confirmation of this doctrine, the globe of 46 inches was filled with fresh spring-water, and two peach-leaves were exposed for 10 days to the sun. In four days the water seemed to be entirely exhausted; but, next day, the water acquired a greenish colour, and again produced air pretty plentifully, which appeared in bubbles on the leaves; and on the 6^th day, 0.34 of a cubic inch of air was produced, of the quality 232. Next day it yielded 2$\backslash frac\{1\}\{2\}$ths of a cubic inch, of the quality 297. The three succeeding days it yielded 1$\backslash frac\{1\}\{2\}$ inches, the quality 307; after which an end was put to the experiment.—

iment.—On making other trials with leaves immersed in water already green and prepared to yield dephlogisticated air, it was found that they produced air in great quantity: but our author is of opinion, that all the appearances may be solved, by supposing that the air was produced in the mass of water by the green matter; and that the leaves, silk, &c. did no more than assist it in making its escape, by affording a convenient surface to which it could attach itself, in order to collect together and assume its elastic form.

Thus we see, that nature is provided with abundant resources for the supplying of this pure part of the atmosphere which is subject to such continual waste; and there is not the least doubt, that in a great number of cases the light of the sun produces pure air from water as well as from vegetables. It is probable, also, that even the waters of the ocean contribute towards this salutary purpose; as Dr Dobson of Liverpool found, that sea-water contained air superior in quality to that of the atmosphere. The purification of atmospheric air by agitating it in water, will be considered in a subsequent section.

As dephlogisticated air is found to support animal life for a much longer time than common air, it has been supposed that it might answer valuable purposes in medicine, provided any cheap method of procuring it in large quantities could be fallen upon. With this view, Mr Cavallo proposes to distil it from nitre with a strong heat; but the experiments already related certainly point out an easier method, free from the expense and trouble which must necessarily attend every chemical operation of this kind.

62. Properties of Dephlogisticated Air.—This kind of air possesses some of the properties of common air in a very eminent degree, but is deficient in others. Those in which it excels, are the support of flame and of animal life. It is equally elastic, or rather more so, than common air; as it likewise exceeds it a little in specific gravity, the proportion betwixt it and common air being that of 160 to 152. On introducing a lighted candle into dephlogisticated air, the flame not only grows larger, but becomes exceedingly bright; and when the air is very pure, the candle burns with a crackling noise, as if the air contained some combustible matter, at the same time that the wax or tallow wastes surprisingly fast.

The heat of the flame is in proportion to its light. If we fill a bladder with dephlogisticated air, and then fasten to its neck a glass tube whose aperture is drawn to a fine point, the dephlogisticated air, if driven out by pressing the bladder, will augment the heat of a candle to such a degree, that if any small bits of metal, placed on a piece of charcoal, be held in the apex of the flame, they will almost instantly be melted. Even grains of platina may by this means be melted; and in a larger fire there is no doubt that the effects of burning mirrors might be equalled.

On mixing dephlogisticated and inflammable air together, an explosion takes place as on mixing common and inflammable air, but with much greater violence. If an ounce vial, which for this purpose should be very strong, be filled with a little more than one-third of dephlogisticated and the rest inflammable air, and the flame of a candle presented to its mouth, it will explode nearly as loud as a small pistol.

All phlogistic processes are promoted much better by dephlogisticated than common air. Dr Priestley put a quantity of pyrophorus into one of the small jars used for making experiments upon air in quicksilver; then filling up the vessel with that fluid, he inverted it in a basin of the same, and threw in dephlogisticated air at different times. It always occasioned a sudden phlogistic and vehement accession, like the flashing of gun-powder, and the air was greatly diminished.

It has been, almost throughout all ages, believed, that combustion in every instance diminished common air, or reduced it to a smaller volume: but the late experiments of Mr Lavoisier have shown, that this is a mistake; and that in ordinary processes attended with the production of fixed and phlogisticated air, the quantity of vapour produced is equivalent to that absorbed, or otherwise made to disappear during the operation. With dephlogisticated air the case is very different. Mr Lavoisier having introduced a burning candle into a glass jar filled with very pure air obtained from calcined mercury, a great heat took place; which at first expelled a small quantity of the air; but afterwards, when the candle was extinguished, it was found that two-thirds of the bulk of air employed had been converted into fixed air, or a quantity of this kind of air equivalent to the former had been produced. The remainder, after taking up the fixed air by caustic alkali, was still as pure as before. In the common processes, he observes, that not more than one-tenth of the air employed is converted into fixed air. In this experiment, the superior gravity of fixed air, and the consequent condensation of the other, must undoubtedly have produced some diminution in the volume of air, though Mr Lavoisier does not take notice of it. In other cases, however, the diminution is much more perceptible. Mr Scheele having introduced some live coals into a matras filled with dephlogisticated air, found that it was diminished by one-fourth of its quantity. Repeating the experiment with sulphur, the flame became larger and more vivid than in common air, and three-fourths of its quantity were lost. Putting a piece of phosphorus into seven ounce-measures of this kind of air, stopping the mouth of the bottle with a cork, and setting fire to the phosphorus within it, the vial broke in pieces, as soon as the flame was extinguished, by the pressure of the external air. Repeating the experiment with a stronger vial, and opening it afterwards under water, the fluid rushed into it in such a manner as almost to fill it entirely. This extraordinary diminution was also perceived on setting fire to inflammable air in the dephlogisticated kind. The way in which he accomplished this was, by filling a matras with dephlogisticated air, and inverting it over a phial containing an effervescing mixture of vitriolic acid and iron-filings plunged into a vessel of hot water, and furnished with a slender tube reaching above the surface of the vessel, as represented Plate VIII. fig. 2. The inflammable air issuing from the orifice of the small tube, was set on fire previous to the inversion of the matras, and the mouth of the latter immersed in the water; on which that fluid soon began to rise, and continued to do so till seven-eighths of the vessel were full. In cases of slow combustion, where common air is diminished and phlogisticated, the dephlogisticated kind was found to be almost entirely.

57
Burns vehemently with pyrophorus.

58
Common air is not diminished by burning.

59
But dephlogisticated air suffers diminution.

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grain weight of loss. The vessel he used had a neck of about two feet long: and he particularly remarks, that, in order to succeed in this experiment, the fire should be managed with very great dexterity; for if that be too strong, part of the precipitate will be volatilized, and then the result of the experiment is precarious.

These experiments were opposed by others made by Dr. Priestley, who in several trials found that a considerable quantity of the metal was always lost. In one of these experiments, out of 11 pennyweights 10 grains of mercury, the loss amounted to one pennyweight two grains. In another experiment, 88 grains were lost, out of a quantity of red precipitate, in the preparation of which half an ounce of mercury had been employed. The quantity of mercury lost in his experiments, or rather the proportion of it to that of the metal employed, was always various, and the difference not very small; whence Mr. Cavallo and others, with great appearance of reason, conclude, that the true reason of any perceptible loss was the strong heat made use of in the distillation, and consequently that there is no reason to suppose that any earth exists in dephlogisticated air.

The next question was, Whether any of the nitrous acid existed in dephlogisticated air? That it contains none in a proper state of acidity, is indeed evident from many decisive experiments; but an idea was naturally entertained, that in the formation of dephlogisticated air the nitrous acid was decomposed, and part of it entered into the composition of the aerial fluid. This gave rise to the theories of Mr. Lavoisier and Mr. Kirwan, which are noticed under the article ACID; as also the experiments of Mr. Watt, which tended to show that no nitrous acid was destroyed in the composition of dephlogisticated air. To these Mr. Kirwan replied in the manner related in that article. We shall here, however, give a quotation from Dr. Priestley as a kind of addition to Mr. Watt's testimony on this head, so that the reader may be the better able to determine the weight of the evidence on both sides.

"At Mr. Watt's request (says he), I endeavoured to ascertain the quantity of acid that was expelled from nitre, in procuring the dephlogisticated air from it. To do this, I put two ounces of purified nitre into a glass retort, and receiving the air in 300 ounce-measures of water, only filled each recipient half full, and agitated the air very much in the water, in order to make the fluid imbibe as much as possible of the acid it contained. Notwithstanding this agitation, however, every vessel of the air retained a strong smell of the acid. The moment the air ceased to come, I filled a large phial with the water, and carried it to Mr. Watt, who carefully examined it; and in a paper which he presented to the Royal Society, and which is published in the Philosophical Transactions, he has given an account of the quantity of acid that was contained in all the 300 ounces of water: whence it may be fairly inferred, that there was no occasion to suppose that any of the acid entered into the composition of the air; but that it was all either rendered volatile or retained in the water." On the other hand, the Abbé Fontana informs us, that, in distilling an ounce of nitre with a strong heat, in order to expel dephlogisticated air from

it, only a few grains of weak nitrous acid are obtained, more or less as the fire applied is weak or strong: but that the quantity of dephlogisticated air extricated from it follows the contrary rule; being greatest when the heat is most violent and suddenly applied, and less when the fire is gradually applied.

On calcining metals in dephlogisticated air, very singular phenomena are observed, which seem to throw great light upon the composition of this fluid. "One of the most simple of all phlogistic processes (says Dr. Priestley), is that in which metals are melted in dephlogisticated air. I therefore began with this, with a view to ascertain whether any water be produced when the air is made to disappear in it. Accordingly, into a glass vessel, containing seven ounce-measures of pretty pure dephlogisticated air, I introduced a quantity of iron turnings, which is iron in thin small pieces, exceedingly convenient for these and many other experiments, having previously made them, together with the vessel, the air, and the mercury by which it was confined, as dry as I possibly could. Also to prevent the air from imbibing any moisture, I received it immediately in the vessel in which the experiment was made, from the process of procuring it from red precipitate, so that it had never been in contact with any water. I then fired the iron by means of a burning lens, and presently reduced the seven ounce-measures to 0.65 of a measure; but I found no more water after this process than I imagined it had not been possible for me to exclude, as it bore no proportion to the air which had disappeared. Examining the residuum of the air, I found one-fifth of it to be fixed air; and when I tried the purity of that which remained by the test of nitrous air, it did not appear that any phlogisticated air had been produced in the process: for though it was more impure than I suppose the air with which I began the experiment must have been, it was not more so than the phlogisticated air of the seven ounce-measures, which had not been affected by the process, and which must have been contained in the residuum, would necessarily make it. In this case, one measure of this residuum, and two of nitrous air, occupied the space of 0.32 of a measure. In another experiment of this kind, ten ounce-measures of dephlogisticated air were reduced to 0.8 of a measure, and by washing in lime-water to 0.38 of a measure. In another experiment, 71 ounce-measures of dephlogisticated air were reduced to half an ounce-measure, of which one-fifth was fixed air, and the residuum was quite as pure as the air with which I began the experiment; the test with nitrous air, in the proportions above mentioned, giving 0.4 in both cases.

"In these experiments the fixed air must, I presume, have been formed by the union of the phlogiston from the iron and dephlogisticated air in which it was ignited; but the quantity of it was very small in proportion to the air which had disappeared; and at that time I had no suspicion that the iron, which had been melted and gathered into round balls, could have imbibed it; a melting heat having been sufficient, as I had imagined, to expel every thing that was capable of assuming the form of air from any substance whatever. Sensible, however, that such a quantity of air must have been imbibed by something, to which it must have given a very perceptible addition of weight, and

Dephlogisticated Air. 63
Dephlogisticated air imbibed by iron.

feeling nothing else that could have imbibed it, it occurred to me to weigh the calx into which the iron had been reduced; and I presently found, that the dephlogisticated air had actually been imbibed by the melted iron, in the same manner as inflammable air had been imbibed by the melted calces of metals in my former experiments, however improbable such an absorption might have appeared a priori. In the first instance, about twelve ounce-measures of dephlogisticated air had disappeared, and the iron had gained six grains in weight. Repeating the experiment very frequently, I always found that other quantities of iron, treated in the same manner, gained similar additions of weight, which was always very nearly that of the air which had disappeared.

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It capable of taking it iron, sufficiently heated, was incapable of saturating itself with pure air from the atmosphere, I then proceeded to melt it with the heat of a burning lens in the open air; and I presently found, that perfect iron was easily capable of being fused in this way, and continued in this fusion a certain time, exhibiting the appearance of boiling or throwing out air; whereas it was, on the contrary, imbibing air; and, when it was saturated, the fusion ceased, and the heat of the lens could make no farther impression upon it. When this was the case, I always found that it had gained weight in the proportion of 7½ to 24, which is very nearly one-third of the original weight. The same was the effect when I melted steel in the same circumstances, and also every kind of iron on which the experiment could be tried. But I have reason to think, that with a greater degree of heat than I could apply, the iron might have been kept in a state of fusion somewhat longer, and by that means have imbibed more than even one-third of its original weight.

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Remarkable phenomenon attending the melting of cast-iron.

“There was a peculiar circumstance attending the melting of cast iron with a burning lens, which rendered it impossible to ascertain the addition that was made to its weight, and at the same time afforded an amusing spectacle: for the moment that any quantity of it was melted, and gathered into a round ball, it began to disperse in a thousand directions, exhibiting the appearance of a most beautiful fire-work; some of the particles flying to the distance of half a yard from the place of fusion; and the whole was attended with a considerable hissing noise. Some of the largest pieces, which had been dispersed in this manner, I was able to collect, and having subjected them to the heat of the lens, they exhibited the same appearance as the larger mass from which they had been scattered.

71
Formation of water from dephlogisticated and inflammable air.

“When this cast iron was melted in the bottom of a deep glass receiver, in order to collect all the particles that were dispersed, they firmly adhered to the glass, melting it superficially, though without making it crack, so that it was still impossible to collect and weigh them. However, I generally found, that, notwithstanding the copious dispersion, what remained after the experiment rather exceeded than fell short of the original weight of the iron.”

On attempting to revive this calx of iron in inflammable air, a very new and unexpected appearance took place. Having put a piece of iron saturated with pure air into a vessel filled with inflammable air confined by water, the inflammable air disappeared and the metal

was revived; but on weighing it, he found that 2½ dephlogisticated grains out of 11½ had been lost, besides the 7½ ounce-measures of inflammable air which had vanished. Considering all these circumstances, the Doctor had now no doubt that the two kinds of air had united and formed either fixed air or water; and with a view to determine this point, he repeated the experiment in a vessel where the inflammable was confined by mercury, both the vessel and mercury having been previously made as dry as possible. In these circumstances he had no sooner begun to heat the iron, than the air was perceived to diminish, and at the same time the inside of the vessel to become cloudy, with particles of dew that covered almost the whole of it. These particles by degrees gathered into drops, and ran down in all places, excepting those which were heated by the sunbeams. On collecting the water produced in this experiment, by means of a piece of filtering paper carefully introduced to absorb it, he found it to be as nearly as possible of the same weight with that which had been lost by the iron; and also in every experiment of this kind, in which he attended to the circumstance, he found that the quantity of inflammable air which had disappeared was about double that of the dephlogisticated air set loose in the operation, supposing that weight to have been reduced into air. Thus, at one time, a piece of this flag absorbed 5½ ounce measures of inflammable air, while it lost the weight of about three ounce-measures of dephlogisticated air, and the water collected weighed two grains. Another time a piece of flag lost 1.5 grains, and the water produced was 1.7 grains. In a third case, where 6½ ounce-measures of inflammable air were reduced to 0.92 of a measure, the iron had lost the weight of 3.3 ounce-measures of dephlogisticated air, or nearly two grains.

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Quantity produced in this manner.

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The Doctor having succeeded so well with iron, next experimented the calx of copper, or those scales which fly off metals with from it by hammering whilst it is red-hot; and found copper, &c. water produced in the inflammable air in the same manner as when the scales of iron were used. On using precipitate per se, he imagined at first that water was obtained from this substance also; but on repeating the experiment to more advantage, he found no more water than might be supposed to have been contained as an extraneous substance either in the inflammable air or in the red precipitate. With iron, however, the case was vastly different. As the Doctor had formerly satisfied himself that inflammable air always contains a portion of water, and also that when it has been some time confined by water it imbibes more, so as to be increased in its specific gravity by that means, he repeated the experiment with inflammable air which had not been confined by that fluid, but was received in a vessel of dry mercury from the vessel in which it had been generated; but in this case the water was produced, to appearance, as copiously as in the former experiment. “Indeed (says he), the quantity of water produced, so greatly exceeding the weight of all the inflammable air, is sufficient to prove that it must have had some other source than any constituent part of that air, or the whole of it, together with the water contained in it, without taking into consideration the corresponding loss of weight in the iron.

“I must here observe, that the iron flag which I had treated in this manner, and which had thereby lost

the weight which it had acquired in dephlogisticated air, became perfect iron as at first, and was then capable of being melted by the burning lens again; so that the same piece of iron would serve for these experiments as long as the operator should choose. It was evident, therefore, that if the iron had lost its phlogiston in the preceding fusion, it had acquired it again from the inflammable air which it had absorbed; and I do not see how the experiment can be accounted for in any other way."

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Experiments of Mr Cavendish, &c. on water.

As these experiments of Dr Priestley tend very much to throw some light on the composition of dephlogisticated air, we shall here give an account of some others made by Mr Cavendish, as well as those of Dr Priestley and the French chemists, upon water: From all which it is concluded by the most celebrated philosophers and chemists, That dephlogisticated air is one of the constituent and elementary parts of water, inflammable air being the other; though the opinion is still contested by some foreign chemists.

"As there seemed great reason," says Mr Cavendish, "to think, from Dr Priestley's experiments, that the nitrous and vitriolic acids were convertible into dephlogisticated air, I tried whether the dephlogisticated part of common air might not be converted into nitrous or vitriolic acid." For this purpose he impregnated some milk of lime with the fumes of burning sulphur, by burning 122 grains of sulphur in a large glass receiver, in which some lac calcis was included. No nitrous salt, nor any thing besides selenite, was produced in the process. Neither was any nitrous acid produced by phlogisticating common air with liver of sulphur, or by treating dephlogisticated air in the same manner. The liver of sulphur used in these experiments was made with lime; and the only observation made on this occasion was, that the selenite produced was much more soluble in water than when made with dephlogisticated vitriolic acid.

To try whether any vitriolic acid was produced by the phlogistication of air, 50 ounces of distilled water were impregnated with the fumes produced on mixing 52 ounce-measures of nitrous air with a quantity of common air sufficient to decompose it. This was done by filling a bottle with some of this water, and inverting it into a basin of the same; and then by a syphon, letting in as much nitrous air as filled it half full; after which, common air was added slowly by the same syphon, till the nitrous air was decomposed. When this was done, the distilled water was further impregnated in the same manner till the whole quantity of nitrous air was employed. The impregnated water was sensibly acid to the taste; and on distillation yielded first phlogisticated nitrous acid, then water, and lastly a very acid liquor consisting of dephlogisticated nitrous acid. By saturation with salt of tartar, 87$\backslash frac\{1\}\{2\}$ grains of nitre, without any mixture of vitriolated tartar, or other vitriolic salt, were obtained.

These experiments having proved unsuccessful, Mr Cavendish next proceeded to try the effects of exploding dephlogisticated and inflammable air together in close vessels. He begins with relating an experiment of Dr Priestley; in which, it was said, that on firing a mixture of common and inflammable air by electricity, in a close copper vessel holding about three pints, a loss of weight was always perceived, on an average

about two grains, though the vessel was stopped in dephlogisticated air. It is also related, that on repeating the experiment, in glass vessels, the inside of the glass, though clean and dry before, immediately became dewy; which confirmed an opinion he had long entertained, that common air deposits its moisture by phlogistication. The experiment, however, did not succeed with Mr Cavendish, at least with regard to the loss of weight; which never exceeded the fifth part of a grain, and commonly was nothing at all. In these experiments the greatest care was taken to observe with accuracy the diminution of air by the explosion, and quality of the remainder; from which it appeared, that 423 measures of inflammable air were nearly sufficient to phlogisticate 1000 of common air, and that the bulk of air remaining after the explosion is very little more than four-fifths of the common air employed; whence he concludes, that "when they are mixed in this proportion, almost all the inflammable, and about one-fifth of the common air, lose their elasticity, and are condensed into the dew which lines the glass."

78
Quantity of inflammable air necessary to phlogisticate common air.

To examine more exactly the nature of this dew, 500,000 grain-measures of inflammable air were burnt with about 2$\backslash frac\{1\}\{2\}$ times the quantity of common air, and the burnt air was made to pass through a glass cylinder eight feet long and three-fourths of an inch in diameter, in order to deposit the dew. The two airs were conveyed slowly into this cylinder by separate copper pipes, passing through a brass plate which stopped up one end of the cylinder; and as neither inflammable nor common air can burn by themselves, there was no danger of the flame spreading to the magazines from which they were conveyed. Each of these magazines consisted of a large tin vessel inverted into another just big enough to receive it. The inner vessel communicated with the copper pipe, and the air was forced out of it by pouring water into the outer vessel; and in order that the quantity of common air expelled should be 2$\backslash frac\{1\}\{2\}$ times that of the inflammable air, the water was let into the outer vessels by two holes in the bottom of the same tin pan; the hole which conveyed the water into that vessel in which the common air was confined being 2$\backslash frac\{1\}\{2\}$ times as big as the other. In trying the experiments, the magazines being first filled with their respective airs, the glass cylinder was taken off, and water let by the two holes into the outer vessels, till the airs began to issue from the ends of the copper pipes; they were then set on fire by a candle, and the cylinder put on again in its place. By this means upwards of 135 grains of water were left in the cylinder, which had no taste nor smell, and which left no perceptible sediment on being evaporated to dryness; neither did it yield any pungent smell during the evaporation; in short, it seemed pure water. In one of his experiments a little foaty matter was perceived, but it was found to proceed from the luting. On repeating the experiment with dephlogisticated, instead of common air, the produce was nitrous acid.

The following conclusion is drawn by Mr Cavendish from all these experiments: "There seem two ways by which the production of the nitrous acid, in the manner above-mentioned, may be explained: first, by supposing that dephlogisticated air contains a little nitrous acid, which enters into it as one of its component parts;

parts; and that this acid, when the inflammable air is in sufficient proportion, unites to the phlogiston, and is turned into phlogisticated air, but does not when the inflammable air is in too small proportion: and, secondly, by supposing that there is no nitrous acid mixed with or entering into the composition of dephlogisticated air; but that, when the air is in sufficient proportion, part of the dephlogisticated air with which it is debased is, by the strong affinity of phlogiston to dephlogisticated air, deprived of its phlogiston, and turned into nitrous acid; whereas, when the dephlogisticated air, is not more than sufficient to consume the inflammable air, none then remains to deprive the phlogisticated air of its phlogiston, and turn it into acid.— If the latter explanation be true, I think we must allow that dephlogisticated air is in reality nothing but dephlogisticated water, or water deprived of its phlogiston; or, in other words, that water consists of dephlogisticated air united to phlogiston. On the other hand, if the former explanation be true, we must suppose, that dephlogisticated air consists of water united to a little nitrous acid, and deprived of its phlogiston; but still the nitrous acid in it must only make a very small part of the whole, as it is found that the phlogisticated air into which it is converted is very small in comparison of the dephlogisticated air. I think the second of these explanations seems much the more likely; as it was found that the acid in the condensed liquor was of the nitrous kind, not only when the dephlogisticated air was prepared from nitrous acid, but when procured from plants or turbith mineral. Another strong argument in favour of this opinion is, that dephlogisticated air yields no nitrous acid when phlogisticated by liver of sulphur; for if this air contains nitrous acid, and yields it when phlogisticated by explosion with inflammable air, it is very extraordinary that it should not do so by other means. But what forms a stronger, and, I think, almost decisive argument in favour of this explanation, is, that when the dephlogisticated air is very pure, the condensed liquor is made much more strongly acid by mixing the air to be exploded with a little phlogisticated air."

The experiments of Dr Priestley alluded to were those in which inflammable air was supposed by Mr Lavoisier to be procured from water by passing its steam through red-hot iron tubes. It was soon discovered, however, by Dr Priestley, that this inflammable air did not proceed from the water, but from the iron of the tube; and might be obtained by transmitting aqueous vapour through charcoal or iron placed in tubes of copper, glass, or earthen ware, made red-hot, but not through these tubes by themselves. In this case, the loss of the water employed exceeded that of the inflammable air produced in the proportion of 1.3 to 2; and the iron which had thus absorbed the water, appeared exactly similar to that which had been burned in dephlogisticated air in the manner already related. His conclusions from thence are these: "Since iron gains the same addition of weight by being melted in dephlogisticated air, and also by the addition of water

VOL. I. Part I.

when red hot, and becomes, as I have already observed, the same substance in all respects, it is evident that this air or water, as existing in the iron, is the very same thing; and this can hardly be explained but on the supposition that water consists of two kinds of air, viz. inflammable and dephlogisticated."

Of these processes he gives the following explanation: "When iron is heated in dephlogisticated air, we may suppose, that, though part of its phlogiston escapes, to enter into the composition of the small quantity of fixed air which is then procured, yet enough remains to form water with the dephlogisticated air which it has imbibed, so that this calx consists of the intimate union of the pure earth of iron and of water; and therefore, when the same calx, thus saturated with water, is exposed to heat in inflammable air, this air enters into it, destroys the attraction between the water and the earth, and revives the iron, while the water is expelled in its proper form."

The whole of the Doctor's opinions on the component parts of this kind of air, however, are summed up in the following sentence in his Observations relating to Theory.—"The only kind of air that is now thought to be properly elementary, and to consist of a simple substance, is dephlogisticated air; with the addition at least of the principle of heat, concerning which we know very little; and as it is not probable that this adds anything to the weight of bodies, it can hardly be called an element in their composition. Dephlogisticated air appears to be one of the elements of water, of fixed air, of all the acids, and many other substances, which, till lately, have been thought to be simple."

The experiments of the French philosophers were of the same nature with those of Mr Cavendish, but conducted on a larger scale. The inference drawn from them was the same with that already mentioned, viz. that dephlogisticated and inflammable air in all cases are the two constituent parts of water. This opinion is adopted by Mr Kirwan in his Treatise on Phlogiston. "The experiments of Mr Cavendish, and of Mr Kirwan," says he, "appear to me to leave no room to doubt, that when very pure dephlogisticated and inflammable air are inflamed, the product is mere water (A); for when these airs are employed in the proper proportion, only 0.02 of the mixture of both airs retains its aerial form. Now it is impossible to suppose that all the water obtained pre-existed in these airs; that is, that 49 parts in 50 were mere water."

Notwithstanding these positive conclusions, however, by some of the most respectable names in this country, the evidences adduced have been unsatisfactory to some French chemists; who maintain, that Messrs Cavendish, Priestley, and Kirwan, are totally mistaken with regard to the production of water from dephlogisticated and inflammable air; contending, that the water obtained had previously existed in the air, and was not originally produced in the operation. The fact, indeed, becomes somewhat dubious from some experiments related by Dr Priestley himself, and of which we shall now proceed to give an account.

X One

(A) The experiments of Mr Cavendish show that nitrous acid is the product in this case. He takes notice of the difference between the result of the French experiments and his, but ascribes it to their using inflammable air prepared from charcoal: His was from zinc.

One consequence of the hypothesis in question is evident, that if water really be produced by the depllogration of either depllogifcated or common air with inflammable air, the quantity of liquid obtained ought to increase in proportion to the quantity of the two airs consumed, and that without any limitation. This, however, is not the case, as Dr Priestley has observed. He had succeeded indeed with scales of iron and copper, as has already been related; and in the experiment with the latter, the production of water was so copious, that when only $3\backslash frac\{1\}\{2\}$ ounce-measures of air were absorbed, the water flood in drops on the inside of the vessel, and some of these ran down it. Water was also procured by firing depllogifcated and inflammable air from iron by the electric spark in a close vessel, an experiment similar to those made by Mr Lavoisier at Paris. In his first experiment he put $3.75$ ounce measures of a mixture of air, of which one-third was depllogifcated and two-thirds inflammable air from iron, in a close vessel, and, after the explosion, found in it one grain of moisture; but on repeating the experiment with half as much depllogifcated as inflammable air, he could perceive no sign of moisture. The greatest difficulty, however, which he says he ever met with respecting the preceding theory, arose from his never having been able to procure any water when he revived red precipitate in inflammable air, or at least no more than might have been supposed to be contained in the inflammable air as an extraneous substance.

In order to make the experiments with the scales of iron and that with the red precipitate as much alike as possible, and compare them both to the greatest advantage, he made them one immediately after the other, with every circumstance as nearly the same as he could. The inflammable air was the same in both experiments, and both the scales of iron and red precipitate were made as dry as possible. They were heated in vessels of the same size and form, and equally confined by dry mercury; and yet, with the former, water was produced as copiously as before, viz. running down the inside of the vessel in drops, when only four ounce-measures of inflammable air were absorbed; but though he heated the red precipitate till eight ounce-measures of the inflammable air were absorbed, and only $0.75$ of an ounce-measure remained, there was hardly any sensible quantity of water produced, "certainly," says he, "not one-tenth of what appeared in the experiment with the scales of iron. In this experiment there can be no doubt but that the depllogifcated air produced from the red precipitate united with the inflammable air in the vessel; and as no water equal to the weight of the two kinds of air was produced, they must have formed some more solid substance, which, in the small quantities I was obliged to use, could not be found.

"The difficulty, with respect to what becomes of the two kinds of air, was not lessened by the attempts which I made to collect all that I could from repeated decompositions of inflammable and depllogifcated air in a close vessel. As I had produced water in this process when no more than a single explosion was made at a time, I thought that by continuing to make explosions in the same vessel, the water would not fail to accumulate till any quantity might be collected; and I intended to have collected a considerable part of an ounce. And as I should know exactly what quantity

of air I decomposed, I had no doubt of being able to ascertain the proportion that the water and air bore to each other. With this view a mixture was made of a large quantity of air, one-third depllogifcated and two-thirds inflammable, from iron and oil of vitriol.—But though I had a sensible quantity of water at the first explosion (in each of which between four and five ounce-measures of the mixture of air were used), I was surprised to perceive no very sensible increase of the quantity of water on repeating the explosions. Having therefore expended $48$ ounce-measures of the mixture, the process was discontinued; and, collecting the water with all the care that I could, I found no more than three grains, when there ought to have been eleven.

"In this process the inside of the vessel was always very black after each explosion; and when I poured in the mercury after the explosion, though there was nothing visible in the air within the vessel, there issued from the mouth of it a dense vapour. This was the case, though I waited so long as two minutes after any explosion, before I proceeded to put in more mercury in order to make another; which, if the vapour had been steam, would have been time more than sufficient to permit it to condense into water. I even perceived this vapour when I had a quantity of water in the vessel, and the explosion was consequently made over it, as well as in contact with the sides of the vessel which were wetted with it; so that, as this vapour had passed through the whole body of water when the vessel was inverted, it is probable that it must have consisted of something else than mere water. But I was never able to collect any quantity of it, though it must have been something produced by the union of the two kinds of air."

In order to collect a quantity of this vapour, he contrived an apparatus, which, by diffusing it through a thin glass vessel, he supposed would condense all the contents whether fluid or solid; but after repeating the experiment as carefully as possible, by taking $20$ explosions, and repeating the whole several times over, he could find nothing in the vessel besides a small quantity of water, which, added to that in the strong vessel, came far short of the weight of the air that was decomposed.

"All the conjecture," say he, "that I can advance, in order to explain this phenomenon is, that since foot yields pure air, part of the foot is formed by the union of the depllogifcated air in the atmosphere, and the inflammable air of the fuel: but smoke, which contains much foot, is soon dispersed, and becomes invisible in the open air. Such, therefore, may be the case here. The foot formed by the union of the two kinds of air, may be diffused through the air, in the vessel in which they are exploded, and be carried invisibly into the common atmosphere; which may account for my not being able to collect any quantity of it in this apparatus."

Not discouraged by this bad success, the Doctor attempted to collect this volatile matter by means of a quantity of water incumbent upon the mercury in the strong glass vessel in which the explosions were made, though he had found that part of it could escape through the water. He decomposed a great quantity of the two kinds of air in these circumstances; and pre-

Dephlogisticated Air. Phlogisticated Air.
fently found that the water became very cloudy, and was at length filled with a blackish matter. This he collected, and found that it remained perfectly black upon the earthen vessel in which the water containing it was evaporated; which would not have been the case if the blackish matter in the water had been that powder of mercury which is produced by agitating it in pure water: For that black mass always became white running mercury the moment the water was evaporated from it. If a sufficient quantity of this matter could have been procured, he could have satisfied himself whether it was foot or not.

89 Water in considerable quantity obtained from dephlogisticated and inflammable air. See Plate VIII. fig. 3.
"That water, in great quantities (says he), is sometimes produced from burning inflammable and dephlogisticated air, is evident from the experiments of Messrs Cavendish and Lavoisier. I have also frequently collected considerable quantities of water in this way, though never quite so much as the weight of the two kinds of air decomposed. My apparatus for this purpose was the following: Into the mouth of a large glass balloon. I introduced a tube, from the orifice of which there continually issued inflammable air from a vessel containing iron and oil of vitriol. This being lighted, continued to burn like a candle. Presently after the lighting of it, the inside of the balloon always became cloudy, and the moisture soon gathered in drops, and settled in the lower part of the balloon. To catch what might issue in the form of vapour, in the current of air through the balloon, I placed the glass tube b, in which I always found some water condensed. It is very possible, however, that in both these modes of experimenting, the water may be converted into a kind of vapour, which is very different from steam, and capable of being conveyed a great way through air, or even water, without condensation along with the air with which it is mixed; and on this account it may not be possible, in either of these modes of experimenting, to collect all the water into which the two kinds of air may be converted. The nature of this kind of vapour into which water may be changed, and which is not readily condensed by cold, is very little understood, but well deserves the attention of philosophers.

"That the water collected in the balloon comes from the decomposition of the air, and not from the fresh air circulating through it, was evident from placing balls of hot iron in the place of the flame, and finding that, though the balloon was as much heated by them as by the flame of the burning of the inflammable air, and consequently there must have been the same current of the external air through it, no moisture was found in the balloon."

89 Phlogistication of air explained.
The universal prejudice in favour of the existence of that principle named Phlogiston, first suggested by Stahl, gave rise, on the first appearance of Dr Priestley's discoveries, to a theory, concerning the action of this substance upon air and other bodies. As it had been observed, that air was diminished, in some cases at least, by burning, universally by respiration, and by some other processes, it was imagined that phlogiston was a body of such a singular nature, that when mixed with air, it always diminished

its bulk, instead of enlarging it, which might have been more naturally expected from the mixture of any vapour whatever. It was also supposed by some, that the phlogiston was not only entirely devoid of gravity, but that it was a principle of positive levity; so that the absolute weight of bodies was diminished by an union with it, and augmented when it was expelled, though their specific gravity was diminished. Various other surprising properties were attributed to phlogiston; such as that of giving elasticity to air, of constituting flame by a chemical combination with air, &c. Its emission into the atmosphere was supposed to be always attended with a diminution of air; and therefore, all processes in which air was diminished and become noxious, such as that by liver of sulphur, a mixture of iron filings and brimstone, &c. were called phlogistic processes. Respiration of animals was taken into the same account; but neither in this, nor in combustion, was it allowed that any kind of vital spirit was absorbed by the blood, or separated from the air by the burning body. On the contrary, it was strenuously argued, that all this was performed by the emission of phlogiston from the lungs or the inflamed substance, which deprived the air, and diminished it in bulk; and as all air was supposed to contain phlogiston, it was likewise imagined, that in all cases where air was mended, as by the growing of vegetables, or agitation in water, the emendation was accomplished, not by the emission of any thing into the atmosphere, but by the mere absorption of phlogiston. In other respects this substance was thought to be an exceedingly powerful principle in nature; the light of the sun itself and the electric fluid being said to be modifications of it, the different kinds of airs to be phlogistic vapours, &c.; so that the whole system of nature seemed ready to be absorbed by it at once.

90 Too great powers attributed to phlogiston.
The formidable powers of this principle were first checked by the discoveries of Mr Lavoisier, though the latter erred equally on the contrary side; and not content with keeping the phlogistic principle within due bounds, would needs deny its existence altogether*. In a treatise published in the year 1782, he first impugns Dr Priestley's theory of respiration, and denies that "the respiration of animals has the property of phlogisticating air in a manner similar to what is effected by the calcination of metals and many other chemical processes; and that it ceases not to be respirable till the instant when it becomes surcharged, or at least saturated, with phlogiston."

91 Doctrine of phlogiston opposed by the foreign chemists. See Plate VIII.
In order to disprove this assertion, he introduced four ounces of mercury to 50 cubic inches of common air, proposing to calcine the metal by keeping it for 12 days in a heat almost equal to that which is necessary to make it boil. After the expiration of the appointed time, 45 grains of precipitate per se were formed, and the air in the vessel was diminished by about 1/2th of its volume. In this state it did not precipitate lime water; but instantly extinguished candles, and killed animals immersed in it; no longer affording any red vapours, or being diminished by mixture with nitrous air: On distilling the precipitate produced, about as much dephlogisticated air was obtained as had been left by the common air in the calcination; and by recombining this with the noxious air left in the vessel, he recomposed a fluid nearly of the same goodness with common air. Hence he draws the following conclusions:

1. That $\backslash frac\{2\}\{3\}$ths of the air we breathe are mephitic, or incapable of supporting the respiration of animals, or the inflammation and combustion of bodies. 2. That the surplus, or only $\backslash frac\{1\}\{3\}$th of the volume of atmospheric air, is respirable. 3. That in the calcination of mercury, this metallic substance absorbs the salubrious part, leaving only the mephitic portion of the air. 4. That by reuniting these two portions which had been separated, we can recompound air similar to that of the atmosphere.

To determine the effects of respiration upon air, a live sparrow was placed under a glass receiver, filled with common air and inverted in mercury, containing 31 cubic inches. In a quarter of an hour it became agitated, and in 55 minutes died convulsed. Notwithstanding the heat of the animal, which necessarily, at first, rarified the air in the receiver, there was a sensible diminution of its bulk; which, at the end of 15 minutes, amounted to one-fortieth: but, instead of increasing afterwards, the diminution became something less in about half an hour; and when the animal was dead, and the air in the receiver had recovered the temperature of the room where the experiment was made, the diminution did not appear to exceed one-sixtieth part.—This air which had been respired by the sparrow, though in many respects similar to that in which the mercury had been calcined, differed from it in this respect, that it precipitated lime-water, and, by introducing caustic fixed alkali to it, was reduced one-sixth in bulk by the absorption of fixed air; after which it appeared exactly the same with that produced by the calcination of mercury or other metals; and atmospheric air was recomposed by mixing this with pure dephlogisticated air in the proportions already mentioned.

That common air is compounded of two kinds of elastic fluids, Mr Scheele has proved by the following experiment: "I dissolved (says he) one ounce of alkaline liver of sulphur in eight ounces of water; of this solution I poured four ounces into an empty bottle, whose capacity was 24 ounces, and worked it well; then I turned the bottle, immersed its neck into a small vessel with water, and kept it in this position a fortnight. The solution had partly lost its red colour, and some sulphur had been precipitated from it during this time. After this I put the bottle in the same position in a larger vessel with water, keeping the mouth and neck under water, and the bottom of the bottle above water, and thus I drew the cork under water, which immediately rushed with violence into the bottle. On examining the quantity of water in the bottle, it was found, that during this fortnight, six parts out of 20 of air were lost." On repeating the experiment with the same materials, and in the same bottle, only four parts out of 20 were lost by standing a week, and no more than six after four months.

From these experiments, and many others similar, it appears that the doctrine of phlogiston had been carried too far by Dr Priestley and other British philosophers, and that the air consists of two kinds of fluids; one perfectly salutary, and friendly in the highest degree to animal life; the other altogether unfit for it. These two appear incapable of being converted directly into one another by any process, natural or artificial: for though both are destructible, yet they are always converted into other substances; from which,

indeed, either the one or the other may be extracted at pleasure by employing the proper methods. The strongest arguments in favour of the transmutation of phlogisticated air into that of a purer kind, were drawn from the purification of noxious air by vegetation, and by agitation in water. In the former case, however, it has been observed in the last section, that this seeming purification is no other than an exchange of the one air for the other; the vegetables absorbing the phlogisticated, and emitting the dephlogisticated air in its stead. With respect to the agitation in water, the matter remained more dubious; and it is only in the last volume of Dr Priestley's treatise that we have any account of this being accomplished by an emission of purer air from the water. "In the infancy of my experiments," says he, "I concluded, that all kinds of air were brought by agitation to the same state; the purest air being partially phlogisticated, and air completely phlogisticated being thereby made purer; inflammable air also losing its inflammability, and all of them brought into such a state as that a candle would just go out in them. This inference I made from all the kinds of air with which I was then acquainted, and which did not require to be confined by mercury, being brought to that state by agitation in a trough of water, the surface of which was exposed to the open air; never imagining that when the air in my jar was separated from the common air by a body of water, generally about twelve inches in depth (adding that within to that without the jar), they could have any influence on each other. I have, however, been long convinced, that, improbable as it then appeared to me, this is actually the case."

This remarkable fact is illustrated by the following experiments: 1. About three ounce-measures of air, phlogisticated by nitrous air, was agitated for a quarter of an hour in a vessel containing 20 ounces of water, which had been boiled for several hours, and which was still very warm. By this process it became diminished one-sixth, and considerably improved in quality. The next day the remainder was agitated for another quarter of an hour, and the water which had been boiled at the same time, when it was also diminished in quantity and improved in quality. 2. An equal quantity of air, phlogisticated by means of iron-filings and brimstone, being agitated for 20 minutes, was diminished by one-seventh, and improved so far that a candle would burn in it. 3. After expelling all the air he could from a quantity of water by boiling, he put to it, in separate phials, air that had been phlogisticated with iron-filings and brimstone, as well as that which the heat had expelled, leaving them with their mouths in water, and agitating them occasionally. On examining the phials in about two months, he found both the air that was confined by water and that which had been expelled by heat completely phlogisticated. 4. That water does imbibe the purer part of the atmosphere, in preference to that which is impure, is evident, he says, from any examination of it: For if the water be clear, and free from any thing that is putrescent, the air expelled from it by heat is generally of the standard of 1; whereas that of the atmosphere, when the nitrous air is the purest, is about 1.2.

Phlogisticated air is equally invisible with common air, and something more elastic. Mr Kirwan procured

Phlogisticated Air. cured some perfectly phlogisticated, so that it was not in the least diminished by nitrous air, from a mixture of iron-filings and brimstone. Having dried it by frequently introducing dry filtering paper under the jar that contained it, he found its weight to be to that of the common air as 985 to 1000, the barometer standing at 30.46 and the thermometer at 60°. The other properties of it are, that it is extremely fatal to animal life, and friendly to that of vegetables, inasmuch that it is now generally believed to be the true and proper nourishment of the latter. It seems to exist originally, in very large quantity, in our atmosphere. It may be separated from the common mass of air by combustion, by respiration, by putrefaction, and in short by every species of phlogistic process; neither is there any other species of air but what may be converted into this by means of fire, dephlogisticated air alone excepted.

100 Nitrous acid procured from phlogisticated air. Phlogisticated air is now generally believed to be a combination of the nitrous acid with phlogiston; and that, in its gradual progress towards this, which is its ultimate stage, it first assumes the character of phlogisticated nitrous acid; then of nitrous air, in which it readily parts with its phlogiston to the atmosphere, or rather to the dephlogisticated part of it; and lastly, it becomes phlogisticated air, in which the union betwixt the principles is so strong, that it cannot be broken by simple exposure to dephlogisticated air without heat; though the experiments of Mr Cavendish show, that this may be done by means of the electric spark, which produces the most violent heat we can imagine.

101 Mr Cavendish's experiments on the production of nitrous acid. It had been frequently observed, that common atmospheric air was always diminished by taking the electric spark in it; and this diminution was supposed to be occasioned by the phlogistication of the air, and separation of its fixed part; in consequence of which it was urged, that lime-water is precipitated by taking the electric spark over it in a small quantity of air. Mr Cavendish, however, who has carefully examined this subject, denies that any fixed air is produced in this manner; and by a set of very curious experiments, published in the 75^th volume of the Philosophical Transactions, has clearly shown that nitrous acid, and not fixed air, is the product of this operation.

The apparatus used in these experiments, was that represented Plate VIII. fig. 4. and consists only of a crooked glass tube, whose ends are plunged into quicksilver contained in two glasses, in the middle part of which the air is confined betwixt the two portions of quicksilver. The air was introduced by means of a smaller tube, fig. 5. the tube M of the former figure being filled with quicksilver, the bent end of which was introduced into a jar DEF, filled with the proper kind of air, and inverted in water. The end C being stopped by the finger, the quicksilver was thus prevented from falling out, let the tube be placed in what position it would, until this pressure was removed. Upon introducing the crooked tube into the jar in the position represented in the figure, and removing the finger from the orifice at C, the quicksilver would descend; and by stopping this orifice again, any quantity of the fluid may be allowed to run out, and the empty space of the tube will be filled with the air desired. Having thus got the proper quantity of air into the tube ABC, it was held with the end C uppermost, and stopped with the finger; and the end A,

Phlogisticated Air. made smaller for that purpose, being introduced into the end of the bent tube M, the air, on removing the finger from C, was forced into that tube by the pressure of the quicksilver in the leg BC. Thus he was enabled to introduce any quantity he pleased of any kind of air into the tube M; and by the same means it was in his power to let up any quantity of soap-ley, or other liquor which he wanted to be in contact with it. In one case, however, in which he wished to introduce air into the tube many times in the same experiment, he made use of the apparatus represented fig. 6. consisting of a tube AB, of a smaller bore, a ball C and a tube DE of a larger bore. This apparatus was first filled with quicksilver; and then the ball C and the tube AB were filled with air, by introducing the end A under a glass inverted into water, which contained the proper kind of air, and drawing out the quicksilver from the leg ED by a syphon. After being thus furnished with air, the apparatus was weighed, and the end A introduced into one end of the tube M, and kept there during the experiment; the way of forcing air out of this apparatus into the tube being by thrusting down the tube ED, a wooden cylinder of such a size as almost to fill up the whole bore, and by occasionally pouring quicksilver into the same tube, to supply the place of that pushed into the ball C. After the experiment was finished, the apparatus was weighed again, which showed exactly how much air had been forced into the tube M during the whole experiment; it being equal in bulk to a quantity of quicksilver, whose weight was equal to the increase of weight of the apparatus. The bore of the tube M, used in these experiments, was about the tenth of an inch in diameter; and the length of the column of air occupying the upper part of the tube was in general from $\backslash frac\{1\}\{4\}$ to $1\backslash frac\{1\}\{2\}$ inches.—In order to force an electrical spark through the tube M, it was necessary to place an insulated ball at such a distance from the conductor as to receive a spark from it, and to make a communication between that ball and the quicksilver in one of the glasses, while the quicksilver in the other glass communicated with the ground.

When the electric spark was made to pass through common air inclined between short columns of a solution of litmus, the solution acquired a red colour, and the air was diminished, as had been observed by Dr Priestley. When lime-water was used instead of the solution of litmus, and the spark was continued till the air could be no further diminished; but not the smallest cloud be perceived in the water, though the air was reduced to two thirds of its original bulk; which is a greater diminution than it could have suffered by any phlogistic process, that being little more than one-fifth of the whole. The experiment being repeated with impure dephlogisticated air, a great diminution took place, but without any cloud in the lime-water. Neither was any cloud produced when fixed air was let up into it; but, on the addition of a little caustic volatile alkali, a brown sediment immediately appeared.

It being thus evident that the lime was saturated by some acid produced in the operation, the experiment was repeated with soap-leys, to discover the nature of it. A previous experiment had been made in order to know what degree of purity the air ought to be of to produce the greatest diminution; and thus it was found,

found, that when good dephlogisticated air was used, the diminution was but small; where perfectly phlogisticated air was made use of, no sensible diminution took place; but when five parts of pure dephlogisticated air were mixed with three of common air, almost the whole was made to disappear.—It must be remembered, that common air consists of one part of dephlogisticated and four of phlogisticated air; so that a mixture of five parts of pure dephlogisticated air and three of common air, is the same thing as a mixture of seven parts of dephlogisticated air with three of phlogisticated. Having made these previous trials, he introduced into the tube a little soap-leys, and then let up some dephlogisticated and common air mixed in the above mentioned proportions, which, rising into the tube M, divided the soap-leys into its two legs. As fast as the air was diminished by the electric spark, he continued to add more of the same kind till no further diminution took place. The soap-leys being then poured out of the tube, and separated from the quicksilver, seemed to be perfectly neutralized, as they did not at all discolour paper tinged with blue flowers. On evaporating the liquid to dryness, a small quantity of salt was left, which was evidently nitre, from the manner in which a paper impregnated with the solution of it burned. On repeating the experiment on a larger scale, with five times the quantity of materials, pure nitre was obtained in proportion, and was found, by the test of terra ponderosa salita, to contain no more vitriolic acid than what might have been expected in the soap-ley itself, and which is exceedingly small.

As, in some former experiments of Mr Cavendish, it had been found, that by deflagrating nitre with charcoal, the whole of the acid was converted into phlogisticated air, he concluded that this kind of air is nothing else than nitrous acid united to phlogiston; according to which, it ought to be converted into nitrous acid by being deprived of its phlogiston. "But (says he) as dephlogisticated air is only water deprived of phlogiston, it is plain, that adding dephlogisticated air to a body, is equivalent to depriving it of phlogiston, and adding water to it; and therefore phlogisticated air ought also to be reduced to nitrous acid, by being made to unite or form a chemical combination with dephlogisticated air; only the acid thus formed will be more dilute than if the phlogisticated air was simply deprived of phlogiston.

"This being premised, we may safely conclude, that in the present experiments, the phlogisticated air was enabled, by means of the electrical spark, to unite to, or form a chemical combination with, the dephlogisticated air, and was thereby reduced to nitrous acid, which united to the soap-leys, and formed a solution of nitre; for in these experiments the two airs actually disappeared, and nitrous acid was formed in their room; and as it has been shown, from other circumstances, that phlogisticated air must form nitrous acid when combined with dephlogisticated air, the above-mentioned opinion seems to be sufficiently established. And a further confirmation is, that no diminution of air is perceived when the electric spark is passed either through pure dephlogisticated or through perfectly phlogisticated air; which indicates a necessity for the combination of the two in order to produce nitrous acid. It was also found by the last experiment, that the

quantity of nitre produced was the same that would have been obtained from the soap-leys, had they been saturated with nitrous acid; which shows, that the production of the nitre was not owing to any decomposition of the soap-leys.

"The soap-leys used in the foregoing experiments were made from salt of tartar prepared without nitre, and were of such a strength as to yield one-tenth of their weight of nitre when saturated with nitrous acid. The dephlogisticated air was also produced without nitre; that used in the first experiment with the soap-leys being procured from the black powder formed by the agitation of quicksilver mixed with lead, and that used in the latter from turbith mineral. In the first experiment, the quantity of soap-leys used was 35 measures, each of which was equal in bulk to one grain of quicksilver; and that of the air absorbed was 416 such measures of phlogisticated air and 914 of dephlogisticated. In the second experiment, 178 measures of soap-leys were used; which absorbed 1920 of phlogisticated air and 4860 of dephlogisticated. It must be observed, however, that in both experiments some air remained in the tube undecomposed, whose degree of purity I had no means of trying; so that the proportion of each species of air absorbed cannot be known with much exactness.

"As far as the experiments hitherto published extend, we scarcely know more of the nature of the phlogisticated part of the atmosphere, than that it is not diminished by lime-water, caustic alkalies, or nitrous air; that it is unfit to support fire or maintain life in animals; and that its specific gravity is not much less than that of common air: so that though the nitrous acid, by being united to phlogiston, is converted into air possessed of these properties; and, consequently, though it was reasonable to suppose, that part at least of the phlogisticated air of the atmosphere consists of this acid united to phlogiston; yet it might be fairly doubted whether the whole is of this kind, or whether there are not, in reality, many different substances confounded by us under the name of phlogisticated air.

I therefore made an experiment to determine whether the whole of a given portion of the atmosphere could be reduced to nitrous acid, or whether there was not a part of a different nature from the rest, which would refuse to undergo that change. For this purpose, I diminished a similar mixture of dephlogisticated and common air in the same manner as before, until it was reduced to a small part of its original bulk; after which some dephlogisticated air was added, and the spark continued until no further diminution took place. Having by these means condensed as much as I could of the phlogisticated air, I let up some solution of liver of sulphur to absorb the dephlogisticated air; after which only a small bubble of air remained unabsorbed, which certainly was not more than $\backslash frac\{1\}\{15\}$th of the bulk of the phlogisticated air let up into the tube; so that if there is any part of the phlogisticated air of our atmosphere which differs from the rest, and cannot be reduced to nitrous acid, we may safely conclude, that it is not more than $\backslash frac\{1\}\{15\}$th part of the whole."

Though these experiments had shown, that the chief cause of this diminution of air is the conversion of the phlogisticated kind into nitrous acid, it seemed

Phlogisticated Air. not unlikely, that when any liquor containing inflammable matter was in contact with the air in the tube, some of this matter might be burnt by the spark, and thereby diminish the air. In order to determine this, the electric spark was passed through dephlogisticated air included between different liquors; and the result of the experiments was, that when dephlogisticated air, containing only $\backslash frac\{1\}\{2\}$th part of its bulk of phlogisticated air, was confined between short columns of soap-leys, and the spark passed through it till no further diminution could be perceived, the air lost $\backslash frac\{1\}\{2\}$ds of its bulk; which is not a greater diminution than might very likely proceed from the decomposition of the small quantity of phlogisticated air contained in it, as the dephlogisticated air might easily be mixed with a small quantity of common air while putting into the tube. When the same dephlogisticated air was confined between columns of distilled water, the diminution was rather greater than before, and a white powder was formed on the surface of the quicksilver beneath: the reason of which, in all probability, was, that the acid produced in the operation corroded the quicksilver, and formed the powder; and that the nitrous air produced by that corrosion united to the dephlogisticated air, and caused a greater diminution than would otherwise have taken place. When a solution of litmus was used instead of distilled water, the solution soon acquired a red colour; which grew paler and paler as the spark was continued, till it became quite colourless and transparent. The air was diminished by almost one-half, and might perhaps have been further diminished had the spark been continued. When lime-water was let up into the tube, a cloud was formed, and the air was further diminished by about one-fifth; the remainder was good dephlogisticated air. In this experiment, therefore, the litmus was, if not burnt, at least decomposed, so as to lose entirely its purple colour, and to yield fixed air; so that, though soap-leys cannot be decomposed by this process, yet the solution of litmus can, and so very likely might the solutions of many other substances be. But there is nothing in any of these experiments which favours the opinion of the air being at all diminished by means of phlogiston communicated to it by the electric spark.

166 Fixed air found in a great variety of substances. THE discovery of this kind of air is as old as Van Helmont; who gave it the name of gas fixe, from its being emitted in great quantity by burning charcoal. Subsequent discoveries showed, that a fluid of the same kind was plentifully produced by fermenting liquor, in almost every kind of combustion, and naturally generated in vast quantity in mines and coal-pits, where it is known by the name of the chak-damp; that it exists in a concrete state in alkaline salts, chalk, limestone, the shells of marine animals, magnesia alba, &c. in a very large proportion, constituting one-half, and sometimes more of their weight; and that it might always be extracted from the atmosphere, in unlimited quantity, by exposing certain substances to it.— On examining the nature of this fluid, it was found to manifestly acid, that it has now obtained a place among these substances under the name of aerial acid;

or, more improperly, cretaceous acid, from its being Fixed Air. contained in great quantities in chalk, as has been already mentioned.

107 Fixed air is the heaviest of all permanently elastic fluids, excepting those derived from the mineral acids. Mr. Kirwan determines it to be to common air as 1500 to 1000, the barometer being at 29.85, the thermometer at 64, and the fixed air being extracted from calcareous spar by marine acid, whose specific gravity was 2.0145. He observes, however, that though this air was obtained in the driest manner possible, and that the globe which contained it appeared perfectly free from moisture; yet, when carried into a room 27 degrees colder, the inside of the globe was covered with dew, which soon formed visible drops.— In its concrete state, fixed air is one of the heaviest bodies in nature. Mr. Kirwan, in the 71st volume of the Philosophical Transactions, gives an account of his ingenious method of finding the specific gravity of fixed air in its fixed state, when combined with calcareous earth; from which it appears, that fixed air, in that state, is prodigiously concentrated, and, were it possible to exist by itself in that concentrated state, it would be the heaviest body known, gold and platina excepted.

Mr. Kirwan first ascertained the specific gravity of a piece of white marble; then expelled the fixed air from a known weight of it finely powdered, by means of diluted vitriolic acid; the bulk and weight of the obtained fixed air being ascertained. Next, he calcined a known quantity of the same sort of marble, by keeping it in a white heat for the space of 14 hours; after which, being weighed again, and from the weight lost by this calcination, the weight of the fixed air, which must have escaped from it according to the above mentioned experiment, being subtracted, the remainder is the weight of water contained in the marble; from which experiments it appears, that 100 grains of the marble contained 32.42 grains of fixed air, 11.66 grains of water, and 55.92 grains of pure calcareous earth.

“ I next (says he) proceeded to discover the specific gravity of the lime. Into a brass box, which weighed 607.65 grains, and in the bottom of which a small hole was drilled, I stuffed as much as possible of the finely-powdered lime, and then screwed the cover on, and weighed it both in air and in water. When immersed in this latter, a considerable quantity of common air was expelled; when this ceased, I weighed it. The result of this experiment is as follows:

“ Hence, dividing the absolute weight of the lime by its loss in water, its specific gravity was found to be 2.3908.

“ From these data I deduced the specific gravity of fixed air in its fixed state; for 100 grains of marble consist of 55.92 of earth, 32.42 of fixed air, and 11.66 of water; and the specific gravity of the marble is 2.717. Now the specific gravity of the fixed air, in its fixed state, is as its absolute weight, divided by its loss of weight in water; and its loss of weight in water is as the

Fixed Air. the loss of 100 grains of marble, minus the losses of the pure calcareous earth and the water.

"Then the loss of the fixed air $36.8\; -\; 35.05\; =\; 1.75$; consequently its specific gravity is $\backslash frac\{32.42\}\{1.75\}\; =\; 18.52$."

108
Its other properties. Fixed air differs considerably in its properties from the airs already mentioned. Its acidity is manifest to the taste, and still more from its neutralising both fixed and volatile alkalis; which it will do in such a manner as not only to destroy their causticity, but to give them a manifestly acid taste, and will moreover enable them to form crystals of a neutral or acidulous salt. It has a considerable antiseptic power, and will even check the putrefaction of animal substances; though it has been observed, that in this case it acts only by absorbing the putrid effluvia already emitted from the body, and becomes itself very offensive, while it sweetens the other. When taken into the lungs, it is equally poisonous with phlogisticated or any other noxious air, and extinguishes flame as effectually; but, when mixed with dephlogisticated air, may be inspired without any danger, and even in its pure state may be swallowed in large quantities, not only without danger, but with the most salutary effects in some diseases, whence it has now become an article of the Materia Medica. As an acid it stands in the lowest rank, being expelled from alkalis by every other; though it is capable of separating oils, sulphur, and the colouring matter of Prussian blue, from the substances with which they are combined.

109
Constituent principles of fixed air. The origin of this acid was for a long time as much unknown as that of the others; and while the general prejudice remained that acids were a kind of primary elements unchangeable in their nature, it was supposed that fixed air was some modification of the others, probably the nitrous. But the discoveries made of late years, have abundantly shown, that the chemical principles are by no means so indestructible as they were imagined; and that the vegetable acids particularly, may be almost totally resolved into fixed air. Hence it was naturally suggested, that fixed air itself might be a compound of some other principles; and it was suggested by Dr Black, that it was a combination of atmospheric air with phlogiston. As the air of our atmosphere, however, is compounded of two substances, one of which naturally contains no phlogiston, and the other as much as it can hold; it seemed unlikely that there should be any possibility of adding to the quantity of phlogiston contained in a portion of the atmosphere, without decomposing it in some manner or other. Succeeding experiments evinced, that it was by a decomposition of the pure part of atmospheric air, and a combination of the phlogiston of the fuel with its basis, that fixed air was produced; and this fact was evinced by numerous experiments made by Mr Kirwan, Mr Lavoisier, and Dr Priestley, so that it is now looked upon to be generally established: and as the experiments

made by Dr Priestley appear fully as convincing as Fixed Air. any, we shall here content ourselves with giving an account of them.

The compound nature of fixed air, and the principles from which it is formed, were first discovered by Mr Kirwan; but Dr Priestley was not convinced by the proofs he adduced, till after making some experiments of his own. The first was, by firing shavings of iron fitted in dephlogisticated air; when he observed a considerable residuum of fixed air, though that in the receiver had been of the purest dephlogisticated kind, and iron could only have yielded inflammable air. The hypothesis of Mr Kirwan was still further confirmed by an experiment in which iron-filings, which could only have yielded inflammable air, were mixed with red precipitate, which is known to yield only pure dephlogisticated air. On heating these in a glass retort, they gave a great quantity of fixed air, in some portions of which nineteen-twentieths were absorbed by lime-water, and the residuum was inflammable; but when the red precipitate was mixed with powdered charcoal, which had been found to yield only inflammable air, the fixed air produced from it was so pure that only one-fortieth part remained unabsorbed by water, which is as pure as that generally prepared from chalk and oil of vitriol. In some of these experiments it appeared, that three ounce-measures of dephlogisticated air went to the composition of two of fixed air: for one ounce of red precipitate gave 60 ounce-measures of dephlogisticated air; and, when mixed with two ounces of iron-filings, it gave about 40 ounce-measures of fixed air that were actually absorbed by water, besides a residuum that was inflammable. The same proportion was obtained when half the quantity of materials were made use of; but on using an ounce of each, only 20 ounce-measures of fixed air, including the residuum, could be got.

In considering this subject farther, it occurred to Dr Priestley, that his experiments, in which charcoal was used, lay open to an objection, that since dry wood, and imperfectly made charcoal, yield fixed air, it might be said, that all the elements of fixed air are contained in charcoal; and though this substance alone, even with the assistance of water, will not yield fixed air, this might be effected by treating it with other substances without their importing any thing to it; especially as the inflammable air procured from charcoal by means of water, appears to contain fixed air when decomposed with the dephlogisticated kind. In order to expel all the fixed air from charcoal, he made a quantity of it from dry oak, and pounding it while hot, instantly mixed four measures of it with one of red precipitate, and, putting them into an earthen retort, got, with a heat no greater than what was sufficient to revive the mercury, a large quantity of air, half of which was fixed. Afterwards the proportion of fixed air was less, and at last no fixed air at all was obtained; but as the residuum was worse than the common atmosphere, he is thence inclined to believe, notwithstanding Mr Caven-dish's experiments, that phlogisticated air may be composed of phlogiston and dephlogisticated air. In another experiment he found a better proportion of charcoal and red precipitate. This was by mixing one ounce of precipitate with the same quantity of perfect dephlogisticated air.

Fixed Air. charcoal hot from the retort in which it was made. Putting these into a coated retort, he expelled from them, by a strong heat, about 30 ounce-measures of air, the whole of which was the purest fixed air, leaving only about one-fortieth part unabsorbed by water, and this almost perfectly phlogisticated.

Having recollected, that in some former experiments he had obtained fixed air from nitrous acid and charcoal, he therefore repeated the experiment with some of the same charcoal which had then been made use of; when fixed air was obtained, in the quantity sometimes only of one-fifth, and sometimes of one-half; to the formation of which he supposed the dephlogisticated air produced by heating the nitrous acid must have contributed. On account of the objections, however, which might be made to the use of charcoal, he next employed iron, which was liable to nothing of this kind; and on mixing an ounce of iron-filings with as much charcoal, and then heating them in a glass retort, he obtained 20 ounce-measures of air, of which one-seventh remained unabsorbed by water. The residuum was of the standard of 1.52, but slightly inflammable. Repeating the experiment with half an ounce of iron filings, he got 26 ounce-measures of air, of which the first part was pretty pure, but afterwards one-tenth remained unabsorbed by water; but on mixing one ounce of precipitate with two ounces of filings, he got about 40 ounce-measures of air, of the first portions of which only one-twentieth was unabsorbed by water, though towards the conclusion the residuum was greater. In this process he got in all 36 ounce-measures of pure fixed air, completely absorbed by water, besides about other four ounce-measures, which, he supposes, might have been absorbed in receiving the air and transferring it into other vessels.

Fixed air was also produced from red precipitate mixed with brass filings, with zinc, from turbith mineral with iron filings, and from the black powder into which mercury mixed with lead is easily converted. In this last case the Doctor supposes that the fixed air was produced from the dephlogisticated kind absorbed by the metals and the phlogiston of the lead; and this is confirmed by an observation that the fixed air always comes first in the process, when the phlogiston is most readily separated, but afterwards the produce becomes quite pure and dephlogisticated. In attempting, however, to increase the quantity of fixed air by heating this black powder in dephlogisticated air, he found only an augmentation of the quantity of dephlogisticated air, and that of the purest kind.

"Perhaps," says he, "as decisive a proof as any of the real production of fixed air from phlogiston and dephlogisticated air, may be drawn from the experiments in which I always found a quantity of it when I burned sulphur in dephlogisticated air. In one of these experiments, to which I gave particular attention, six ounce-measures and an half of the dephlogisticated air were reduced to about two ounce-measures, and one-fifth of this was fixed air. When both the vitriolic acid and fixed air produced by this operation were absorbed by water, the remainder was very pure dephlogisticated air.

"I had always concluded, that no fixed air could be procured by the decomposition of inflammable air which had been produced by mineral acids, because I

had not been able to do it with that which I had got Fixed Air. by means of vitriolic acid; but I learned from Mr. Methier, that this is peculiar to the vitriolic acid, the remains of which, diffused through the inflammable air, procured by it, he conjectures, may actually decompose the fixed air produced in the process. For, as I have hinted before, when the inflammable air is produced from iron by means of spirit of salt, there is a very perceivable quantity of fixed air when it is united with dephlogisticated air. When I decomposed these two kinds of air in equal quantities, they were reduced to about 0.5 of a measure, and of this not more than about one fortieth part was fixed air. This experiment ought, however, to be added to the other proofs of fixed air being produced by the union of dephlogisticated air and phlogiston.

"The last instance, which I shall mention, of the ¹¹²Proportion of fixed air produced from dephlogisticated air, is of a much more striking nature than any that I have yet recited. Having made what I call charcoal of copper, by passing the vapour of spirit of ¹¹²fixed air produced from dephlogisticated air. wine over copper when it was red-hot, I heated a piece of it in different kinds of air. In common air, observing neither increase nor decrease in the quantity, I concluded, perhaps too hastily, that no change was made in it: for when I repeated the experiment in dephlogisticated air, the charcoal burned very intensely; and when a part of it was consumed, which (like common charcoal in the same process, was done without leaving any sensible residuum) I found that no heat which I could apply afterwards, had any farther effect on what was left of the charcoal. Concluding, therefore, that some change must be made in the quality of the air, I examined it, and found about nine-tenths to be the purest fixed air; and the residuum was such as would have been made by separating the absolutely pure part of the dephlogisticated air, leaving all the impurities behind.—Having ascertained this fact, I repeated the experiment, weighing the piece of charcoal very carefully before and after the process; and then found, that by the loss of one grain of charcoal, I reduced four ounce-measures of dephlogisticated air till one-ninth only remained unabsorbed by water; and again, with the loss of one grain and an half of the charcoal, I reduced six and an half-measures of dephlogisticated air till five and an half-measures were pure fixed air. In this process there was a diminution of bulk after the experiment, as might have been expected from the change of the air into one of a heavier kind by means of a substance or principle that could not add much to the weight of it. In one of the experiments, 4.3 ounce-measures of dephlogisticated air were reduced about one-thirtieth part of the whole; and in this case, when the fixed air was separated by water, there was a residuum of 0.75 of a measure of the standard of 1.0, whereas the dephlogisticated air, before the experiment, had been of the standard of 0.2.

"That dephlogisticated air actually enters into the composition of the fixed air, in this experiment, is evident from the weight of the latter, which far exceeds that of the charcoal dispersed in the process. For, in this last experiment, the weight of the fixed air produced was 4.95 grains. Consequently, supposing the charcoal to be wholly phlogiston, as it is very nearly so, fixed air may be said to consist of 3.45 parts of dephlogisticated

gificated air, and 1.5 of phlogiston; so that the dephlogificated air is more than three times the proportion of phlogiston in it.—I must not conclude, however, without observing, that, in one experiment, I never failed to produce fixed air; though it is not easy to see how one of its supposed elements, viz. dephlogificated air, could enter into it. This is by heating iron in vitriolic acid air. In one of these experiments, four ounce-measures of the vitriolic acid air were reduced to 0.65 of an ounce-measure; and of the quantity lost three and an half measures were fixed air absorbed by lime-water, and the remainder weakly inflammable.

Fixed air, even when pure and unmixed, is remarkably altered by the electric spark, part of it being thus rendered immiscible in water. Dr Priestley, having taken the electric spark for about two hours in a small quantity of fixed air confined by mercury, found, that after the operation one-fourth of it remained immiscible with water; though, before it, only one-thirtieth part had remained unabsorbed. The inside of the tube had become very black; which, in other experiments of a similar kind with vitriolic acid air, he had observed to arise from the adhesion of a small quantity of mercury supersaturated with phlogiston. In another experiment, in which the spark was taken an hour and ten minutes in about half an ounce-measure of fixed air, one-fifth remained unabsorbed, and the standard of the residuum was 0.9; though, before the operation, only one-thirtieth part had been absorbed, and the standard of the residuum was 1.0. In this experiment, also, he observed, that the air was increased about a twentieth part. On taking the electric spark an hour in half an ounce of fixed air, as much residuum was left as had remained in five times the quantity of the same fixed air in which no spark had been taken. This residuum was also much purer than that of the original fixed air, the standard being 0.8; whereas that of the original fixed air had been, as before, 1.0. On repeating the experiment, he found the residuum still greater, but equally pure; and, in this case, a good quantity of black matter was observed adhering to the tube. Having taken the spark in a small tube containing $\backslash frac\{1\}\{4\}$th of an ounce-measure of fixed air, the inside of the tube was clouded with black matter, and in the bottom was a small quantity of yellowish matter resembling sulphur; the residuum was between one-fourth and one-fifth of the whole, and less pure than formerly. This circumstance he also supposes to be a proof that fixed air may be composed of phlogiston and dephlogificated air. Pursuing this experiment, by taking the electric spark three hours in a small quantity of fixed air, he observed that it was first increased, and then diminished about one-eighth of the whole; the inside of the tube being very black on the upper part, and below the mercury very yellow, for the space of a quarter of an inch all round the tube; but this space had been above the mercury in the beginning of the operation. One-third of the air remained unabsorbed by water; but so impure, that the standard of it was 1.8, or almost completely phlogificated.—Varying the process by using water impregnated with fixed air instead of mercury, the quantity of air was much augmented by that which came from the water; but thus the far greater part of it was incapable of being absorbed by lime-water; and on this occasion he obser-

ved, that water impregnated with fixed air is a much worse conductor of electricity than the same fluid impregnated with mineral acids. On still varying the circumstances of the experiment, by using common water instead of that which had absorbed fixed air, he found that the quality of the residuum was evidently better than that of the original fixed air.

In order to discover whether the heat or light of the electric spark were the circumstances which effected the change, the Doctor threw a strong light, by means of a lens, for some hours, on a quantity of pounded glass confined in some fixed air; but though the volume of residuum was thus somewhat increased, yet as it was of the same quality with common air, he suspected that it might be only that portion which had been introduced among the particles of the glass. The quantity of air was increased after the operation. With glass-house sand made very hot, the quantity of air was likewise increased; but the experiment was not more satisfactory than the former. Heated bits of crucibles increased the quantity of residuum in the proportion of 10 to 6.6; but the quality was injured either directly by a comparison with nitrous air, or by producing a larger quantity of residuum equally bad. By heating iron, however, in fixed air, part of it was evidently converted into phlogificated air. On heating turnings of malleable iron for some time in fixed air, one-tenth part of it was rendered immiscible with water; and on repeating the process with the remainder, there was a residuum of one-fourth of the whole. There was also a small addition to the quantity of air after the first part of the process, but none after the second; nor could he, after a third and fourth process, render more than one-fourth immiscible with water. In two experiments, the residuum was inflammable, and burned with a blue flame.

With regard to the quantity of fixed air which may be expelled from different substances, Dr Priestley observes, that from seven ounces of whiting, the purest calcareous substance we are acquainted with, he expelled by heat 630 ounce-measures of air; by which means the whiting was reduced to four ounces. One third of this was somewhat phlogificated; the standard being 1.36 and 1.38. Repeating the experiment, he obtained 440 ounce-measures of air from six ounces of whiting; about one-half of which was fixed air, and the remainder of the standard of 1.4. On moistening some calcined whiting with water impregnated with vitriolic acid air, he obtained 90 ounce-measures; of which the first portions were three-fourths fixed air, and the standard of the residuum 1.5; the latter had less fixed air, and the standard of the residuum was 1.44. The whiting was rendered black and hard, but became soft and white with spirit of salt. Three ounces and a quarter of lime fallen in the air, yielded 375 ounce-measures; of which about one-fifth was fixed air, and the standard of the residuum 1.4. Four ounces of white lead had yielded 240 measures of air when the retort melted. The residuum of the first process was one-third, the standard 1.36; and of the last the standard was 1.28, that with the common atmosphere being 1.23. Two ounces and three quarters of wood-ashes yielded, in a very strong heat, 430 ounce-measures of air; of the first portion of which one-tenth, of the second one-third, and of the third one-half, was fixed

Fixed Air. air. The standard of the residuum of the first portion was 1.6, and of the second 1.7. It extinguished a candle; so that the air came properly from the ashes, and not from any remaining particles of the charcoal mixed with them. After the process, the ashes weighed 839 grains; but by exposure to the air for one day, the weight was increased to 842 grains; and, perhaps with more heat than before, yielded 50 ounce-measures of air; of which about one-eighth was fixed air, and the standard of the residuum 1.38 and 1.41. A candle burned in this residuum, and the ashes were reduced to 789$\backslash frac\{1\}\{2\}$ grains. Two ounce-measures of Homberg's pyrophorus burned in the open air, and then distilled in a retort, yielded 144 ounce-measures of air; of which one-half at first was fixed air, but at the last very little. The residuum of the first portion extinguished a candle, but that of the last burned with a blue lambent flame. The standards of both with nitrous air were about 1.8. The pyrophorus was then kept two days in the retort, with the mouth immersed in mercury; after which, on being taken out, it burned as strong as ever. Immediately before the burning, it weighed 428 grains; immediately after it, 449; but being spread thin and exposed to the atmosphere for a night, the weight was increased to 828 grains; though, on being well dried, it was again reduced to 486. Subjecting it to a greater heat than before, the matter yielded 110 ounce-measures of air; the first portions of which were half fixed air, but the last contained very little, and burned with a blue lambent flame. It was then reduced to 396 grains. The experiment was then repeated with a quantity of pyrophorus, which would not take fire in the open air; and on heating this substance in an earthen retort, five-sevenths of the first part of the produce was fixed air: but this proportion gradually diminished; till at last nine-tenths of the whole was inflammable air, burning with a lambent blue flame. This inflammable air being decomposed with an equal quantity of dephlogisticated air, yielded 0.86 of a measure of fixed air. Another quantity of pyrophorus, which burned very well, and which by exposure to the atmosphere had gained 132 grains, being again exposed to heat in an earthen retort, gave 180 ounce-measures of air; three-sevenths of the first portion of which was fixed, and the rest phlogisticated air; but afterwards only one-half was fixed and the rest inflammable, burning with a lambent blue flame; and at last it was wholly inflammable. This pyrophorus took fire again after being poured out of the retort, but not without the assistance of external heat. It had been red-hot through the whole mass at the first burning, and the surface was covered with white ashes; but all the inside was as black as ever it had been. Four ounces of dry ox-blood yielded 1200 ounce-measures of air, and it was conjectured that not less than 200 measures had escaped. It contained no fixed air. The first portion burned with a large lambent white flame, the middle portion fainter, and the last was hardly inflammable at all. The remaining coal weighed 255 grains, and was a good conductor of electricity.

We owe the knowledge of the existence, and of some remarkable properties, of this air, to Mr Cavendish, by

whom they were first published in 1767. Its effects, however, had long before been fatally experienced by miners; in whose subterranean habitations it is often collected in such quantities as to produce the most dreadful effects. It is produced in abundance from putrid animal and vegetable substances; and, in general, by all those which part with their phlogiston easily. Being much lighter than common air, it always rises to the top of those places where it is generated; so that it cannot be confined except in some vaulted place, but always strives to ascend and mix with the atmosphere. By itself it is very noxious, and will instantly put an end to animal life; but when mixed with atmospheric air, may be breathed in much greater quantity than fixed air. Its great inflammability in this state, however, renders it very dangerous to bring any lights, or even to strike a flint with steel, in those places where it abounds. But this only takes place when the inflammable air is mixed with common atmospheric or with dephlogisticated air; in which case, the explosion is much more violent than the former; for pure inflammable air extinguishes flame as effectually as fixed or phlogisticated air.

Besides the subterranean places already mentioned, this kind of air is found in ditches; over the surface of putrid waters, out of which it escapes; in burying-places; in houses of office, where putrid animal and vegetable matters are accumulated; and may, by standing or boiling, be extracted from the waters of most lakes and rivers, especially those in which great quantities of fermenting and putrefying matters are thrown: and as putrefaction thus seems to be the principal source of inflammable air, it thence happens, that much more of it is produced in warm than in cold climates. In those countries, we are informed by Dr Franklin, that if the mud at the bottom of a pond be well stirred, and a lighted candle brought near to the surface of the water immediately after, a flame will instantly spread a considerable way over the water, from the accension of the inflammable air, affording a very curious spectacle in the night-time. In colder climates, the generation of inflammable air is not so plentiful as to produce this phenomenon; nevertheless Mr Cavallo informs us, that it may be plentifully procured in the following manner, in all the ponds about London. "Fill a wide-mouthed bottle with the water of the pond, and keep it inverted therein; then, with a stick, stir the mud at the bottom of the pond, just under the inverted bottle, so as to let the bubbles of air which come out of it enter from ponds, into the bottle; which air is inflammable. When by thus stirring the mud in various places, and catching the air in the bottle until this is filled, a cork or glass stopper must be put over it whilst standing in water; and then the bottle must be taken home, in order to examine the contained inflammable fluid at leisure."

The great quantity of inflammable air produced in warm climates has given occasion to some philosophers to suppose, that it may possibly have some share in producing certain atmospheric meteors. The weak lightnings without any explosion, which are sometimes perceived near the horizon in serene weather, are by them conjectured to proceed from inflammable air fired by electric explosions in the atmosphere. Mr Volta supposes that the ignes fatui are occasioned by the inflammable air which proceeds from marshy grounds,

grounds, and is set on fire by electric sparks: but these phenomena can be accounted for in a more probable manner from the action of the electric fluid itself.

This kind of air is more common than any of the other noxious airs; for there is hardly any inflammable substance on earth, out of which it may not be extracted by one means or other. The fluids, however, which go by the general name of inflammable air, have scarce any other property in common to them all, besides those of inflammability, and being specifically lighter than the common atmospheric air. In other respects, the differences between them are very considerable. The smell, weight, power of burning, of preserving their properties, and the phenomena attending their combustion, are by no means the same in them all; some burning in an explosive manner; others quietly, and with a lambent flame of a white or blue colour. It is, however, necessary to make a proper distinction between an inflammable elastic fluid or inflammable gas, which may be properly called so, and that which is evidently made by combining an inflammable substance with common air; which being easily separable from the air, leaves that fluid in the state it was before. Thus a drop of ether, put into a quantity of common air, mixes itself with it, and takes fire on the approach of flame, like a mixture of inflammable and common air; but if the air to which ether is added be washed in water, the latter is soon separated from it. Common air becomes also inflammable by being transmitted through several essential oils; and thus the air contiguous to the plant called fraxinella becomes inflammable in calm and hot weather, by the emission of its inflammable air.

By heat alone, a considerable quantity of this kind of air may be extracted from most inflammable substances, and even from some of the metals. Dr Hales obtained inflammable air by simply distilling wax, pitch, amber, coals, pease, and oyster shells; and Mr Fontana informs us, that he obtained a considerable quantity of inflammable air from spathose iron, by the action of fire only applied to it in a matras. Dr Priestley, however, obtained it from a vast number of other substances, by distilling them in a gun-barrel; to the extremity of which was luted a tobacco-pipe, or small glass tube, with a flaccid bladder tied on the end. He observes, that the heat must be suddenly applied, in order to get a considerable quantity of air from these substances. "Notwithstanding (says he) the same care be taken in luting, and in every other respect, fix, or even ten, times more air may be got by a sudden heat than by a slow one, though the heat that is last applied be as intense as that which was applied suddenly. A bit of dry oak, weighing about twelve grains, will generally yield a sheep's bladder full of inflammable air with a brisk heat, when it will only yield two or three ounce-measures if the same heat be applied gradually." When he wanted to extract inflammable air from metals, a glass was used, the focus of which afforded a more intense heat than any furnace he could apply: and in this way he obtained inflammable air from several metals; as iron, brass, and tin; but with the metallic calces he had no success.

In the infancy of his experiments, and even after very considerable practice, the Doctor imagined, that

the inflammable air produced in this way came only from the metal, without attending to the share which water had in the production. Some late experiments

of Mr Lavoisier, however, showed, that water had a great share in the production of inflammable air; informed much that it gave occasion to a supposition, that the water was the only source from whence it was derived. This mistake, however, was detected by Dr Priestley, who, by his numerous and accurate experiments, seems in a manner to have exhausted the subject. The method which Mr Lavoisier had followed, was to send the steam of boiling water through a red-hot iron tube; in doing which, the intense heat acquired by the water occasioned the production of a great quantity of inflammable air. Dr Priestley repeated his experiments not only with water, but with other fluids. Sending the vapour of two ounces of spirit of wine through a red-hot earthen tube, he obtained 1900 ounce-measures of inflammable air, which burned with a white lambent flame. It contained no fixed air; and 30 ounce-measures of it weighed eight grains less than an equal quantity of common air. He collected also 0.35 of an ounce-measure of water. In this experiment, the weight of the water collected was 168 grains, of the inflammable air 633 grains, and that of the spirit of wine originally was 821 grains; so that as little was lost in the process as could be expected.—Repeating the experiment with vitriolic ether, an ounce of it treated in the same manner in an earthen tube almost filled with pieces of broken earthen retorts and crucibles, one tenth part of an ounce of water was collected, and 740 ounce-measures of inflammable air were procured, without any mixture or fixed air, burning with a white lambent flame like that of wood, and not exploding with dephlogisticated air. Twenty-nine ounce-measures of this weighed five grains less than an equal quantity of common air. Vapour of spirit of turpentine yielded inflammable air mixed with much black smoke, which soon collected on the surface of the water in the receiver. The smell of this air was exceedingly offensive, and its flame was much less luminous than that of the former. Its specific gravity was the same with that of the air procured from spirit of wine. Olive oil yielded a considerable quantity of air on being mixed with calcined whiting; the first portions burning with a large white flame, and the last with a lambent blue one.

In extracting air from solid substances, the steam of water was always necessary; and thus inflammable air was produced from a great number of different ones. From sulphur treated in this manner in an earthen tube, inflammable air was obtained of a nature similar to that from oil of vitriol and iron. From arsenic, the produce was one-seventh of fixed air; but all the rest strongly inflammable, with a smell scarcely distinguishable from that of phosphorus. Twenty ounce-measures of this air weighed 4½ grains less than an equal quantity of common air. Both these experiments, however, were very troublesome, on account of the volatility of the matters, which sublimed and choked up the tubes. From two ounces of the scales of iron, or fining cinder, which he has found to be the same thing, Dr Priestley obtained 580 ounce-measures of air; one-tenth of the first part of which was fixed air, but afterwards it was all inflammable.

Forty

Forty ounce measures of this air weighed two grains more than an equal quantity of common air. From charcoal exposed to the red-hot steam of water, inflammable air was procured in great quantities. From ninety-four grains of perfect charcoal, that is, prepared with a strong heat so as to expel all fixed air from it, and 240 ounces of water, 840 ounce-measures of air were obtained, one-fifth part of which was fixed air; and the inflammable part appeared likewise, by decomposition, to have a quantity of fixed air intimately combined with it.—Three ounces of bones burnt black, and treated in this manner in a copper tube, yielded 840 ounce-measures of air; the water expended being 288 grains, and the bones losing 110 grains of their weight. This air, he observes, differs considerably from that of any other kind of inflammable air; being in several respects a medium betwixt the air procured from charcoal and that from iron. It contains about one-fourth of its bulk of uncombined fixed air, but not quite one-tenth intimately combined with the remainder. The water that came over was blue, and pretty strongly alkaline; owing to the volatile alkali not having been totally expelled by the heat which had reduced the bones to blackness.

A variety of substances, said not to contain any phlogiston, were subjected to the same process, but without yielding any inflammable air. The experiments with iron, however, were the most satisfactory, as being subject to less variation than those with charcoal; and clearly evincing, that the air in the process does not come from the water alone, but from the iron also; or, as Dr Priestley says, "only from the iron; the weight of water expended, deducting the weight of air produced, being found in the addition of weight in the iron as nearly as could be expected in experiments of this kind. And though the inflammable air procured in this process is between one-third and one-half more than can be procured from iron by solution in acids, the reason may be, that much phlogiston is retained in the solutions; and therefore much more may be expelled from iron when pure water, without any acid, takes place of it. The produce of air, and likewise the addition of weight gained by the iron, are also much more easily ascertained in these experiments than the quantity of water expended in them; on account of the great length of the vessels used in the process, and the different quantities that may perhaps be retained in the worm of the tub.

The following are the results of some of the Doctor's experiments.—Two hundred and sixty-seven grains, added to the weight of a quantity of iron, produced a loss of 336 grains of water, and an emission of 840 ounce-measures of air; and in another experiment, 140 grains added to the weight of the iron produced a loss of 240 grains of water, and the emission of 420 ounce-measures of air. "The inflammable

air produced in this manner (says he) is of the lightest kind, and free from that very offensive smell which is generally occasioned by the rapid solution of metals in oil of vitriol; and it is extricated in as little time in this way as it is possible to do it by any mode of solution. The following experiment was made with a view to ascertain the quantity of inflammable air that may be procured in this manner from any given quantity of iron. Nine hundred and sixty grains of iron, when dissolved in acids, will yield about 800 ounce-measures of air; but, treated in this manner, it yielded 1054 measures, and then the iron had gained 329 grains in weight" (A).

Inflammable air having been at first produced only from metals by means of acids, it was then supposed that part of the acid necessarily enters into its composition; but this hypothesis is now found to be ill grounded. "That no acid (says Dr Priestley), is necessarily contained, or at least in any sensible quantity, either in inflammable air, though produced by means of acids, or in the dephlogisticated air of the atmosphere, is evident from the following experiment, which I made with the greatest care: Taking a basin which contained a small quantity of water tinged blue with the juice of turnsole, I placed it in a bent tube of glass, which came from a vessel containing iron and diluted oil of vitriol; and lighting the current of inflammable air as it issued from this tube, so that it burned exactly like a candle, I placed over it an inverted glass jar, so that the mouth of it was plunged in the liquor. Under this jar the inflammable air burned as long as it could; and when extinguished for want of more pure air, I suffered the liquor to rise as high as it could within the jar, that it might imbibes whatever should be deposited from the decomposition of either of the two kinds of air. I then took off the jar, changed the air in it, and, lighting the stream of inflammable air, replaced the jar as before. This I did till I had decomposed a very great quantity of the two kinds of air, without perceiving the least change in the colour of the liquor, which must have been the case if any acid had entered as a necessary constituent part into either of the two kinds of air. I also found no acid whatever in the water, which was procured by keeping a stream of inflammable air constantly burning in a large glass balloon, through which the air could circulate, so that the flame did not go out. Neither was there any acid produced in the decomposition of inflammable and dephlogisticated air in a strong close glass vessel.

"With respect to inflammable air, I have observed, that when sufficient care is taken to free it from any acid vapour that may be accidentally contained in it, it is not in the smallest degree affected by a mixture of alkaline air. On the whole, therefore, I have at present no doubt, but that pure inflammable air, though it certainly contains water, does not necessarily contain any

(A) In these experiments, the Doctor seems not to have supposed that any particular kind of water was necessary for this production of inflammable air: but in the Memoirs of the Philosophical Society at Haarlem, it is asserted by Dr Deiman and M. Paets Van Troostwyk, that the experiment will not succeed when boiled or distilled water, or any other than that containing fixed air, is made use of; and to this air they attribute the calcination of the iron and production of inflammable air. This assertion, however, is contrary to what we find related by Mr Kirwan. See n° 138.

any acid: yet an acid vapour may be easily diffused through it, and may perhaps in many cases be obliterated retained by it, as no kind of air seems to be capable of so great a variety of impregnations as inflammable air is."

Mr Cavendish first perceived the necessity of moisture to the production of inflammable air; but it was not until after making several experiments that Dr Priestley could adopt the same idea. He had observed some very remarkable circumstances relating to the production of inflammable air from charcoal, by which he was induced to suppose that the former was pure phlogiston in a volatile state without any moisture whatever. The Doctor observes, that "charcoal is generally said to be indestructible, except by a red heat in contact with air. But I find (says he), that it is perfectly destructible, or decomposed, in vacuo, and, by the heat of a burning lens, almost convertible into inflammable air; so that nothing remains besides an exceedingly small quantity of white ashes, which are seldom visible, except when in very small particles they happen to cross the sun-beams as they fly about the receiver. It would be impossible to collect or weigh them; but, according to appearance, the ashes thus produced, from many pounds of wood, could not be supposed to weigh a grain. The great weight of ashes produced by burning wood in the open air arises from what is attracted by them from the air. The air which I get in this manner is wholly inflammable, without the least particle of fixed air in it. But in order to this, the charcoal must be perfectly well made, or with such a heat as would expel all the fixed air which the wood contains; and it must be continued till it yield inflammable air only, which, in an earthen retort, is soon produced.

"Wood or charcoal is even perfectly destructible, that is, resolvable into inflammable air, in a good earthen retort, and a fire that would about melt iron. In these circumstances, after all the fixed air had come over, I several times continued the process during a whole day; in all which time inflammable air has been produced equally, and without any appearance of a termination. Nor did I wonder at this, after seeing it wholly vanish into inflammable air in vacuo. A quantity of charcoal made from oak, and weighing about an ounce, generally gave me about five ounce-measures of inflammable air in twelve minutes."

Although from these experiments it did not appear that water was in any way essentially necessary to the production of this kind of inflammable air, it appeared manifestly to be so in the following: "At the time (says he) when I dispersed any quantity of charcoal with a burning lens in vacuo, and thereby filled my receiver with nothing but inflammable air, I had no suspicion that the wet leather on which my receiver stood could have any influence in the case, while the piece of charcoal was subject to the intense heat of the lens, and placed several inches above the leather. I had also procured inflammable air from charcoal in a glazed earthen retort for two whole days successively, during which it continued to yield it without intermission. Also iron-filings in a gun-barrel, and a gun-barrel itself, had always given inflammable air whenever I tried

the experiment. These circumstances, however, deceived me, and perhaps would have deceived any other person; for I did not know, and could not have believed, the powerful attraction between water and charcoal or iron, when the latter are intensely hot. They will find, and attract it, in the midst of the hottest fire, and through any pores that may be left open in a retort; and iron-filings are seldom so dry as not to have as much moisture adhering to them as is capable of enabling them to give a considerable quantity of inflammable air. But my attention being now fully awakened to the subject, I presently found that the circumstances above mentioned had actually misled me; I mean with respect to the conclusion which I drew from the experiments, and not with respect to the experiments themselves, every one of which will, I doubt not, be found to answer, when properly tried.

"Being thus apprised of the influence of unperceived moisture in the production of inflammable air, and willing to ascertain it to my perfect satisfaction, I began with filling a gun-barrel with iron-filings in their common state, without taking any particular precaution to dry them, and I found that they gave air as they had been used to do, and continued to do so many hours: I even got ten ounce-measures of inflammable air from two ounces of iron-filings in a coated glass retort: At length, however, the production of inflammable air from the gun-barrel ceased; but, on putting water to it, the air was produced again; and a few repetitions of the experiment convinced me that I had been too precipitate in concluding that inflammable air is pure phlogiston. I then repeated the experiment with the charcoal, making the receiver, the stand on which I placed the charcoal, and the charcoal itself, as dry and hot as possible, and using cement instead of wet leather, in order to exclude the air. In these circumstances I was not able, with the advantage of a good sun and an excellent burning lens, to decompose quite so much as two grains of the piece of charcoal which gave me ten ounce-measures of inflammable air; and this, I imagine, was effected by means of so much moisture as was deposited from the air in its state of rarefaction, and before it could be drawn from the receiver. To the production of this kind of inflammable air, therefore, I was now convinced that water is as essential as to that from iron."

In his analysis of different kinds of inflammable air, the Doctor observes, that the difference most commonly perceived is, that some of them burn with a lambent flame, sometimes white, sometimes yellow, and sometimes blue; while another kind always burns with an explosion, making more or less of a report when a lighted candle is dipped into a jar filled with it. The inflammable air extracted from metals by means of acids is of this last kind; and that from wood, coal, or other inflammable substances by means of heat, belongs to the former. It has also been observed, that these kinds of inflammable air have different specific gravities; the purest, or that which is extracted from iron, &c. being about ten times as light as common air; but some of the other kinds not more than twice as light (A).

This difference was for some time attributed to a quantity

(A) Here the Doctor's calculation differs somewhat from that of Mr Kirwan; who, in his Treatise on Phlogiston,

quantity of fixed air intimately combined with the heavier kinds, so that it could not be discovered by lime-water, while the lightest contained no fixed air at all. In order to ascertain this point, he had recourse to decomposition; which was performed by mixing with the inflammable air to be tried an equal quantity of common or dephlogisticated air, and then confining them in a strong glass vessel previously filled either with water or mercury; making afterwards an electric spark in some part of the mixture by means of wires inserted through the sides of the vessel, and nearly meeting within it. Thus he supposed that he might be able to determine the quantity of combined fixed air, and likewise the relative quantity of phlogiston contained in each of them. The former appeared by washing the air with lime-water after the explosion, and observing how much of them was observed; and the latter by examining the residuum with the test of nitrous air, and observing the purity of it. Finding, however, that, in some cases, more fixed air was found after the explosion than could have been contained in the inflammable air, he was thence led to observe the generation of fixed air from the principles mentioned in the last section.

In prosecuting this subject, it was found, that one measure of inflammable air produced by steam from metals, and one of dephlogisticated air, such as by mixture with two measures of nitrous air was reduced to 0.72 of a measure, were reduced by explosion to 0.6 of a measure; the residuum, by an equal quantity of nitrous air, was reduced to 0.87. With the same dephlogisticated air, the inflammable air from fining-cinder and charcoal was reduced only to 1.85 of a measure; but by washing in lime-water, to 1.2. The residuum examined by nitrous air appeared to be of the standard of 0.9. In another process, the diminution after the explosion was to 1.55, and that after washing in lime-water to 0.65, of a measure; in a third, by explosion to 1.6, and by washing to 0.66; and in a fourth, the first diminution was to 1.6, and the second to 0.6. In this last experiment there was a generation of an entire measure of fixed air; and that this had not been contained originally in any latent state in the original fluid, was evident from the specific gravity of the inflammable air made use of. This, indeed, was one of the heaviest kinds of the fluid: but 40 ounce-measures of it weighed only two grains more than an equal bulk of common air; whereas, had all the fixed air found in the residuum been contained in the original air, it must have been at least one-half heavier. "Indeed (says the Doctor) if any quantity of inflammable air, of about the same specific gravity with common air (which is the case with that species of it I am now considering), yield so much as seven-tenths of its bulk of fixed air in consequence of its explosion with dephlogisticated air, it is a proof that at least part of that fixed air was generated in the process, because seven-tenths of such fixed air would weigh more than the whole measure of inflammable air."

Equal parts of dephlogisticated air and the inflammable kind produced from spirit of wine, were reduced to one measure, and by washing in lime-water to 0.6 of a measure. The standard of the residuum was 1.7.—In another experiment, in which the vapour of the spirit of wine had passed through a tube filled with bits of crucibles, the first diminution was to 1.6, the second to 1.4, and the standard of the residuum was to 1.84; but in a third, the first diminution was to 1.2, the second to 0.9.—Air procured by steam from red-hot platina was reduced to 0.72 of a measure, and the standard of the residuum was 0.9. It contained no fixed air.—Air from brimstone, with an equal part of dephlogisticated air, was diminished to 0.6, and no fixed air was found in the residuum. Its standard was 0.95.—With inflammable air from arsenic, the first reduction was to 1.15, the second to 0.95. The standard was 0.82.—With the inflammable air procured by a decomposition of alkaline air, the diminution by explosion was to 0.96, and no fixed air was contained in the residuum; the standard of which was 0.8.—Inflammable air from ether resembles that from spirit of wine. The first diminution was to 1.56, the second to 1.2; and the standard was 1.9.

Inflammable air procured by means of steam from charcoal of metals produces a considerable quantity of fixed air; the first diminution being to 1.12, the second to 0.8, and the standard of the residuum 1.9. This analysis was of the first portion that came over, the second was somewhat different; the first diminution being to 1.0, the second to 0.75, and the standard of the residuum 1.9.—From coak, or the charcoal of pitcoal, the first diminution was to 1.15, the second to 0.95, and the standard 1.9; but the dephlogisticated air in this experiment was by no means pure.

With inflammable air from spirit of turpentine, the first diminution was to 1.7, the second to 1.6, and the standard 1.9.—From bones, the first diminution was to 0.67, the second to 0.58; the standard 1.47.—From common charcoal, the first diminution was to 1.5, the second to 0.74, and the standard 1.7. In another experiment, the first diminution was to 0.82, the second to 0.63, and the standard of the residuum 1.37.

Inflammable air procured by distilling some rich mould in a gun-barrel had a very offensive smell, like that procured from putrid vegetables; it contained one-twentieth part of uncombined fixed air. When this was separated from it, and the remainder decomposed with dephlogisticated air, the first diminution was to 1.4, the second to 0.67, and the standard of the residuum was 0.6.—The air procured from cast-iron has likewise a peculiarly offensive smell; and, on this account, the Doctor imagined, that it might contain more phlogiston than common inflammable air, so as to absorb more dephlogisticated air than the other. But this conjecture did not appear to be well founded; for on exploding it with dephlogisticated air in the proportions

giston, informs us, that in his experiments he used "inflammable air extracted from clean newly-made filings of soft iron, in the temperature of 59°, by vitriolic acid whose specific gravity was 1.0973, and obtained over mercury, having very little smell, and what it had being very unlike the usual smell of inflammable air."—The weight of this air, when the barometer stood at 29.9, and the thermometer at 60°, was found to be to that of common air as 84.3 to 1000; and, consequently, near 12 times lighter.

proportions already mentioned, the diminution was the same as with inflammable air produced from the malleable kind, viz. 1.56.

In these experiments, it seemed evident, that at least part of the fixed air found after the explosion was produced by its means; but the following seem no less convincing proofs, that fixed air may be converted into the inflammable kind, or at least that the elements of fixed air may remain in inflammable air in such a manner as to be imperceptible. On heating in an earthen retort a quantity of slaked lime, which had long been kept close corked in a bottle, it gave air, of which one-fifth was generally fixed air; but in the gun-barrel the same lime yielded no fixed air at all, but a great quantity of inflammable air of the explosive kind, like that which is got from iron alone by means of water. As this total disappearance of the fixed air appeared extraordinary, the Doctor was induced to repeat it several times with all possible care; and the following was the result of his experiments: Three ounces of slaked lime, which had for some time been exposed to the open air, heated in an earthen tube, yielded 14 ounce-measures of air, of which only two and an half remained unabsorbed by water; the residuum was slightly inflammable, but not perfectly phlogisticated. Three ounces of the same lime, heated in a gun-barrel, gave 20 ounce-measures of air, all of which was inflammable, and no part fixed. It was expected, however, that the fixed air would have appeared on the decomposition of this inflammable air with the dephlogisticated kind; but after this process, it appeared to be exactly such inflammable air as is procured from metals by the mineral acids, or by steam; the diminution of the two kinds of air being exactly the same: and tho' some fixed air was found in the residuum, it was no more than is usually met with in the decomposition of inflammable air procured by means of spirit of salt.— Supposing that the two kinds of air might incorporate, when one of them was generated within the other, a gun-barrel was filled with fixed air, and the closed end of it put into a hot fire. Inflammable air was instantly produced; but when the fixed air was separated from it, it burned like inflammable air with which no other kind had ever been mixed.

On heating iron-turnings in five ounce-measures of fixed air, the quantity of it was increased about one ounce-measure, and there remained one and three-fourths unabsorbed by water. The experiment was repeated with the same result; and it was farther observed, that though the inflammable air procured in this manner did not appear by the test of lime-water to contain any fixed air, yet when it was decomposed by firing it with an equal quantity of dephlogisticated air, the residuum contained one-third of fixed air. The diminution was to 1.45. Hence the Doctor conjectures, that though, in some cases, the fixed air appears to be generated by the decomposition of dephlogisticated and inflammable air, yet that inflammable air, when thus produced in contact with fixed air, may combine with it, so as to be properly contained in it, and in such a manner that it cannot be discovered by lime-water.

Inflammable air, when produced in the driest way possible, is exceedingly light, as has been already observed: but Dr Priestley has found, that by standing

on water, a very considerable increase is made in its specific gravity; so that from being ten or twelve times lighter than atmospheric air, it soon becomes only seven times lighter. This great propensity to unite with water is also taken notice of by Mr Kirwan; who tells us, that the bulk of inflammable air obtained over water with the assistance of heat towards the end, was one-eighth greater than when produced over mercury; but the weight of it in the former case was only eight or nine times less than common air.—

“From 85 cubic inches of inflammable air obtained over water, I extracted,” says he, “by oil of vitriol exposed to it for 55 hours, two grains of water; and, though undoubtedly there is an error in all these experiments, yet there can be little doubt but this inflammable air contained one-half its weight of water. The inflammable air, by the subtraction of its water, lost its smell, but continued as inflammable as ever; and therefore there is no reason to think that it was decomposed, or that water is any way essential to it.”

This conclusion is directly contrary to that of Dr Priestley, that water is an essential ingredient in the composition of inflammable air; nor do the experiments of the latter, already recited, seem to have had any weight with him, as he concludes his Treatise on Phlogiston in these words. “To the proofs I have heretofore given, that inflammable air and phlogiston are the same substance, just as ice and the vapour of water are called the same substance, no objection of any weight has since been made. Some have thought that I should have included the matter of heat or elementary fire in the definition of inflammable air: but as fire is contained in all corporeal substances, it is perfectly needless, except where bodies differ in the quantity of it they contain; and in this respect I expressly mentioned its difference with phlogiston to consist.— Others, attending to the quantity of water contained in inflammable air, have supposed it to be an essential ingredient in the composition of this air, and have called it phlogisticated water; but they may as well suppose water to be an essential ingredient in common air, or fixed air, and call this last acidulated water: for inflammable air, equally as other airs, may be deprived of its water without any limitation, and yet preserve all its properties unaltered; which shows the presence of water to be no way essential to it. Lastly, others have thought, that it essentially requires an acid or an alkali, or some saline substance, for its basis; as if there were any more repugnance in the nature of things, that phlogiston should exist in an aerial state without any basis, than marine air, alkaline air, or dephlogisticated air; when it is evident, that an aerial state requires no more than a certain proportion of latent heat: but the production of inflammable air from iron by means of distilled water, without any acid or salt, has effectually done away any suspicion of that sort.”

On the other hand, Dr Priestley informs us, that inflammable air seems now to consist of water and inflammable air: which, however, seems extraordinary, as the two substances are hereby made to involve each other; one of the constituent parts of water being inflammable air, and one of the constituent parts of inflammable air being water; and therefore, if the experiments would favour it (but I do not see that they do so),

Inflam-
mable Air. (so), it would be more natural to suppose, that water, like fixed air, consists of phlogiston and dephlogisticated air, in some different mode of combination.

“There is an astonishing variety in the different kinds of inflammable air, the cause of which is very imperfectly known. The lightest, and therefore probably the purest kind, seems to consist of phlogiston and water only. But it is probable that oil, and that of different kinds, may be held in solution in several of them, and be the reason of their burning with a lambent flame, and also of their being so readily resolved into fixed air when they are decomposed by dephlogisticated air; though why this should be the case, I cannot imagine.

“When inflammable and dephlogisticated air are burned together, the weight of the water produced is never, I believe, found quite equal to that of both kinds of air. May not the light, therefore, emitted from the flame, be part of the phlogiston of the inflammable air united to the principle of heat? And as light accompanies the electric spark, may not this also be the real accension of some phlogistic matter, though it is not easy to find the source of it?”

The French chemists, who deny the existence of phlogiston, are of opinion, that inflammable air is a simple uncomposed element; but for a more full discussion of this subject, see the article PHLOGISTON.

Inflammable air is absorbed by water in considerable quantity, but by the application of heat may be expelled again in equal quantity. By agitation in water Dr Priestley was formerly of opinion that this kind of air might be rendered as good as common air; but this undoubtedly proceeds from the atmospheric air transmitted by the water, as is the case with phlogisticated air mentioned in the last section. After a quantity of water, which had absorbed as much inflammable air as it could, had been suffered to stand a month, it was expelled by heat, and found to be as strongly inflammable as ever. The water, after the process, deposited a kind of filmy matter; which he supposed to be the earth of the metal that had been employed in producing it.

Plants in general grow tolerably well in inflammable air, and the willow plant has been observed to absorb great quantities of it. Its inflammability is not diminished by the putrefaction of animal substances, nor does their putrefaction seem to be retarded by it. Animals confined in it are killed almost as soon as in fixed air; but insects, which can live a considerable time in phlogisticated air, live also a considerable time in this kind of air; but at last they become torpid, and appear to be dead, though they will still recover if removed into the open air. Mr Cavallo relates, that the Abbé Fontana, having filled a large bladder with inflammable air, began to breathe it in his presence; after having made a very violent expiration, in which case the effects are most powerful. The first inspiration produced a great oppression in his lungs, the second made him look very pale, and the third was scarcely accomplished when he fell on his knees through weakness. Birds and small quadrupeds, inclosed in small vessels of this air, died after a very few inspirations. Lastly, inflammable air appears to have a smaller share of refractive power than common air; for Mr Warltire informs us, that having placed an hollow triangular prism, of which the

angle was 72 degrees, so as to half cover a large object-glass in one of Mr Dollond's perspectives, and so turned round as to make the frame of a window, at the distance of 1280 feet, seen partly through the prism and partly through common air, appear undivided. The inflammable air was then blown out of the prism, but no part of the apparatus was moved; when the frame of the window seen through the object-glass and the prism as before, seemed to separate about four inches.

The inflammability of this species of air has given occasion to various projects concerning it; such as that of employing it to give light and heat; and lamps have been described, which may be lighted by the electric spark in the night-time. By its means also very pretty artificial fires are made, with glass tubes bent in various directions, and pierced with a great number of small apertures. The inflammable gas is introduced into these tubes, from a bladder filled with that fluid, and fitted with a copper cock. When the bladder is pressed, the inflammable air, being made to pass into the tube, issues out of all the small apertures, and is set on fire by a lighted taper. None of these contrivances, however, have ever been applied to any use; and the scheme of Mr Volta, who proposed to substitute its explosive force instead of gun-powder, is found insufficient, on account of the weakness of the explosion, except when the two airs are fired in very great quantity, which would be incompatible with the small bulk necessary for warlike engines.

SECT. VII. Sulphurated Inflammable Air.

This was discovered by Dr Priestley at the time when he was engaged in the experiments of which some account has been given in the last section, of transmitting the steam of water and other fluids through red-hot tubes containing some solid material. Having, among others, treated manganese in this manner, by cored from stopping one end of the heated tube with a cork before the steam was applied, he received forty ounce-measures of air, of which one-sixth was fixed air and the rest of the standard of 1.7, lambently inflammable. Having then opened the other end of the tube in order to admit the steam, air was produced more copiously than before. Of 50 ounces of this air, one-seventh was fixed, and the rest, of the standard of 1.8, explosively inflammable. The last portions were very turbid; and the smell, especially that of the last portion, was very sulphureous, tinging the water of a very dark colour, by depositing in it a quantity of blackish water. However, the air itself became presently transparent, and had no other appearance than that of any other kind of air. On looking at the jar in about ten minutes after, it was quite black and opaque; so that nothing could be seen in the inside of it. Filling afterwards another jar with the same kind of air, in order to observe the progress of this uncommon phenomenon, he found, that when the water was well subsided, black specks began to appear in different places, and, extending themselves in all directions, at length joined each other, till the whole jar was become perfectly black, and the glass opaque. When this was done, he transferred the air into another jar; and it soon produced a similar effect upon this, though it never became

so black as the jar in which it had been first received. It also frequently happened, that only the lower part of the jar would become black, as if the matter with which it was loaded had kept subsiding, though invisibly, in the mass of air, and occupied only the lower regions, leaving the upper part entirely free from it. On exposing to the open air the vessels thus turned black, the colour presently disappeared, and a yellow or brown incrustation was left upon it. The same change took place when the vessels were inverted in water, in order to observe the alteration of the air within them; but on examining this air, no sensible change was perceived. In some cases, indeed, he thought the air was injured, but it was much less so than he had expected. After depositing the black matter, the air still retained its sulphureous smell, and he did not imagine that it would ever leave it entirely.

On trying other specimens of manganese, no air of this kind was obtained; but some time after, having occasion to make a large quantity of inflammable air, he used, instead of fresh iron, some that had been already melted in vitriolic acid air. Dissolving this with a considerable quantity of fresh metal in diluted vitriolic acid, he found that the water in which the air was received became very black, and deposited more sediment than had appeared in the experiment with the manganese. The jars were as black as ink, but became yellow on exposure to the air as before; so that there could be no doubt of its being the same thing he had got before. On burning a quantity of it, this kind of air appeared to contain some vitriolic acid, the balloon being filled with a very dense white fume, which rendered the water sensibly acid to the taste. On decomposing it with dephlogisticated air, however, he found the diminution exactly the same as when common inflammable and dephlogisticated air were used; so that it appeared to contain neither more nor less phlogiston than the other; only there was a small quantity of fixed air produced, which is never the case with common inflammable air from vitriolic acid and iron.

When the sulphurated inflammable air is received over mercury, very little black matter is produced on the jars; and it is remarkable, that though the black matter collected on them, when the air is taken through water, soon grows yellow upon exposing it to the air, it is not the case with that which remains in the water; it adheres to the evaporating vessel in form of a black incrustation, which does not burn blue until it has been digested in the nitrous acid, which deprives it of its superfluous phlogiston, and leaves it both of the colour and smell of sulphur.

SECT. VIII. Of Alkaline Air.

This was procured by Dr Priestley, in the beginning of his experiments, from common spirit of sal-ammoniac with quicklime, or the materials from which it is made. He did not at that time prosecute the discovery farther than by impregnating water with it; by which means he could make a much stronger alkaline spirit than any to be met with in the shops. His method of procuring it was by mixing one part of pounded sal-ammoniac with three parts of slacked lime; and for common experiments the same quantity of materials would last a considerable time.

This kind of air, when pure, is instantly fatal to animal life, and extinguishes flame; though, when mixed with common atmospheric air, it is slightly inflammable, and also medicinal in faintings and other cases of debility. A candle dipped into a jar of this air is extinguished; but just before the flame goes out, it is enlarged by the addition of another flame of a pale yellow colour, and sometimes a weak flame spreads for a considerable way, or even through the whole body of the alkaline air. The electric spark taken in it appears of a red colour. Every spark taken in it augments its bulk, and by degrees turns the whole into inflammable air. It is readily absorbed by water, as has been already observed, and dissolves ice almost as fast as an hot fire. On confining some water impregnated with alkaline air in a glass tube, and thus exposing it to a strong heat in a sand-furnace for some days, he observed that a white sediment or incrustation was formed on the surface. The Doctor remarked, that bits of linen, charcoal, and sponge, admitted into a quantity of alkaline air, diminished it, and acquired a very pungent smell; especially the sponge, a bit of which, about the size of an hazle-nut, absorbed an ounce-measure. It is remarkable that copper, which is so easily corroded by the common volatile alkalis, is not affected by alkaline air. The specific gravity of this kind of air is, by Mr Kirwan, determined to be to that of common air as 600 to 1000; though, as he justly observes, this must differ very considerably according to the quantity of moisture it contains.

In prosecuting his experiments on alkaline air, Dr Priestley concluded that it contains phlogiston, both its containing being convertible into inflammable air by electric explosions, and likewise from its reviving the calces of metals. In attempting to ascertain the quantity of lead revived in alkaline air, he met with two difficulties; the first, on account of some part of the calx being blackened and imperfectly revived; the second, that the lead completely revived was dissolved by the mercury employed to confine the air. To prevent this last inconvenience, he put the powdered massicot (the substance he chose to employ on this occasion) into small earthen cups, contriving to place them with their mouths upwards, in such a manner, that when the lead was revived by means of a burning lens, it would remain in the cup, and not mix with the mercury which supported it. The proportions of metal then revived, were six grains of lead in three ounce-measures, 16$\backslash frac\{1\}\{2\}$ in three measures and an half, 13 in two and an half, and 12 in three and three-fourths; but the experiment on which he laid the greatest stress, was that in which 26$\backslash frac\{1\}\{2\}$ grains of lead were revived in 7$\backslash frac\{1\}\{2\}$ ounce-measures of alkaline air. In this proportion, 100 ounce-measures of alkaline air would revive 352 grains of lead; but an equal quantity of inflammable air from iron would have revived 480 grains of metal. This deficiency appeared somewhat surprising to the Doctor, considering that alkaline air is resolved into more than twice its bulk of the inflammable kind; though it is possible, that inflammable air from iron may contain more phlogiston than that into which alkaline air is resolvable.

On heating red precipitate in alkaline air, the mercury was revived as in other cases, and a considerable quantity of water was produced, though none appears.

Alkaline Air. on reviving it with common inflammable air. "It has even (says he) run down in drops in the inside of a vessel which contained five ounce-measures of the air; and a considerable quantity of dephlogisticated air was found in the residuum." On throwing the focus of the lens on red precipitate, inclosed in this kind of air, till three measures of it were reduced to two, water was produced as usual, and the standard of the residuum was 1.7. In another experiment, a violent explosion took place before he could observe whether any water was produced or not.

148 Conversion of alkaline into inflammable air. In examining the phenomena which attend the conversion of alkaline air into the inflammable kind, the Doctor was induced to believe that it was occasioned by heat alone, without the concurrence of light. The effects of the former were first perceived on heating some ochre of iron in alkaline air; when, though the matter turned black, as in an incipient reduction of the metal, he found a considerable increase of quantity instead of decrease in the air, as he had expected; and, on examining the quality of it, he found that it contained no fixed air, but was entirely inflammable. With scales of iron a similar enlargement was perceived; but in this way he could never increase the quantity to more than double that which had been originally employed, and even after this the whole smelled strongly of volatile alkali; the iron had undergone no change.

The Doctor now, concluding from these experiments that the change of alkaline into inflammable air was produced by this cause alone, proceeded to repeat the experiment, by heating in the alkaline air bits of dry crucibles, or of earthen retorts, which had been just before exposed to very great heats, so that they could not be supposed to give out any air themselves, and therefore could only serve to communicate a strong heat to the alkaline air; and in these experiments the result was the same as when ochre and iron were made use of. The bits of white earthen ware were always turned black; but finding the same effect of augmenting the air and giving it an inflammable quality, though he used the bit of crucible over and over again, he was thoroughly convinced that the change was effected by heat alone.

In all these experiments, however, with a burning-glass, as a strong light was also concerned, he heated a quantity of alkaline air in a green glass retort, receiving in a glass tube, filled with water, all the air that could be expelled from it by heat. At first it was all absorbed by the water, being merely alkaline air expelled by the rarefaction; but when the bulb of the retort became red-hot, he found that the bubbles driven out were not wholly absorbed, and at last none of them were so. These were altogether inflammable; so that no doubt remained of the change being produced by heat alone, without any intervention of light.

It was farther observed, that whenever the alkaline air was changed into inflammable by means of bits of retorts or crucibles containing clay, they always became black during the process. He inclined therefore to suppose, that something might be deposited from the air which might attach itself to the clay. "Indeed, (says he) if this was not the case, I do not see why the clay should become black; though, perhaps, part of the same phlogiston which forms the inflammable air may be attracted by the red-hot clay, with-

out there being any proper decomposition of the air. Nitrous Air. That this is the case seems probable from an experiment in which I used porcelain instead of common earthen ware; which did not become black in the process, though inflammable air was produced."

149 Volatile alkali produced from nitrous air. In some of Dr Priestley's experiments, he had observed that iron, which had long rusted in nitrous air, gave out a strong smell of volatile alkali. This extraordinary phenomenon, however, was only perceived and iron, where the nitrous air and iron had been in contact for a very long time; but he found that it was much sooner produced by making use of a weak solution of copper; by putting iron into which he obtained that species of nitrous air called dephlogisticated. A phial containing some of this iron, which had been used only once for the purpose just mentioned, having been kept close corked for about two months, was accidentally broken; when some pieces of the iron were found covered with a green crust, and these had a strong smell of volatile alkali. On making some more experiments on this subject, he found that two months standing was requisite to produce the alkaline smell desired.

SECT. VIII. Of Nitrous Air.

150 This kind of air is plentifully obtained in all cases How produced. where the nitrous acid is combined with phlogiston: Thus, when it is mixed with metals, or animal or vegetable substances, nitrous air is produced in great quantities; but very sparingly when treated with metallic calces, earths, or other matters which are said to contain little or no phlogiston. All the metals, excepting gold, platina, and regulus of antimony, which are not soluble in the pure nitrous acid, yield nitrous air on being treated with it; and even from these, when dissolved in aqua regia, some quantity of this air may be obtained. Every metal, however, does not yield it in equal quantity, with equal facility, or equally good. Silver, copper, iron, brass, bismuth or nickel, when put into nitrous acid, yield this air in considerable quantity: Mercury yields it but slowly without the application of heat, though no great degree of it is necessary. Copper and iron, especially the latter, require the acid to be cautiously applied on account of the violent emission of fumes. Gold, platina, and regulus of antimony, when put in aqua regia, yield nitrous air pretty readily; but lead yields it in smaller proportion than any other metal, and zinc does the same among the semimetals, the elastic fluid produced from it being mostly phlogisticated air.

In the production of this kind of air, great differences are perceived by a diversity in the strength of the acid. Thus, if we dissolve copper in strong nitrous acid, no nitrous air is produced, though the same materials will yield air in great quantity by the mere affusion of water to dilute the acid. This is very properly explained by Doctor Priestley, from the property that the nitrous acid has of attracting phlogiston, which is evident from what happens in the solution of mercury. When strong spirit of nitre is poured upon this metal, the solution soon begins, and is very rapid, yet not a single bubble of elastic fluid is produced; but in a short time the acid next to the mercury is changed of an orange colour, which is an indication of its having acquired phlogiston, probably from the nitrous air.

Nitrous Air. air which is decomposed the moment it is formed, and before its particles are united into visible bubbles. The bubbles of air indeed break through the coloured acid, but they disappear the moment they come in contact with the pale-coloured acid. As soon as the whole quantity of acid has assumed the orange colour, nitrous air escapes from it in considerable quantity; but the mixture of water deprives the acid of its power of decomposing nitrous air. The strong and pale-coloured nitrous acid ought to be diluted with at least two or three parts of water to one of the acid, for the easy production of nitrous air from copper and mercury.

In common experiments, no other degree of heat is necessary than that produced by the effervescence itself, except mercury be used, which requires the application of some degree of heat; but when the metal exposes a very great surface to the acid, as is the case when the filings of the metal are used, the effervescence and production of nitrous air are often much quicker than can be conveniently managed. The most proper method of producing nitrous air, however, is explained in the last section of this treatise.

Nitrous air by itself is equally transparent and invisible with common air, excepting at its first production, when it is somewhat coloured, owing to a little superfluous nitrous acid, or to some earthy particles which are carried up with it. Its smell resembles that of nitrous acid, or indeed is the very same; because, in passing through the common air to our nostrils, it is decomposed, and converted into nitrous acid. The same is to be said of its taste; though Mr Fontana, who tasted it without any contact of external air, affirms that it has no taste whatever. The method in which he ascertained this fact was as follows. Having first introduced the nitrous air into a bottle of elastic gum in water, as is done with glass bottles, he brought his mouth, that, while the neck of the elastic-gum bottle was under water, near the neck of it; and then, by pressing the bottle, introduced the nitrous air into his mouth. The experiment, however, is by no means void of danger; for if the person happens to draw any quantity of this air into the lungs, he may be nearly suffocated, as nitrous air is exceedingly noxious. In performing of it, he recommends to exhaust the mouth entirely of common air, though he does not inform us how this can be done; nor indeed is it easy to conceive the possibility of doing so.

Though nitrous air extinguishes flame, it may by certain processes be brought in to such a state that a candle will burn in it with an enlarged flame; and it becomes what Dr Priestley calls dephlogisticated nitrous air, which is treated of in the next section. It is remarkable, however, that when a candle is extinguished, as it never fails to be in common nitrous air, the flame seems to be a little enlarged about its edges by the addition of another bluish flame before the former goes out.

Nitrous air seems to be the most fatal to animal life of any. Even insects, which can bear phlogisticated and inflammable air, generally die the moment they are put into it. Frogs, snails, and other animals which do not respire very frequently, die in a few minutes, and generally do not recover even when taken out of this noxious fluid before they are dead. Plants

perish very soon in nitrous air, and even in common air saturated with nitrous air; but Dr Priestley informs us, that "though in general plants die almost immediately in water impregnated with nitrous air, yet in one case of this kind, when the superfluous nitrous air was let out under water, so that no part of it was decomposed in contact with the water, the plant grew in it remarkably well."

Water, by agitation in nitrous air, may be made to imbibe one tenth part of its bulk; and afterwards the nitrous air may be expelled again by boiling, though not in the same quantity as it was absorbed; but for this purpose the water should be previously deprived of its air. Dr Priestley informs us, that having carefully pumped all the air out of a quantity of rain-water, letting it stand 24 hours in a good vacuum, and then impregnating it with nitrous air, he instantly expelled it again by boiling, when he obtained only about one fourth part of it, though sufficiently pure, and without any mixture of fixed air. Water may also be deprived of the nitrous air it contains, though it does not freeze quite so readily when impregnated with this air as in its natural state.

Nitrous air is absorbed by strong oil of vitriol nearly in the same quantity as by water; the acid acquiring a purple colour, by reason of the phlogiston contained in the nitrous air. The strong nitrous acid absorbs it in great quantity; and becomes smoking, orange coloured, and afterwards green, on account of the phlogiston contained in it. Marine acid imbibes but a small quantity, and very slowly, acquiring at the same time a light-blue colour. Both nitrous air and common air phlogisticated by it are meliorated by agitation in nitrous acid.

Nitrous air is absorbed in considerable quantity by radical vinegar, and the concentrated vegetable acid.—Solution of green vitriol imbibes it in much greater quantity than water, and acquires a black colour; which, however, soon goes off by exposure to the common air. Its taste also becomes acid.—Very little is absorbed by caustic alkalis. Oil-olive slowly absorbs a considerable quantity, but oil of turpentine absorbs much more. By a little agitation, it will imbibe more than ten times its quantity of nitrous air; acquiring at the same time a yellowish or orange colour, and becoming a little glutinous. The part which is not absorbed appears to be converted into phlogisticated air.—Ether and spirit of wine absorb it very quickly, but no nitrous air is obtained by the application of heat after they have absorbed it. It is greatly diminished by oil of turpentine, liver of sulphur, and pyrophorus; all of which leave it in a phlogisticated state. It is also diminished and phlogisticated by being kept in a bladder, alternately exposed to moisture and dryness. Nitrous acid air has the same effect.

One of the most remarkable properties of nitrous air, is its diminution with dephlogisticated air; by dephlogisti-
cated air. which means it becomes a test of the quantity of that kind of air contained in the atmosphere. With pure dephlogisticated air, the diminution is almost to nothing, at the same time that some quantity of nitrous acid is reproduced by the decomposition of the nitrous air; but as our atmosphere is always mixed with a considerable quantity of phlogisticated air, on which
nitrous.

Nitrous Air. nitrous air has no effect, the diminution in this case is never so considerable. Upon this principle the EUDYOMETER is constructed.

155
Its antiseptic power. Another very remarkable property of nitrous air is its strong antiseptic power; inasmuch that animal matters may, by its means, be preserved for many months without corruption. This property, it was thought, might have been extremely useful on many occasions; but Dr Priestley, after a number of experiments on the subject, concludes in the following manner. "Nitrous air will indeed preserve meat from putrefaction; but after long keeping, it becomes very offensive both to the nostrils and palate, though the smell is not altogether that of putrefaction; and indeed the substance continuing quite firm, it could not be properly putrid. —Having formerly experienced the remarkable antiseptic power of nitrous air, I proposed an attempt to preserve anatomical preparations, &c. by means of it; but Mr Key, who made the trial, found, that, after some months, various animal substances were shrivelled, and did not preserve their forms in this kind of air."

156
Specific gravity of nitrous air. The specific gravity of nitrous air, as well as of other kinds, has been ascertained by Mr Kirwan. As it corrodes metals, he endeavoured to find its weight by comparing the loss sustained by the materials which produce it. Thus he found, that 14 grains of the materials produced 38.74 inches of nitrous air; and, consequently, by proper calculation, that the specific gravity of nitrous air is to that of atmospheric air as 1195 to 1000. — "If this air (says he) had been obtained over water, or in strong heat, its weight would probably have been very different; as it is liable to be mixed with phlogisticated air, nitrous vapour, and a variable quantity of water. Nitrous vapour would render it heavier, and phlogisticated air or water probably lighter."

157
Component parts of nitrous air. With regard to the constituent principles, or elements of nitrous air, all those who look upon phlogiston to be a distinct substance, have believed that the former is a compound of nitrous acid and phlogiston. By the opposite party, it is supposed to be a substance entirely simple, and one of the constituent parts of the nitrous acid. This opinion seems in part now to be entertained by Dr Priestley himself, notwithstanding his former sentiments on the subject. "I had no doubt on the subject (says he) until I read the work of Mr Metherie; who asserts, that nitrous air contains no proper nitrous acid, but only one of the elements of it; the other being dephlogisticated air, which had before been considered by Mr Lavoisier as the principle of all acidity. — Among other observations in support of his assertion, Mr Metherie has the following. 1. Nitrous air burnt together with inflammable air, produces no nitrous acid. 2. Though nitrous air be obtained from a solution of mercury in the nitrous acid, almost all the acid is found in the solution. 3. Nitrous air, absorbed by marine acid, does not make aqua regia. 4. He is of opinion, that a small portion of the nitrous acid being decomposed, furnishes a pure air, so altered, that, uniting with inflammable air, it changes it into nitrous air."

"In reviewing the experiments I had formerly made on this kind of air, I could not recollect any of them in which the pure nitrous acid was produced, ex-

cepting that with dephlogisticated air, besides the experiment in which it was decomposed by the electric spark; which furnishes a strong objection to this hypothesis." To ascertain the matter more fully, the following experiments were made.

"When nitrous air is decomposed by iron, or by a mixture of iron and sulphur, the water, over which the process is conducted, acquires no acidity; but I had supposed that all the acid was absorbed by the iron. Having by me a quantity of this iron which had been reduced to perfect rust in nitrous air, and which, I knew, must have imbibed more than its weight of this air, I thought that the acid might be obtained from it by distillation; but a quantity of this rust of iron, distilled in an earthen retort, yielded neither nitrous air nor nitrous acid, at least in any quantity that could favour the common hypothesis.

"I then endeavoured to decompose nitrous air by heating iron in it with a burning lens; and in this process I succeeded far beyond my expectation: for the air was presently diminished in quantity, while the iron became of a darker colour, was sometimes melted into balls, and gathered considerable weight, though it had no appearance of containing any nitrous acid. — In the first experiment, the original quantity of nitrous air was diminished to about one-third; and after this, it was increased." The increase was found to arise from a production of inflammable and dephlogisticated nitrous air.

The Doctor proceeded to try various other experiments on the decomposition of nitrous air, particularly that of burning Homberg's pyrophorus; but without any success, or obtaining the smallest particle of nitrous acid. His conclusions from the whole are the following.

"Water seems to be a necessary ingredient in nitrous air as well as inflammable air; at least, without a quantity of water, nitrous air cannot be formed. For example, copper will be dissolved in strong nitrous acid without producing any nitrous air, just as iron and water, may be dissolved in concentrated vitriolic acid without producing inflammable air."

"That nothing is necessary to the formation of nitrous air besides phlogisticated nitrous acid and water, is evident from the production of it by the impregnation of pure water with phlogisticated nitrous vapour formed by the rapid solution of bismuth; an experiment which I mentioned before. However, to make it in a more unexceptionable manner, I interposed a glass vessel between that in which the solution was made and that in which the water to be impregnated with the phlogisticated vapour was contained, that whatever was distilled over by the heat of the process might be prevented from reaching the water. In these circumstances, however, when nothing but the dry phlogisticated vapour could enter the water, it began to sparkle and yield nitrous air very copiously as soon as it had received a bluer tinge from the impregnation. — Nitrous air is also produced by pouring a highly coloured or phlogisticated nitrous acid into pure water, in which no metal or earthy matter is any way concerned."

"I have formerly observed, how readily nitrous air is diminished by taking the electric spark in it. This experiment I have frequently repeated, in order more particularly

Nitrous Air. particularly to ascertain the quantity and quality of the residuum. In one experiment half an ounce of nitrous air was reduced, an less than half an hour, to one quarter of its bulk. One-fourth of the residuum was still nitrous, and the rest phlogisticated. Taking the electric spark in a quantity of nitrous air till it was diminished to one-third, the whole was completely phlogisticated, not affecting common air at all, and extinguishing a candle. A white matter was formed with the mercury over which the spark was taken, which made the water admitted to it extremely turbid. In another process, the electric spark was taken in a quantity of nitrous air till it could be no more diminished, when it was reduced in bulk in the proportion of 104 to 24. Letting it stand all night upon the mercury, it was increased in the proportion of 114 to 24; seemingly by the acid uniting to the mercury and generating more nitrous air, since it had that smell. No water appeared after the process; and the water admitted to it acquired no acid taste, but an astringent one like that of water impregnated with nitrous air. There was a white powder formed, as in the former experiments.—To try if it were possible to make water imbibe the acid from the nitrous air, the electric spark was taken in it, with a small quantity of water over the mercury. But even this water did not acquire any acid taste, but only astringent one."

The Doctor concludes his experiments on this subject with a conjecture, that the phlogiston, and neither the heat nor light of the electric, contributes to the decomposition of the nitrous air. As his final sentiments on the matter, however, are merely conjecture, without any certain experiments to confirm them, we shall here refer the reader to his Section on Theory, at the end of his sixth volume of Experiments, &c.

SECT. IX. Of Dephlogisticated Nitrous Air.

This species differs from common nitrous air in being able to support flame, though it still continues fatal to animal life. Common nitrous air may be converted into the dephlogisticated kind by particular processes; though, when zinc is dissolved in the nitrous acid, if the air be taken at different times, that which comes about the middle, or rather the latter end of the process, will be of this kind; in which it not only supports the burning of a candle, but the flame is enlarged (sometimes to four or five times its original bulk) by the addition of a weaker and bluish flame round the former; and this burning is sometimes accompanied with a crackling noise, as if the candle was burning in dephlogisticated air. It may also be obtained in some part of the process of procuring nitrous air from iron, though with this metal the success is uncertain; but tin yields a considerable quantity of it. By exposing iron to nitrous air, it may be so far dephlogisticated as to admit a candle to burn in it. Dr Priestley filled an eight-ounce phial with nails, and then with mercury; and displacing the mercury with nitrous air, left the phial inverted in a quantity of the same fluid. Two months after, the nitrous air was found to be changed in such a manner as to admit a candle to burn in it with its natural flame; and by continuing still longer in contact with the iron, a candle would burn in it with an enlarged flame. These changes, however,

are very irregular, so that they seldom produce the like effects with the regularity one might expect. Dr Priestley once found, that by the contact of iron in quicksilver, it was so changed as to be fired with an explosion like a weak inflammable air; whilst another quantity of nitrous air, which had been treated in like manner for about the same length of time, only admitted a candle to burn in it with an enlarged flame.

In that section of his last volume in which the Doctor treats of this kind of air, he observes, that water is absolutely necessary to its composition, or rather to the decomposition of the common nitrous air by iron. He had decomposed it before, either by previously filling the vessels that were to contain the nitrous air with water or with mercury; though it had always required a much longer time when the latter was made use of. The reason of its being formed at all in this last way, was a small quantity of moisture adhering to the inside of the vessel containing the mercury.

To try the influence of water in this case, he now procured a number of very clean small needles; and water on having made a phial, and likewise a proper quantity of mercury, quite clean and dry, he put the needles into the phial, and, filling it up with mercury, introduced the nitrous air: but it continued in this way for six or eight months without the smallest alteration. Introducing a few drops of water, a diminution of about one-third of the air took place, and the remainder appeared to be phlogisticated. On the 26th of May 1782, he examined a quantity of nitrous air, which had been confined with iron-shavings from the 27th of August preceding, when he found one-half of it absorbed; the remainder supported the flame of a candle better than common air, though a mouse died in it; and yet this air had continued several months in the same state with regard to quantity, nor was it at all probable that its quality would have been altered by any length of time.

Though this kind of air is produced by the contact of iron and nitrous air, the Doctor has never been able to ascertain the quantity of nitrous air which a given quantity of iron can decompose; and though iron soon becomes so much affected by this process that it crumbles into powder, it still seems equally capable of decomposing a fresh quantity. Having made a comparative experiment, by putting together one quantity of nitrous air with fresh iron and another with rust, he found that in both the air was diminished to about one-third, and a candle burned in both equally well; but neither of them had the properties of fresh nitrous air in any degree.

As the process for obtaining dephlogisticated nitrous air by means of iron is very tedious, the Doctor endeavoured to find another which might be attended with less inconvenience. This he accomplished by dissolving turnings of iron in a dilute solution of copper in nitrous acid (the same that remains after the production of nitrous air), mixing it again with an equal quantity of water. Without this precaution, he tells us, that though the iron will at first be acted upon very slowly, yet the mixture will at length grow so hot as actually to boil, and the process will be exceedingly troublesome; however it will be necessary, previous to any attempt to dissolve the iron, to heat the solution of copper, in order to expel all the nitrous air and superfluous

Dephlogisticated Nitrous Air. fluous nitrous acid. Without this precaution a quantity of common nitrous air will be produced.

Dephlogisticated nitrous air is absorbed by water almost as readily as fixed air, and in considerable quantity; the liquid taking up about one-half its bulk of air. After being thus saturated, the whole quantity of dephlogisticated nitrous air may be expelled pure by heat, and is easily received in vessels containing mercury. It was likewise observed, that as this kind of air much resembles fixed air in its properties of being imbibed by water, and expelled again by heat, it resembles it also in this farther property, that all the air which has been actually incorporated with the water will not be imbibed by water again. But the proportion of this part is three or four times greater than the corresponding part of fixed air; it is also considerably more phlogisticated. Water impregnated with it very soon parts with it again on being exposed to the atmosphere.—It discovers not the smallest trace of containing either acid or alkali. Its specific gravity is less than that of common air. On heating red precipitate in this kind of air, pure dephlogisticated air was produced without affecting, or being affected by, the nitrous air. Repeating the experiment with malleable iron, the quantity of it was enlarged, and the whole phlogisticated, without any mixture of fixed air. By heating bits of clean crucibles or retorts in this kind of air, it seemed to approach in quality to common atmospheric air; and the effects were always found to be the more considerable the longer the process was continued. On attempting, however, to determine whether this change in the constitution of dephlogisticated nitrous air was occasioned by means of heat or light, he heated it in earthen tubes; but found, that though these were glazed both on the outside and inside, and seemed perfectly air-tight both before and after the experiment, the air had escaped. By the electric spark it was rendered wholly immiscible with water, and brought to the standard of 1.45; so that the Doctor had no doubt of its being respirable. Yet this kind of air, though it admits a candle to burn so well in it, will not kindle pyrophorus, though the nitrous air from which it is produced would instantly set it on fire.

SECT. X. Of Vitriolic, Nitrous, Marine, and other Acid Airs.

§ 1. Vitriolic acid Air.—This is always a combination of vitriolic acid with phlogiston, and consequently may be procured from any mixture of that acid in its highly concentrated state with phlogistic matter. Hence it is obtained from all the metals, gold and platina excepted, on boiling them with strong oil of vitriol. It is also procurable from the same acid rendered black by any phlogistic matter. No greater heat is required to expel this kind of air than that produced by the flame of a candle. It is the heaviest of all aerial fluids, next to fluor acid air, being to common air as 2265 to 1000. Dr Priestley informs us, that a quantity of vitriolic acid thus impregnated with phlogiston, will yield many times more air than an equal quantity of the strongest spirit of salt.—When the vitriolic acid air is produced in great plenty, the top of the phial in which it is generated is commonly filled with white vapours. The air has also the same appearance as it is transmitted through

the glass tube; and it is sometimes discoverable in the recipient. When such substances are put to the oil of vitriol as cause a great effervescence with that acid, care should be taken to add them by very small quantities at a time, and likewise to apply the heat by very slow degrees, lest the rapid production of air, and the heat attending it, should break the vessels. It is most equally produced by using strong oil of vitriol and charcoal; but in most cases the production of vitriolic acid air is attended with that of inflammable, and sometimes fixed or phlogisticated air. With ether about one-half of the first produce is inflammable; but the quantity lessens as the process goes on. The Doctor observed, that, when quicksilver was used, the acid was not turned black, as in other experiments of the like nature. He also observed, that iron yielded a little inflammable air together with the acid gas; but that the elastic fluid produced when zinc was used, contained about two parts of inflammable and one of acid air. Copper, silver, and lead, when heated in vitriolic acid, yield the purest vitriolic acid air, without any mixture of inflammable air; but the lead yields only a very small quantity, and requires a great degree of heat. It is procured in the greatest abundance from the fumes of burning sulphur, and is then called the volatile vitriolic, or sulphureous acid; for an account of the properties of which, see CHEMISTRY, (Index).

§ 2. Of Nitrous Acid Air.—This is the pure nitrous acid by itself, without any addition of phlogiston. It tained. is procured by heating the strong spirit of nitre in a phial, and then receiving the vapour in glass vessels filled with quicksilver. It is, however, extremely difficult, or rather impossible, to preserve it for a length of time by means of any fluid hitherto known. Water absorbs it immediately, and quicksilver is corroded, and produces nitrous air. "But (says Dr Priestley) tho' the acid vapour very soon unites with the quicksilver, yet, the jar in which it was received being narrow, the saline crust which was formed on the surface of the quicksilver, impeded the action of the acid upon it till I had an opportunity of admitting water to the air I had produced, and of satisfying myself, by its absorption, of its being a real acid air, having an affinity with water similar to other acid airs."

The most remarkable property of this vapour is, that its colour may be made more or less intense by the mere circumstance of heat. It may be confined in glass vessels with ground-stoppers, or in tubes hermetically sealed, and thus exposed to the action of heat: in which case it will be found, that the colour of the vapour becomes considerably more intense in proportion as the glass vessel containing it is more or less heated; and that, on the contrary, the intensity of the colour diminishes as it is cooled. "It seems probable (says Dr Priestley), that if this vapour was not confined, but had room to expand itself, it would become colourless with heat. This at least is the case when it is combined with water. The phenomena I refer to are very common in the process for making dephlogisticated air, in which I first observed them. But the same things are observable in the process for producing any other kind of air in which much spirit of nitre is made use of; and likewise constantly in the common process for making spirit of nitre itself. It is, that when the heat is moderate, the vapour within the

the glass tube or retort is red; but that, as the heat increases, it becomes transparent." The Doctor having observed that red lead, impregnated with nitrous vapour, may be preserved a long time without deliquescing or losing its acid, made use of a composition of this kind for procuring the nitrous vapour with which he filled his tubes. By imbibing this vapour the minimum lost its red colour and became white. "I put (says he) a small quantity of this white minimum into a glass tube closed at one end; then holding it to the fire, make it emit the red vapour till the whole tube is filled with it; and having the other end of the tube drawn out ready for closing, as soon as the vapour begins to issue out of that end, I apply my blowpipe and seal it. By this means I conclude that the tube is filled with a pure red vapour, without any mixture of nitrous air, and perhaps common air also." For a further account of the properties of nitrous acid air, see CHEMISTRY, (Index.)

§ 3. Of Marine Acid Air.—The marine acid, by heat, may be resolved into a permanently elastic and transparent invisible vapour, which, however, is more easily preserved in its aerial state than nitrous acid air, as the former has no effect upon quicksilver. An easy and cheap method of obtaining this kind of air is by filling a phial, fitted with a glass tube and stopper, with common salt, and then pouring a small quantity of oil of vitriol upon it; which, by the assistance of heat, will disengage the acid principle, or the marine acid air, from the salt. "A phial (says Dr Priestley) prepared in this manner will suffice, for common experiments, many weeks; especially if some more oil of vitriol be occasionally put to it. It only requires a little more heat at the last than at the first. Indeed, at first, the heat of a person's hand will often be sufficient to make it throw out the vapour. In warm weather it will even keep smoking many days without the application of any other heat. On this account it should be placed where there are no metallic utensils which it can corrode; and it may easily be perceived when the phial is throwing out this acid vapour, as it always appears in the open air in form of a light white cloud."

After the marine acid has yielded all the air that can be expelled from it, it is extremely weak, so that it can but barely corrode iron. The gas itself is considerably heavier than common air, the specific gravity of the two being in the proportion of five to three; a cubic inch weighing 0.654 grains. It is very fatal to animal life, but less so than pure nitrous air; for flies and spiders live longer in marine acid than in nitrous air. In dipping a candle into a jar of this air the flame is extinguished; but the moment before it goes out, and also when it is afterwards first lighted again, it burns with a green or light-blue flame, like that of common salt thrown into a fire. Its diminution by the electric spark is barely perceptible. Ice is dissolved by it as fast as if it touched a red-hot iron. It is partly absorbed by almost every substance containing phlogiston, and the remaining part becomes inflammable. Oil of olives absorbs it very slowly, and oil of turpentine very fast; by which they both become almost black, and the remainder of the air is inflammable. Essential oil of mint absorbs marine air pretty fast, becoming brown, consistent, and so heavy

as to sink in water; and its smell is in great measure altered. Ether absorbs it very fast, and has its colour altered by the impregnation, becoming first turbid, then yellow, and at last brown. The air over the ether is strongly inflammable. A small bit of phosphorus smoked and gave light in this acid air; and the elastic fluid was but little diminished in twelve hours. On the admission of water, about four-fifths of the gas were absorbed, and the rest was inflammable. This change was also effected by a great number of other substances: some of which, however, required a considerable time to produce their effect; such as crusts of bread not burned, dry wood, dry flesh, roasted pieces of beef, ivory, and even flints. See CHEMISTRY, (Index.)

§ 4. Of Fluor Acid Air.—The discovery of fluor acid air was made by Mr Scheele, who obtained it by distilling the spar called fluor with vitriolic acid. Dr Priestley, who made several experiments upon the subject, was of opinion that this new acid was only the vitriolic disguised by its connection with the fluor. He even supposed that he had produced it by pouring vitriolic acid on other phosphoric spars; both these opinions, however, he has now retracted, and believes the fluor acid to be one of a peculiar kind. Its most remarkable property is the great attraction it has for siliceous earth, so that it even corrodes and makes holes in the retorts in which it is distilled. See CHEMISTRY, (Index.)

§ 5. Of the Vegetable and another Acid Air.—By means of heat alone, the concentrated vegetable acid emits a permanently elastic and aerial fluid. This has the properties of the acid of vinegar; but, like it, is weaker than the rest of the mineral acid airs, though it agrees with them in its general characters. Water imbibes it as readily as any of the other acid airs; oil-olive readily absorbs it, and in considerable quantity, losing at the same time its yellowish colour, and becoming quite transparent. Common air is phlogisticated by it, as it is also by the liquid vegetable acid. As the vegetable acid, however, from which this air had been obtained, was distilled by oil of vitriol, the Doctor suspected that what he had examined might derive most of its properties from the oil of vitriol, and rather be vitriolic than vegetable acid air.

An acid air, somewhat different from any hitherto described, was obtained by Dr Priestley from the vapour arising on distilling to dryness a solution of gold in marine acid impregnated with nitrous acid vapour, which makes the best kind of aqua regia. "The produce (says he) was an acid air of a very peculiar kind, partaking both of the nature of the nitrous and marine acids; but more of the latter than of the former, as it extinguished a candle; but it was both extinguished and lighted again with a most beautiful deep blue flame. A candle dipped into the same jar with this kind of air went out more than 20 times successively, making a very pleasing experiment. The quantity of this acid air is very great; and the residuum I have sometimes found to be dephlogisticated, sometimes phlogisticated, and at other times nitrous air."

This species of air, first particularly taken notice of by Mr Bergman, who obtained it from an ore of

Atmospheric Air
176
Produced first from an ore of zinc.
177
B-Brain-
ed from he-
par sulphur-
tis.

zinc called Pseudogalena nigra Dannemorensis, and which was found to contain 29 parts of sulphur, one of regulus of arsenic, six of water, six of lead, nine of iron, 45 of zinc, and four of siliceous earth. The hepatic air was produced but in small quantity by pouring oil of vitriol on this mineral; spirit of salt produced it in much larger quantity; but nitrous acid produced only nitrous air. The most proper method of obtaining this air is by pouring marine acid on hepatic sulphur, which extricates it in vast quantity. It is said also to be sometimes produced naturally from putrefying matters. It is the characteristic of all livers of sulphur, whether they be made with alkalis or earths. The smell of the pure gas is intolerable; and the vapour has a disagreeable effect on many metallic substances, particularly silver, lead, copper, &c. destroying their colour, and rendering them quite black. It is suddenly fatal to animal life, renders syrup of violets green, and is inflammable, burning with a very light blue flame. It is decomposed by vitriolic and nitrous air, by dephlogisticated air, and by the contact of atmospheric air, in which case it deposits a small quantity of sulphur; being indeed, as is supposed by Mr Bergman and Mr Kirwan, no other than sulphur kept in an aerial form. Its specific gravity, compared with that of atmospheric air, is as 1106 to 1000. It combines readily with water, and gives the smell to the sulphureous medicinal waters. Its great attraction for some of the metals and their calces makes it the basis of some Sympathetic Inks. See also CHEMISTRY, (Index.)

178
Proportion of the two ingredients of which it is composed.

THE two component parts of our atmosphere, viz. dephlogisticated and phlogisticated air, have been so fully treated of under their respective sections, that little remains to be said in this place, excepting to determine the proportion in which they are usually met with in the common air. The only regular set of experiments which have been made on this subject are those of Mr Scheele. He constructed an eudiometer, consisting of a glass receiver, which could contain 34 ounces of water, and a glass cup containing a mixture of one pound of iron-filings, and an equal weight of flowers of sulphur moistened; which cup standing upon a glass supporter, was inserted in the former receiver, which, when this was in it, could contain 33 ounces of water. To the outside of the glass tube or receiver, was affixed a slip of paper, to the height of a third of the tube, containing 11 divisions, each corresponding to one ounce of water. This paper was varnished over with oil varnish, to prevent its being spoiled by water. The whole then was placed in water, which gradually rose as the air was diminished. This mixture would serve four times before the power of diminishing air was lost. He carefully compared the height of the air therein with the barometer and thermometer, both before and after the experiment; in eight hours the experiment was completed. With this instrument he examined the goodness of the common air in Stockholm every day for a whole year, and found the diminution never to exceed $\backslash frac\{1\}\{2\}\backslash \%$, nor to fall short of $\backslash frac\{1\}\{3\}\backslash \%$; so that upon a medium it may be estimated at $\backslash frac\{2\}\{3\}\backslash \%$. During the months of January and February it

VOL. I. Part I.

was $\backslash frac\{2\}\{3\}\backslash \%$. The 23d of March it was $\backslash frac\{1\}\{3\}\backslash \%$, though the cold increased, and the barometer stood higher than before. The 19th of April it was $\backslash frac\{1\}\{2\}\backslash \%$, though the barometer and thermometer did not vary, and so stood till the 21st. In May and June it stood between $\backslash frac\{2\}\{3\}\backslash \%$ and $\backslash frac\{1\}\{3\}\backslash \%$. The 30th of July it stood at $\backslash frac\{1\}\{2\}\backslash \%$. From the 3d to the 15th of September at $\backslash frac\{1\}\{3\}\backslash \%$. The 6th of October at $\backslash frac\{1\}\{2\}\backslash \%$, during a high storm; but after it stood between $\backslash frac\{1\}\{3\}\backslash \%$ and $\backslash frac\{2\}\{3\}\backslash \%$, till the 4th of November, when it fell to $\backslash frac\{1\}\{3\}\backslash \%$, and continued between $\backslash frac\{1\}\{3\}\backslash \%$ and $\backslash frac\{1\}\{2\}\backslash \%$ to the 20th, when it rose to $\backslash frac\{1\}\{2\}\backslash \%$. The 21st it fell to 8, and stood between $\backslash frac\{1\}\{3\}\backslash \%$ and $\backslash frac\{1\}\{2\}\backslash \%$ till the 8th of December, when it rose to $\backslash frac\{2\}\{3\}\backslash \%$; and from thence to the 31st it stood between $\backslash frac\{1\}\{3\}\backslash \%$ and $\backslash frac\{2\}\{3\}\backslash \%$.

As it has already been shown that the pure dephlogisticated part of the atmosphere is entirely consumed by phlogistic processes, such as that of fermenting brimstone and iron-filings, this eudiometer must be considered as an exact test of the proportion of dephlogisticated air contained in the atmosphere. The small variation in the quantity shows, that the processes in nature which destroy this air, are nearly balanced by those which produce it; though it must appear surprising, that both these fluids, so extremely different, should be produced at all seasons of the year in a proportion nearly equal; nor is it less surprising that two fluids of unequal specific gravity should remain incorporated together without any tendency to separate, which it is certain they never do, either in the atmosphere itself, or when confined in vessels in any quantity whatever.—As phlogisticated air is somewhat lighter than dephlogisticated, it might be supposed that the former would occupy the higher regions of the atmosphere in such a manner as to render them considerably more unwholesome than the lower parts; but this seems not to be the case: On the contrary, it appears that by experiments with the eudiometer, that the upper parts of the air contain a greater proportion of dephlogisticated air than those near the earth. See EUDIOMETER.

§ 1. FIXED AIR, or AERIAL ACID. The artificial methods of producing this are principally three, viz. by fermentation, by heat, and by acids.

(1.) By fermentation. When vegetable or animal substances, especially the former, are fermented, they yield a great quantity of fixed air. In breweries, on the surface of the fermenting liquor, there is always a stratum of fixed air reaching as high as the edge of the vats; so that if these vessels are deep, and the fermenting liquor much below their edges, the above-mentioned stratum may be some feet in thickness. The same phenomenon is observable in the fermentation of wines in general; and it is owing to the production and elasticity of fixed air, that fermenting liquors, when put into close vessels, often burst them with great violence. The case is the same whatever substance it is that undergoes the vinous fermentation, though the quantity of fixed air produced is not the same in all substances, nor even in the same substance at different times. From 42 cubic inches of beer Dr Hales obtained 639 cubic inches of air in 13 days. From a quantity of sugar

A a undergoing

undergoing the vinous fermentation, Mr Cavendish obtained so much fixed air, that out of 100 parts of the former 57 appeared to have been volatilized and converted into fixed air.

But though a vast quantity of fixed air escapes during this process of fermentation, a very considerable portion still remains united with the fermented liquor, and to this it owes all its briskness and agreeable pungent acidulous taste; for when the fixed air is totally evaporated, the liquor becomes entirely rapid and flat. Hence also we are furnished with a method of restoring the briskness to these liquors after they have lost it in consequence of being exposed to the atmosphere, viz. by impregnating them again with fixed air, either naturally or artificially produced.

Dr Priestley has made several experiments in order to determine the quantity of fixed air contained in several sorts of wine. His method was to take a glass phial (fitted with a ground stopple and tube), capable of containing $1\backslash frac\{1\}\{2\}$ ounce-measure. This he filled with wine, plunging it into a proper vessel of water. The whole was then put over the fire, and the water, into which the phial was plunged, suffered to boil. The end of the tube being placed under the mouth of an inverted receiver filled with quicksilver, the heat expelled the fixed air from the wine, which entering into the receiver, ascended in bubbles through the quicksilver to the top, pushing out part of the metal and taking its place. The result of his experiments was as follows:

During the acetous fermentation also, liquors emit a vapour, great part of which is fixed air, though the nature of its other component parts has not yet been thoroughly ascertained.

Fixed air is likewise produced, though in no great quantity, by putrefaction. In this case, however, a great part of the elastic fluid consists of inflammable and phlogisticated air, and the fixed air itself seems to be intimately connected with a putrid offensive effluvium. It seemed to Dr Priestley to "depend in some measure upon the time and other circumstances in the diffusion of animal or vegetable substances, whether they yield the proper putrid effluvium, or fixed or inflammable air."

The elastic fluid produced by putrifying vegetables, when kept in a moderate degree of heat, is almost all fixed air; while that from animal substances contains several times more inflammable than fixed air. Vegetable substances yield almost all the permanently elastic fluid in a few days, but animal bodies continue to emit it for several weeks. When the elastic fluid yielded by animal substances is absorbed by water, and that water boiled, the fixed air may then be obtained without any mixture of the putrid effluvium. It is also to be observed, that the quantity of elastic fluid producible from animal substances is various according to the nature of the parts of the animal employed. Thus the muscular parts will yield less elastic fluid, and also

less mixed with any putrid or offensive effluvium, than Of Artificial a whole animal, or than the liver, &c. The propor- Artificial Airs.
tion of inflammable and of fixed air is also various, according to the various parts employed.

(2.) By heat. In every combustion, except that of sulphur or of metals, a quantity of fixed air is generated. This may be observed by fixing a lighted candle in an inverted receiver over a basin of lime-water, for a precipitation of the lime will presently ensue; and the same precipitation (which is one of the characteristics of fixed air) will always ensue, whether a candle, a burning piece of wood, or, in short, any other combustible substance, except sulphur or metals, be made use of.

During this production or extrication of fixed from atmospheric air, the latter is commonly supposed to be considerably diminished, though Mr Lavoisier and Mr Scheele have now rendered that opinion doubtful. If a piece of charcoal be burned by throwing the focus of a lens upon it when contained in a glass-receiver inverted in water, after the apparatus is cooled, the water will have mounted a small way into the receiver. The diminution, however, is limited, and depends on several circumstances. Dr Hales has observed, that, in equal receivers, the air suffers a greater diminution by burning large candles than small ones; and likewise that, when equal candles are made use of, the diminution is greater in small than in large receivers. The cause of this phenomenon probably is, that the air contained in the receiver cannot all come into contact with the flame of the candle; whence, as soon as the air which is nearest the flame becomes contaminated, the candle is extinguished. Thus the author of a Concise Treatise on the Various Kinds of Permanently Elastic Fluids, has diminished the air of an inverted receiver one sixth part, by moving the candle whilst it burned through the different parts of the vessel, so that the flame was brought into contact with a greater quantity of the confined air than if it had remained in one situation till it became extinct. Dr Mayow observed, that by the burning of a candle the air was diminished of one thirtieth only; Dr Hales found it to be diminished of one twenty sixth part; and Dr Priestley found it to be diminished of one fifteenth or sixteenth. Mr Cavendish observed, that air suffered a diminution of one-tenth of the whole quantity, by passing through an iron-tube filled with red-hot powder of charcoal. A candle, or any other combustible body, will cease to burn by itself, and consequently to contaminate a quantity of confined air much sooner than when it is, in some manner, forced to burn by the external application of heat. "The focus of a burning mirror," says Dr Priestley, "thrown for a sufficient time either upon brimstone or wood, after it has ceased to burn of its own accord, and has become charcoal, will have a much greater effect of the same kind, diminishing the air to its utmost extent, and making it thoroughly noxious." The combustion of the phosphorus of urine diminishes air in a great degree. Mr Lavoisier has observed, that by the combustion of phosphorus, air may be diminished of about one-fifth or one-sixth. This accurate philosopher has also observed, that the acid of phosphorus thus formed, acquires the weight lost by the diminished air; finding that about three inches of air were absorbed by every

Of Artificial Air. one grain of phosphorus, when the experiment was tried with a receiver inverted in water, upon the surface of which a small quantity of oil had been introduced; but when the receiver was inverted in quicksilver, the absorption was constantly between two one-fourth and two three-fourth inches for each grain. Mr Cavallo mentions his having often repeated the experiment of burning phosphorus in a glass tube inverted in water, by applying the closed part of the tube, wherein the phosphorus was contained, to a pretty strong fire, when he always observed that the utmost diminution of the inclosed air effected by this means was full one-fifth.

Dr Hales remarked, that after the extinction of candles in a receiver, the air continued to diminish for several days after. This may be owing to the gradual absorption of part of it by the water; it having been remarked by Dr Priestley, "that this diminution of air by burning is not always immediately apparent, till the air has passed several times through water; and that when the experiment was made with vessels standing in quicksilver instead of water, the diminution was generally inconsiderable till the air had passed through water."

In these experiments of burning combustible bodies in a quantity of air, and measuring the diminution, we should always remark two causes of mistake, viz. the absorption of air by the coaly residuum of the burned matter, which sometimes is very considerable, or by the fluid in which the receiver is inverted, and the production of elastic fluid from the burning substances; thus gunpowder generates a great quantity of elastic fluid when inflamed, &c.

Even the electric spark separates fixed air from common atmospheric air; for when a number of these sparks are taken in a small quantity of common air over lime-water, a diminution will take place, the lime will be precipitated, and if we put a blue vegetable juice instead of the lime-water, it will be turned red by the acidity of the fixed air deposited upon it. Dr Priestley having cemented a wire into one end of a glass tube, the diameter of which was about one-tenth of an inch, and having fixed a brass ball to that extremity of the wire which was out of the tube, filled the lower part of it with the juice of turnsole or archil, so that a quantity of common air was contained in the tube between the extremity of the wire and the surface of the liquor. Then taking electric sparks between the said wire and liquor for about one minute, the upper part of the liquor began to look red, and in about two minutes it was manifestly so. The air, at the same time, was diminished in proportion as the liquor became red; but when the diminution arrived to be one-fifth of the quantity of the air contained, then a longer electrization produced no sensible effect. "To determine," says the Doctor, "whether the cause of the change of colour was in the air or in the electric matter, I expanded the air which had been diminished in the tube by means of an air-pump, till it expelled all the liquor, and admitted fresh blue liquor in its place; but after that, electricity produced no sensible effect, either on the air or on the liquor; so that it was evident that the electric matter had decomposed the air, and had made it deposit something that was of an acid nature."

The calcination of metals, as already observed, phlogisticates, and consequently diminishes common air;

but does not produce any fixed air, since the lime-water, over which the calcination is made, does not become turbid; and when metallic calxes are exposed to a sufficient strong heat, they in general yield some fixed air: so that it seems that the fixed air which is formed in the act of the calcination of metals is absorbed by the calx. Some fixed air may be obtained from red lead, by no greater degree of heat than that of the flame of a candle applied to the phial that contains it.

Of Artificial Air. The calcareous earths, which, when acted on by Obtained acids, yield a vast quantity of fixed air, produce a very small quantity of it when exposed to a strong heat by themselves, in a proper vessel, even when exposed to the focus of a lens. Dr Priestley, in his experiments relating to the production of dephlogisticated air from various substances, when moistened with nitrous acid, and afterwards exposed to a sufficient degree of heat, generally found that some fixed air was produced together with the dephlogisticated air; but often obtained fixed air only, without any dephlogisticated air being mixed with it, or fixed and nitrous air together. From half an ounce of rust of iron, moistened with spirit of nitre, and dried, he obtained about a quart of elastic fluid, about one-third of which was fixed and the rest nitrous air. From ashes of pit-coal, treated in the same manner, he obtained nearly the like result. But in those experiments, the Doctor mostly used a gun-barrel, into which he introduced the substances to be tried; so that it is very probable, as he justly observes, that the iron might have contributed to the formation of the fixed air. In fact, when he tried substances of the same sort, first in a gun-barrel and then in glass vessels, he obtained much more fixed air in the former than in the latter case. One of those experiments he made with tobacco pipe-clay, which, after being moistened with spirit of nitre, was when dry exposed to the fire in a gun-barrel, and yielded some elastic fluid, which appeared to be wholly fixed air; but repeating the experiment in a glass-phial with a ground stopple, and taking the produced elastic fluid at eight different times, found that on the beginning some fixed air was produced, but afterwards the produce was dephlogisticated air. He made a similar experiment with flints carefully calcined in close vessels, and obtained a similar result.

187 Most minerals contain fixed air, which may be extracted to a certain degree by means of heat. Mr Krenger, distilling a greenish fusible spar, which was luminous in the dark, obtained from it some permanently elastic fluid, which, like fixed air, crystallized a solution of fixed alkali. Mr Fontana, in his analysis of the malachite, finds that that mineral contains a vast quantity of fixed air, as pure as that which is extracted from chalk by means of vitriolic acid.

From almost every metallic ore and earthy mineral some fixed air may be obtained, as well as from chalk, lime-stone, marble, marine shells, fixed and volatile alkali, and from magnesia alba, by means of a violent fire, or of acids.

In Mr Boyle's, Dr Boerhaave's, and Dr Hales's works, and in other books, the quantities of elastic fluid generated in various processes, and by diverse substances, are mentioned with distinction; but as those writers were not acquainted with the characteristic properties of fixed air, we do not know whether the elastic fluid mentioned by them was pure fixed air or not.

From animal substances, mixed with spirit of nitre, and sometimes heated a little, in order to facilitate the production of elastic fluid, Dr Priestley obtained, in general, fixed air; but whereas the fixed air produced by a similar process with vegetable substances is mostly mixed with nitrous air, this is mixed with an elastic fluid, which is seldom nitrous in a very slight degree, but is often phlogisticated air, viz. in such a state as extinguishes a candle, does not diminish common air, nor is itself diminished by nitrous air. Towards the end of the process, the Doctor remarks, "that when, by means of a strong heat, the produce of air is very rapid, and the air full of clouds, it is, like air, produced from vegetable substances in the same circumstances, slightly inflammable, burning with a lambent, greenish, or bluish flame."

(3.) By acids. Calcareous substances in general produce abundance of fixed air when acted upon by any acid, only the strongest acids will expel from them more fixed air than the weakest; and it happens to be peculiarly advantageous for those who want to produce a great quantity of fixed air, that the vitriolic acid is both the cheapest and strongest acid, and, upon the whole, the fittest for this purpose. The phenomena attending the production of fixed air from calcareous substances, &c. are themselves very remarkable, and furnish the subject of much speculation in philosophy.

The principal facts are the following. 1. When calcareous earths, alkalies, and magnesia, in their usual state, are mixed with acids, they cause an effervescence; and consequently the production of a permanently elastic fluid, namely, fixed air. 2. These substances retain the fixed air very obstinately; so that a strong fire is necessary to expel it from magnesia, and the strongest is not sufficient to expel it entirely from fixed alkalies, and especially from calcareous earths (A). When these substances are treated with acids, they yield the fixed air, because they have a stronger attraction to those acids than to the fixed air. 3. The calcareous earths which are insoluble in water, when deprived of the fixed air become soluble in it. Thus lime-stone is not soluble in water, but lime (viz. lime-stone deprived of its fixed air) is soluble in water. And if those substances, deprived of their fixed air, are put in a situation proper to recover their lost fixed air, they lose the property of being soluble in water. Thus, when lime-water is exposed to fixed air, the lime absorbs the fixed air; and, losing at the same time its property of being soluble in water, is precipitated from it in the state it was before calcination, viz. of a calcareous earth insoluble in water, and capable of effervescing with acids. 4. Alkalies, both fixed and volatile, when deprived of their fixed air, become more caustic, and more powerful solvents, incapable of crystallization, and of effervescing with acids. But if to those alkalies, and also earths rendered more caustic, their fixed air be restored, they acquire at once all the properties they had before they were deprived of the fixed air, viz. they become more mild, effervesce with acids, recover their weight, &c.

Those properties of calcareous earths and alkalies were ascertained by the learned Dr Black, who performed a variety of decisive and well-contrived experiments, upon which he formed a just theory, viz. that the causticity, sharpness, solubility, &c. of those substances, was owing to the fixed air being expelled from them; and that when they were combined with a proper quantity of fixed air, they were mild, &c. The Doctor gives the epithet of mild to those substances when they are combined with air, and of caustic when deprived of it; as caustic calcareous earth, caustic fixed alkali, &c. Among the other experiments, he connected two phials by means of a bent tube; in one of which he put some caustic spirit of sal ammoniac, and in the other some mild alkali, or mild calcareous earth; then pouring, through a hole made in the side of the latter phial, some acid upon the mild alkali, so as to produce some fixed air, which, passing through the tube into the other phial, combined with the spirit of sal ammoniac, and rendered it mild.

Easy methods of obtaining Fixable Air for occasional Experiments, &c.

(1.) By Fermentation. Mix together equal parts of brown sugar and good yeast of beer, to which add about twice the bulk of water. This mixture being put into a phial, to which a bent tube with a cork may be adapted, will yield a considerable quantity of fixed air, which may be received into a phial filled with quicksilver or water, as in the following process.

(2.) By Acids. Let a glass tube, open at both ends, be bent, by means of a blow-pipe and the flame of a candle, nearly into the shape of an S, as it is represented by AB, and fix a cork D to one of its extremities, so as to fit the neck of a common phial, that may hold about four or five ounce measures. The hole through the cork may be made with an iron wire red-hot, and the tube may be fastened in it with a bit of soft wax, so as not to let any air go through. Fill a similar phial, or any glass receiver K, with water, and invert it after the manner shown above, in a basin HI, about half filled with water. Now put some chalk or marble, grossly powdered, into the bottle E, so as to fill about a fourth or fifth part of it, and upon it pour some water, just enough to cover the chalk; then add some oil of vitriol to it, which needs not be more than about the fourth or fifth part of the water. Immediately after, apply the cork D, with the tube AB, to the bottle, and putting it in the situation FG, let the extremity B of the tube pass through the water of the basin into the neck of the bottle K, which now must be kept up with the hand, or other convenient support, as it cannot rest upon the bottom of the basin. The mixture of chalk, &c. in the bottle FG, will immediately begin to effervesce, showing a frothing, and an intestine motion accompanied with heat, that may be felt by applying the hand to the outside of the fluid. The elastic fluid called fixed air is copiously emitted from this mixture, and passing through the bent tube, will go into the bottle K, as appears by the bubbles which come out of the tube, and, passing

(A) Chalk, lime-stone, &c. after being kept in a very strong fire for many hours, if they are put into acids, yield a considerable quantity of fixed air; which shows that the purest quick-lime contains some fixed air.

Of Artificial Air. Of Artificial Air.
ing through the water, ascend to the top of the inverted bottle. In proportion as the elastic fluid fills the bottle K, the water gradually descends, and at last is quite expelled from it; the bottle K then is filled with fixed air, and being corked under water, may be removed from the basin, and kept for use. Another bottle may then be filled with water, and may be inverted over the extremity of the bent tube in the place of K, which other bottle may be filled in a similar manner, and so on till the mixture in FG has finished to yield any fixed air.

If one of these bottles filled with fixed air be uncorked, and, holding it with the mouth upwards, a lighted wax taper, bent like L, or a small piece of it affixed to the extremity of a wire, be immediately let down in it, the flame will be instantly extinguished. The same thing will happen if a lighted piece of wood is let down in it.

Take a clean bowl, and putting the mouth of a bottle, filled with fixed air, in it, uncork it, and keep it in that situation for about a minute. The fixed air being specifically heavier than common air, will come out of the bottle, and will remain at the bottom of the bowl, whilst common air enters into the bottle; which bottle may now be removed; and, in order to show the real existence of the fixed air, which will immediately show its being heavier than common air, put a lighted wax-taper into the bowl, pretty near its bottom, which taper will be extinguished immediately. The air in this experiment must be agitated as little as it is possible. That the flame of the wax taper was really extinguished by the fixed air, may be easily proved in the following manner:—Blow once or twice into the bowl, by which means the fixed air will be expelled from it; and then, on letting down a lighted wax-taper in it as before, it will be found that it is no longer extinguished, but will burn very well, the bowl being now filled with common air. This experiment never fails of surprising the spectators, as it clearly exhibits two remarkable properties of a fluid, which they can neither see nor distinguish by the feeling.

Can be en Air.
When the bottle K is about half filled with fixed air, put a mark with a bit of soft wax on the outside of it, just coinciding with the level of the water in it, and immediately after shake the bottle; but taking care that its mouth be not lifted above the surface of the water in the basin. After having shaken it for about a minute, on intermitting the agitation, it will be found that the water is above the mark; which shows that some of the fixed air has been absorbed by it. Let this absorption be carried on as far as possible, by agitating the bottle repeatedly, and allowing time to let more fixed air be produced and enter into the bottle in proportion as the water absorbs it. Then apply the hand, or a finger, to the mouth of the bottle whilst under water; bring the bottle out, and turn it with the mouth upwards. The water then will be found to have acquired a pleasant acidulous taste. The water thus impregnated with fixed air changes the blue infusion of some vegetable substances into red; so that if a weak solution of heliotrope is mixed with it, or indeed if it is simply exposed to fixed air, the liquor acquires a reddish appearance. It also corrodes iron, and some other metals, much more easily than common water. But the greatest and most useful property of

this acidulated water, or water impregnated with fixed air, is its being a powerful antiseptic. As the most used mineral waters are medicinal principally on account of their being impregnated with fixed air, besides which they generally contain some small portion of metal or salt dissolved; they may be imitated by impregnating water with fixed air, and then adding that quantity of salt or of metal, that by analysis the original mineral waters are found to contain.

183
It is for its great property of hindering putrefaction, that fixed air by itself, or incorporated with various fluids, especially with water, and that vegetables, sugar, and other substances which abound with fixed air, are very powerful remedies in putrid diseases. Sir John Pringle supposes, with great probability, that the frequent use of sugar and fresh vegetables, which at this time make up a considerable part of the diet of the European nations, prevents those putrid diseases and plagues which formerly were rather frequent.—Dr Macbride, showing experimentally that fixed air is discharged by such substances as form our common food, ascribes the preservation of the body from putrefaction in great measure to the fixed air, which in the ordinary process of digestion is disengaged from the aliment, and incorporates with the fluids of the body.

From the same property it may be also usefully applied to several economical purposes. Mr Henry found, that fixed air can preserve fruit for a considerable time. He tried a bunch of Italian grapes, which being suspended in the middle part of Dr Nooth's apparatus, and being supplied with plentiful streams of fixed air every day, was preserved without any signs of decay for about one month longer than a similar bunch suspended in a decanter containing common air. Strawberries and cherries he also found to be preserved without decay some days longer in fixed than in common air. Indeed, fixed air preserves not only fruit, but resists putrefaction in general. Dr Macbride, in his elegant Essays on Medical and Philosophical Subjects, has published various experiments which demonstrate this property of fixed air. He found, that not only good meat was preserved incorrupt for a considerable time, when exposed to fixed air; but that the putrefaction of substances actually putrid was impeded by this means, and even that those substances were restored from the putrescent to a sound state. That putrefaction was checked by fermentation, was discovered by Sir John Pringle; and Dr Macbride observed, that this effect was owing to the fixed air produced in the act of fermentation. But it must be observed, that when the sound, or even putrid substances, expose a very great surface to the fixed air, as is the case with milk, bile, and other fluids impregnated with fixed air, and also with small bits of meat, then they are preserved for a considerable time: but large pieces of solid animal substances, as for instance roundish pieces of flesh of about half a pound weight, do not seem to remain incorrupt much longer in fixed than in common air; at least the difference is inconsiderable. Sir William Lee, baronet, in two of his letters to Dr Priestley, informs him of his having found, that flesh-meat, even in the hot season, could be preserved wholesome for several days, by only washing it two or three times a-day in water impregnated with fixed

Of Artificial fixed air. "We have been enabled," says he, "to preserve meat as perfectly sweet and good to the extent of ten days, as at the first killing; and there seems no doubt it might be preserved much longer." He has even recovered some meat that had begun to change. This useful discovery, Sir William justly observes, may be very beneficial to the public, especially to butchers. "Particularly a butcher," says he, "who deals pretty largely, assures me he found the greatest success from it, and only objects that the veal was a little discoloured, though kept perfectly sweet."

Fixed air, as it combines with water, so it may be combined with other liquors. Beer, wine, and other fermented liquors, may be impregnated with fixed air, and by this means their sharpness may be restored, when they are become vapid, or, as it is commonly said, dead. The acidulous taste communicated by the impregnation of fixed air, cannot be discovered in beer, wines, and, in short, in such liquors which have much taste of their own. Milk acquires an acidulous taste by being impregnated with fixed air, and is thereby preserved incorrupt for some days; which affords a very easy expedient of preserving milk in those places where it cannot be had new very often.

62. To produce INFLAMMABLE AIR.—The process for making this sort of gas is the same as that for making fixed air: one of the materials only must be different, viz. iron-silings, or grossly powdered zinc, must be used instead of chalk; to which silings some oil of vitriol and water must be added, in the same proportion as in the fixed air, or rather a little more of oil of vitriol.

N.B. Instead of the silings of iron, small nails, or small bits of iron-wire, answer equally well.

The inflammable elastic fluid produced by this mixture has a displeasing smell, even when mixed with a very large quantity of common air; so that if any considerable quantity of it comes out of the bottle, before the cork with the bent tube be applied to it, &c. its smell may be perceived all over the room in which the experiment is made, but this smell is not particularly offensive.

When a bottle has been filled with this elastic fluid, stop the mouth of it with your thumb, or any stopper, and taking it out of the basin, bring it near the flame of a candle; and when the mouth of the bottle is very near it, remove the stopper, and the elastic fluid contained in the bottle will be immediately inflamed; and if the capacity of the bottle is nearly equal to four ounce-measures, it will continue burning quietly for about half a minute, the flame gradually descending lower and lower, as far as about the middle of the bottle, in proportion as the inflammable gas is consumed.

In this experiment we see, that inflammable air follows the general rule of all other combustible substances, namely, that of burning only when in contact with common air: thus the flame of this gas, whilst burning, is observable only on that surface of it which is contiguous to the common air; so that if the bottle be closed, the flame is put out immediately, because the air is intercepted from it. But if the inflammable air were put in such a situation as to expose a very great surface to the common air, it is plain, that by

this means its combustion would be accelerated, so as Of Artificial to let it burn instantly, and go off with an explosion, Airs. caused by the sudden rarefaction of the air. In fact, this effect may be easily observed in the following manner: When the bottle is to be inverted into the basin, in order to let it be filled with the inflammable gas, instead of filling it entirely with water, let half of it remain filled with common air; then invert it, and let the other half, which is now filled with water, be filled with inflammable air after the usual manner; and when the bottle is full, remove it in the manner shown above, and approach it to the flame of the candle, by which means the inflammable air takes fire; but now it explodes all at once with a large flame and a considerable report, sometimes breaking the bottle in which it is contained. In this case, the bottle being filled with equal parts of inflammable and common air, these two elastic fluids were mixed together, so that almost every particle of the one touched every particle of the other, and hence the sudden combustion was occasioned. The force of this explosion is so very considerable, that some pistols have been contrived, which are charged with a mixture of air and inflammable gas, and being fired by means of an electric spark, are capable to drive a leaden bullet with great violence. Sometimes those pistols are made of glass (but in this case they are not charged with a bullet), and it is very diverting to show that pistols are charged and explode by the combustion of an invisible substance.

When a slender pipe is tied to the neck of a bladder, and the bladder is filled with inflammable air, after the manner described in the preceding experiment (viz. when the bladder was required to be filled with fixed air), two very pleasing experiments may be performed with it. First, the inflammable gas may be inflamed by applying the flame of the candle to the extremity of the pipe; and squeezing at the same time the bladder, a stream of fire will be formed in the air, which will last as long as the bladder contains any inflammable air; for this gas coming out of the pipe with violence, will continue inflamed for a considerable way in the air. Secondly, the extremity of the pipe may be dipped into a solution of soap, then removing it from the solution, and squeezing the bladder very gently, a ball of soap-water may be formed, including inflammable air; which ball, on account of the inflammable gas being much lighter than common air, as soon as it is detached from the pipe will ascend upwards, and will break by dashing against the ceiling, contrary to those commonly made by children, which in still air go downwards.—Whilst the ball is ascending, if the flame of the candle be approached to it, the film of soap-water will be instantly broke, and the inflammable air will take fire; thus a flame may be shown to be seemingly produced from a soap-ball.

By taking electric sparks in any kind of oil, spirit 187 of wine, ether, or spirit of sal ammoniac, Dr Priest-inflammable air ob- tained from various substances. The oil, or other liquor, was confined in a glass tube by quicksilver, and a wire was cemented in the upper part of the tube, through which the sparks being sent, went to the quicksilver through the oil; but after that a few sparks had been taken, a quantity of inflammable air was generated, &c. Left the production of inflammable air should be attributed to the cement which fastened the

^{Of Artificial Air.} the wire, the Doctor repeated the experiment with ether in a glass syphon; but the inflammable air was generated as before. This elastic fluid does not lose its inflammability by being passed several times from one vessel into another through water.

Alkaline air, by taking electric explosions in it, is changed into inflammable air.

By means of acids, inflammable air is obtained in greater abundance, and more readily. Iron, zinc, or tin, yield plenty of inflammable air when acted on by diluted vitriolic or marine acids.

If iron is put into strong vitriolic acid, the quantity of elastic fluid that is produced is very little, except heat be applied to the phial, for then the production of elastic fluid is more copious; but this elastic fluid is vitriolic acid air, mixed with a small portion of inflammable air, the proportional quantity of it being less when the acid is more concentrated.

Zinc, treated after the same manner, produces the like effects, except that it gives more elastic fluid, without the application of heat, than iron does; and the greatest part of the produced elastic fluid is inflammable.

In order to obtain the greatest quantity of inflammable air from iron or zinc, the vitriolic acid must be diluted with much water, as about one part of strong oil of vitriol to five or six parts of water. Dr Priestley found, that 11 grains of iron yielded 8½ ounce-measures of inflammable air. According to Mr Cavendish, one ounce of zinc, dissolved either in the vitriolic or marine acid, yields a quantity of inflammable air equal to the bulk of 356 ounces of water; one ounce of iron, dissolved by means of vitriolic acid, yields a quantity of inflammable air equal to the bulk of 412 ounces of water; and one ounce of tin yields half as much inflammable air as iron does.

The solutions of iron, tin, copper, lead, and zinc, in the marine acid, produce marine acid air, and inflammable air, but in various quantities. The proportion of the former to the latter is as one to eight in iron, as one to six in tin, as three to one in copper and lead, and as one to ten in zinc. Regulus of antimony, dissolved in marine acid, with the application of heat, yields a small quantity of elastic fluid, which is weakly inflammable.

Dr Priestley obtained inflammable air, not only by dissolving various substances in marine acid, but also by exposing divers bodies to marine acid air, which is probably the purest part of the marine acid. Having admitted iron-filings to this acid air, they were dissolved by it pretty fast; half of the elastic fluid disappeared, and the rest was rendered unabsorbable by water, and inflammable. The same effect was produced by almost every substance which contains phlogiston, as by spirit of wine, oil of olives, spirit of turpentine, charcoal, phosphorus, bees-wax, sulphur, dry cork-wood, pieces of oak, ivory, pieces of roasted beef, and even some pieces of a whitish kind of flint.

A greater or smaller portion of the acid air was absorbed, and the rest sometimes was all inflammable, and often was partly acid air, which was soon absorbed on the admission of water, and partly inflammable. In short, it seems as if this acid air, having a great affinity with phlogiston, separates it from all those substances which contain it even in small quantity, and from that combination becomes inflammable.

By means of nitrous acid, inflammable air may be obtained from various substances containing phlogiston; but it is always mixed with nitrous air, and sometimes also with fixed and common or phlogisticated air. If two parts of spirit of wine, mixed with one part of nitrous acid, are put into a phial with a ground-stopper and tube, and the flame of a candle be applied to it, so as to heat it gradually, the inflammable air will be produced very readily; the inflammability of which is, however, not very permanent, for by a little washing in water it may be annihilated. In the solution of most substances in nitrous acid, it generally happens, that the elastic fluid, which is obtained towards the latter end of the process, possesses the property of being inflammable: thus iron, dissolved in nitrous acid, yields nitrous air; but when the nitrous air ceases to be produced, if the heat of a candle be applied to the solution, more elastic fluid will be produced which is inflammable. "The nitrous acid (says Dr Ingenhoufz) when mixed with iron-filings in a very diluted state, gives, by the assistance of a moderate degree of heat, a mixture of different airs, partly fixed, partly common air, and partly phlogisticated air. See further the article AEROSTATION.

§ 3. To produce Nitrous Air.—This permanently elastic fluid is never found naturally, like fixed or inflammable air, but is entirely artificial.

Either silver, copper, brass, iron, mercury, bismuth, or nickel, when mixed with nitrous acid, yield nitrous air in great quantities. Some of them, especially mercury, require the aid of heat in order to produce the elastic fluid; the flame of a candle applied to the phial is sufficient; but others, especially copper and iron, do not want the application of any heat. Gold, platinum, and the regulus of antimony, when put in aqua regia, yield nitrous air pretty readily. Among the metals, lead yields nitrous air in the smallest quantity. "I poured (says Dr Priestley) smoking spirit of nitre into a phial with a ground-stopper and tube, containing 1½ ounce-measure filled with small leaden shot, so as to leave no common air at all, either in the phial or in the tube; and I placed it so as to receive the air that might come from it in water. After waiting an hour, in which little or no air was produced, I applied the flame of a candle, though not very near, to it: and in these circumstances I got produced about an ounce-measure of air: but upon some water rushing into the phial while the candle was withdrawn, air was produced very plentifully. I collected in all about a quarter of a pint; and might probably have got much more, but that the salt formed by the solution of the lead had so nearly closed up the tube, that I thought proper to discontinue the process. The air, both of the first and of the last produce, was of the same quantity; and so far nitrous, that two measures of common air, and one of this, occupied the space of two measures only; excepting that the very first and very last produce, mixed with common air, took up a little more room than that which I got in the middle of the process. When the air was produced very fast, it was exceedingly turbid, as if it had been filled with a white powder."

Among the semi-metals, zinc gives the weakest nitrous air, when dissolved in nitrous acid. The elastic fluid

Of Artificial Acid produced from it is mostly phlogisticated air. From four pennyweights and 17 grains of zinc, dissolved in spirit of nitre diluted with an equal quantity of water, Dr Priestley obtained about 12 ounce-measures of very weak nitrous air. It occasioned a very slight effervescence when mixed with common air. The Doctor obtained nitrous air even from some flowers of zinc. "Having (says he) mixed a quantity of blue spirit of nitre with flowers of zinc, which were of a dull colour, and appeared from several experiments to contain a portion of phlogiston, it yielded, with the heat of a candle applied to the phial which contained it, strong nitrous air; when the common spirit of nitre, applied in the same manner, gave only phlogisticated air; the phlogiston of which came probably from the calx itself, though a small portion of it might have been in the nitrous acid, which I believe is never entirely free from it."

The quantity of nitrous air that may be obtained from various metals, is difficult to be ascertained, on account of the diversity occasioned by the strength of the acid, the various nature of the metallic substance, and the method of performing the experiments. The following is a table of the produces of nitrous air from various metals, extracted from Dr Priestley's first volume of Experiments and Observations; but which, as the author himself intimates, is far from being very accurate.

The various strength of the nitrous acid produces great diversity in the production of nitrous air. Thus, if copper is dissolved in strong nitrous acid, it will not produce the least quantity of nitrous air; but when dissolved in diluted nitrous acid, it produces a great quantity of that elastic fluid. The strong and pale-coloured nitrous acid should be diluted with at least two or three parts of water to one of the acid, for the easy production of nitrous air from copper and mercury.

The briskness of the effervescence, and the production of nitrous air, are promoted by heat, and also by letting the metallic substance present a great quantity of surface to the acids.

For the generality of experiments, no other degree of heat is required than that produced by the effervescence itself, except mercury be used, which requires the application of some heat. When the metal exhibits a very great surface to the acid, as is the case when filings are used, the effervescence and production of nitrous air are often much quicker than can be conveniently managed.

Copper or brass, when clipped into flat bits, each about two or three grains in weight, and about a quarter of a square inch in surface, and when dissolved in nitrous acid properly diluted, yield nitrous air very equally; but if iron be used, the pieces of it should be larger and fewer; in short, it should present a much less surface to the diluted acid; otherwise the increase of heat in the process, and the rapid production of

elastic fluid, render the operation both difficult and dangerous for the operator.

As the nitrous air is mostly necessary to try the goodness of respirable air, it is of great consequence to make it always of one constant degree of goodness; but this object is answered by dissolving substances of a very homologous nature in the nitrous acid; therefore it is plain, that the metals whose nature is more uniform must be preferred for this purpose. Accordingly, brass yields nitrous air of a more uniform nature than iron: copper is superior to brass; but pure mercury is still superior to copper: and indeed this is the metal which, considering its nature, uniformity of substance, and easy solution, is upon the whole the most useful for this purpose.

It has been generally observed, that solid vegetable substances, when dissolved in nitrous acid, yield more nitrous air than the animal substances, though this nitrous air is not so pure as that obtained from metals.

Sometimes it contains some fixed air, and a good deal of inflammable air, which is mostly produced towards the end of the process. On the other hand, the nitrous air extracted from animal-substances generally contains a good deal of phlogisticated air, and sometimes some fixed air. In order to obtain nitrous air from the solution of animal and vegetable substances in nitrous acid, often some degree of heat must be applied to the phial. The acid also sometimes must be very concentrated, and in other cases it must be diluted; but it is hardly worth while, or practicable, to determine with exactness all those particular cases.

To make Nitrous Air.—The metal, viz. copper, brass, or mercury, is first put into the bottle (which, as well as the whole process, is the same as that described for fixed Air), so as to fill about one-third of the same; then some water is poured into the bottle, so as just to cover the metal-filings; and lastly, the nitrous acid is added, the quantity of which, when strong, should be about one-third or half the quantity of the water. The smell of the nitrous gas is very penetrating and offensive, and occasions a red smoke as soon as it comes into contact with the common air; hence, whenever any of it escapes from the bottle, it may be observed not only by the smell, but also by the slight red colour.

In order to observe the principal property of this elastic fluid, which is that of diminishing the bulk of common air, let a glass tube, closed at one end, and about nine inches long, and half or three quarters of an inch in diameter, be filled with water, and inverted in water; then take a small phial, of about half an ounce-measure, filled with common air, and plunging it under the water contained in the same basin where the inverted tube is kept, let that quantity of air enter into the tube, which will go to the top of it, the water subsiding accordingly. Let a mark be made, either with a file or by sticking soft wax on the tube, just opposite to the surface of the water in it, which will mark how much of the tube is filled by that given measure of air. After the same manner, fill the same small phial (which we shall call the measure) again with air; throw that air into the tube, and put a mark on the tube coinciding with the level of the water in it. In this manner let four or five measures be marked on the tube. Now, if three measures of common air are

^{Of Artificial Air.} put into this tube, when filled with water and inverted, they will fill a space of it as far as the third mark. The same thing will happen if three measures of nitrous instead of common air be put in it; but if two measures of common air and one measure of nitrous air, or one measure of the former and two of the latter, be introduced in it, they will fill a space much shorter than the third mark. On the moment that these two kinds of elastic fluids come into contact, a reddish appearance is perceived, which soon vanishes, and the water, which at first nearly reaches the third mark, rises gradually into the tube, and becomes nearly stationary after about two or three minutes; which shows that the diminution is effected gradually. See EUDIOMETER.

§ 4. To procure DEPHLOGISTICATED AIR.—This is no other than exceedingly pure atmospheric air, entirely free from those heterogeneous vapours which contaminate the air we commonly breathe. The easiest method of procuring this air is to put some red-lead into the bottle, together with some good strong oil of vitriol, but without any water. Let the red-lead fill about a quarter of the bottle, and the vitriolic acid be about the same quantity or very little less; then apply the bent tube to the bottle, and proceed in the same manner as above. But it must be remarked, that without heat this mixture of red-lead and vitriolic acid will not give any dephlogisticated air, or it yields an inconsiderable quantity of it; for which reason the flame of a candle (that of a wax taper is sufficient) must be applied under the bottom of the bottle; which for this purpose must be rather thin, otherwise it will be easily cracked (A). In this manner the red-lead will yield a good quantity of elastic fluid, the greatest part of which is dephlogisticated air; but not the whole quantity of it, for a good portion of fixed air comes out with it. In order to separate the fixed from the dephlogisticated air, the inverted bottle, when filled with the compound of both, as it is emitted from the red-lead, must be shook in the basin for impregnating water with fixed air; by which means the water will absorb the whole quantity of fixed air, and leave the dephlogisticated air by itself.

From every experiment it appears, that dephlogisticated air, if it could be readily obtained, and at a cheap rate, would be a most valuable manufacture. The heat communicated by means of it to burning fuel is incredible.

These are not the only advantages which might be expected from dephlogisticated air. It has been found by experience, that animals will live much longer in this kind of air than in an equal quantity of common air; whence it is supposed, that the breathing of it must be much more healthy, and contribute to longevity much more than the common atmosphere. Nay, there are not wanting some who attribute the longevity of

VOL. I. Part I.

the Antediluvians to the great purity of the atmosphere ^{Of Artificial Air.} at that time; the whole mass being afterwards tainted by the deluge, in such a manner that it could never regain its former purity and salubrity. But all this as yet is mere conjecture; and excepting the single fact, that animals live much longer in a quantity of dephlogisticated than of common air, there is no evidence that the former contributes more to longevity than the latter. Dr Priestley even throws out a conjecture, that the use of dephlogisticated air might perhaps wear out the system much sooner than common air, in the same manner as it consumes fuel much faster than common air. The great quantity, however, even of the purest air, which is requisite to support animal life, and the expense and trouble of the most ready methods of procuring it, have hitherto prevented any fair trial from being made. Yet philosophers, considering the probability there is of this kind of air being salutary in many diseases, have bestowed some pains in attempting to find out methods of procuring it easily and in large quantity; concerning which we have the following observations in Cavallo's Treatise on Air.

"A man makes in general about 15 inspirations in a minute, and takes in about 30 cubic inches of aerial fluid. But the air which has been once inspired is not thereby much injured, and it may be respired again and again; so that, upon a very moderate calculation, and as appears from actual experiments often repeated, we may safely assert, that a person can breathe 400 cubic inches of good ordinary atmospheric air, at least 30 times, without any inconvenience, i. e. it would serve for two minutes; after which that air, though much depraved, is still in a state of being breathed, but then it would occasion some uneasiness. Now, supposing the dephlogisticated air employed to be four times more pure than common air, 400 cubic inches of dephlogisticated air would serve for at least 120 respirations or eight minutes.

"But supposing that 30 inches of common air are completely phlogisticated by a single inspiration, and changed for such as is quite fresh, which indeed is the case in common respiration, then 450 cubic inches of common air will be requisite for one minute's respiration, and 27,000 for one hour; and as dephlogisticated air is supposed to be four times as good, the same quantity of it will serve for four hours. Indeed, if we could depend on the assertions of Mr Fontana, that by adding lime-water to absorb the fixed air produced by respiration, an animal can live 30 times as long as without it, no doubt a much smaller quantity would serve."

But it is certain such assertions cannot be true; because, though the fixed air should be absorbed as soon as produced, the remaining quantity would still be contaminated by phlogiston. Nay, we are informed by Dr Priestley, who repeated Fontana's experiments,

B b

that

(A) In this operation the flame of the candle, when once applied, must be kept continually near it; and when the mixture does not produce any more elastic fluid, or the operation is required to be intermitted, care should be taken to remove the extremity of the bent tube from the water first, and then to take off the flame of the candle from under the bottle; otherwise, if the flame of the candle be first removed, the materials within the bottle condensing by cold, the water immediately enters, which in an instant fills the bottle, and generally breaks it.

Of Artificial Air. that animals will not live longer in a quantity of dephlogisticated air when it stands in contact with lime-water, than they will when no lime-water is used. In what manner a difference so enormous can take place, between philosophers in other respects so accurate, we can by no means determine. It is plain, however, that if 27,000 inches of common air are necessary for a person in one hour, the same quantity of dephlogisticated air cannot be breathed longer than four hours, nor even so long, with any real advantage. Mr Cavallo indeed allows only 12,000 inches for four hours; but though this might no doubt sustain life for that time, the person must at best expect nothing from it superior to the common atmosphere, if he was not materially injured by it.

A very ready method of procuring dephlogisticated air in large quantity, is by means of nitre; and on the supposition that 12,000 inches are sufficient for four hours, (or for 40 hours, as he limits the Abbé Fontana's supposition), Mr Cavallo proceeds in the following manner: "The instruments necessary for the production of dephlogisticated air from nitre are the following; viz. earthen retorts, or earthen vessels with a straight neck, somewhat in the shape of Florence flasks, but with a longer neck, these being cheaper than the retorts, and answering as well;—a small furnace, in which the earthen retort must be kept red-hot; a common chimney-fire is not sufficient. These furnaces may be very easily made out of large black lead crucibles. The nitre must be put into the retort or other vessel, so as to fill half or nearly three quarters of its belly; then a bent glass tube is luted to the neck of the earthen vessel, in such a manner as not to let any elastic fluid escape into the open air. The best lute or cement for this or similar purposes is made by mixing together whiting and drying oil. The retort being put into the furnace, must be surrounded with lighted charcoal, which is to be supplied according as it wastes: in short, the belly of the retort must be kept quite red-hot, or rather white-hot, for about three hours at least. If, instead of the retort, the other described earthen vessel be used, care should be had to place it with the neck as little inclined to the horizon as possible, lest the nitre should stop the neck and break it." The air is then to be received into large glass jars, as is usual in other experiments on air.

"The retort or other earthen vessel that is used for this purpose cannot serve for more than once, because it generally breaks in cooling; and besides, the decomposed nitre cannot easily be taken out of it. The retort capable of holding a pound of nitre (the quantity necessary for producing 12,000 cubic inches of dephlogisticated air) for this operation, costs at least half-a-crown; the other earthen vessels in the shape of Florence flasks, but with longer necks, cost about 18d. a-piece, or 2s.; so that the price of these vessels forms a considerable part of the expence. If glass vessels are employed, the nitre will not yield near so much air, though of a purer sort, because the glass vessels cannot endure such a great fire as the earthen ones. The retorts of metal, or at least of those metals which are most usually employed for this purpose, viz. iron and copper, phlogisticate in a great measure the air as soon as produced. Considering, then, all these circumstances, it appears, that when a person has all the

usual apparatus and furnace, the expences at present Of Artificial Air. necessary in London for the production of 12,000 cubic inches of dephlogisticated air, (viz. the price of one pound of nitre, of an earthen retort or other vessel, and of charcoal), amount to about 4s. or 4s. 6d."

Another method of preparing dephlogisticated air is, by blowing that of the common atmosphere thro' melted nitre. In this process the phlogiston contained in the atmosphere is gradually consumed, by detonating with the acid of the nitre, and therefore issues much more pure than before. This method has the appearance at first of being much easier and more commodious than the former; but as it is impossible to mix the atmospheric air so exactly with the melted nitre that every particle of the one may come in contact with every particle of the other, it is plain that the former method must be preferable; not to mention that it will be found exceedingly troublesome to blow the air through the nitre, as the latter will be perpetually apt to cool and concrete into lumps by the cold blast.

§ 5. To procure VITRIOLIC Acid Air.—This consists of the vitriolic acid, united with some phlogiston, which volatilizes and renders it capable of assuming the form of a permanently elastic fluid. To obtain it, some strong concentrated vitriolic acid must be put into the usual bottle, together with some substance capable of furnishing phlogiston. Olive oil answers very well. The oil of vitriol should be about three or four times as much as the sweet oil, and both together should fill about one-third or half the bottle. A gentle degree of heat is then required, in order to let these materials yield any elastic fluid; which may be done by applying the flame of a wax taper, as directed above for the production of dephlogisticated air.

§ 6. To procure MARINE Acid Air, which is no other than the marine acid itself, and which without any addition becomes a permanently elastic fluid; put some sea-salt, or common kitchen salt, into the usual bottle in which the materials for producing elastic fluids are generally put, so as to fill about a fourth part of it, and upon this salt pour a small quantity of good concentrated vitriolic acid; then apply the bent tube to the bottle, and introduce it through the quicksilver into the receiver, filled with and inverted in quicksilver after the usual method, and the elastic fluid is copiously produced.

§ 7. To procure NITROUS Acid Air.—This may be obtained from heated nitrous acid, the vapour of which acquires a permanent elasticity, and it has been found to remain uncondensed into a visible fluid by any cold to which it has been hitherto exposed. The great difficulty is to find a fluid capable of confining this acid air; because it is easily and abundantly absorbed by water, which is one of its properties by which it differs from nitrous air. It acts upon quicksilver, and also upon oils: hence its examination cannot be made but very imperfectly; for substances must be exposed to it, or mixed with it, whilst it is actually changing its nature by acting on the mercury or other fluid that confines it.

When water has absorbed a good quantity of this elastic fluid, it acquires the properties of nitrous acid; and when heated, it yields a large quantity of nitrous air,

Of Artificial air, viz. a quantity many times greater than that which water is wont to imbibe of it by agitation, or by any known means.

When the nitrous acid air is combined with effential oils, a considerable effervescence and heat are produced, nearly in the same manner as when the nitrous acid itself is poured upon those oils.

§ 8. FLUOR ACID AIR.—Put some of those minerals called fluors, or fusible spars, pulverized, into the usual bottle, and upon it pour some concentrated oil of vitriol; then adapt the bent tube, &c. The fluor acid

air is at first produced without the help of heat; but in Of Artificial Air. a short time it will be necessary to apply the flame of a candle to the bottle, by which means a considerable quantity of this elastic fluid is obtained.

§ 9. ALKALINE AIR.—Let the usual bottle be about half filled with volatile spirit of sal ammoniac; and after applying the bent tube, &c. let the flame of a candle be brought under the bottle, by which means the alkaline air will be produced copiously.

HEPATIC AIR. See Sect. XI. supra.

I N D E X.

A.
Aerial acid, a name for fixed air, n^o 106.
Air, supposed anciently to be homogeneous, 1. Not so in reality, 2. Has some way of purifying itself, 3. Halley's calculation of the quantity of water evaporated into it from the sea, 4. Dr Watson's of the moisture evaporated from dry ground, ibid. How it is purified from the aqueous vapour, 4. From phlogistic vapours, 5. Why a dry air is always wholesome, but a moist one is not, ibid. Contaminated in certain places by various kinds of vapours, ibid. How purified from vapours heavier than itself, ibid. Its specific gravity compared with water, 6. Its pressure as a gravitating fluid, 7. Effects of its gravity on vegetables and animals, ibid. Of its elasticity, 8. Whether this can be impaired, 9. Its elasticity is always in proportion to its density, ibid. How far a quantity of air may be compressed, 10. Is capable of vast dilatation by its elastic force, ibid. In what proportion it is expanded by heat, 11. Its elasticity supposed to be the cause of earthquakes, ibid. Effects of its elasticity on various bodies, 12. Great solvent power of the air, 13. Its chemical effects, 15. Air contained in mineral waters, 19, 20. Decomposed in the calcination of metals, 29. Is not diminished in common cases of combustion, 58. A kind of air procured from solution of gold, 175.

Alkaline air: Its properties, 146. Contains phlogiston, 147. Converted into inflammable air, 148.
Animals: Cause of their death in dephlogisticated air, 61. Effects of inflammable air on them, 141.
Arsenic: Inflammable air produced from it by the red-hot steam of water, 124.
Asbes gain most of their weight by absorption from the atmosphere, 122.
Atmosphere consists of two very different kinds of fluids, 24, 93. The proportions of these, 178. The upper parts of it more salubrious than the lower, 179.
B.
Black's (Dr) discoveries, 21. His theory concerning fixed air attacked at first, but now universally received, 23.
Boyle's discoveries, 17.
C.
Calcination of metals: Mr Lavoisier's experiments on it, 92. His conclusions therefrom with regard to the composition of atmospheric air, 93.
Cast iron: Remarkable phenomenon attending its calcination with a burning-glass, 70.
Cavallo's conclusions from Dr Ingenhouz's experiments, 38. His method of collecting inflammable air from ponds, 119.
Cavendish's experiments on water, 75. On the production of nitrous acid, 101, 102.
Charcoal yields a great quantity of fixed air, 16.—totally convertible into inflammable air,

129. Its excessive attraction for water, 132.
Combustion, whether common air is diminished by it, 58, 183.
Contagion of the plague, of a heavy sluggish nature, 5.
Copper: Dr Priestley's experiments to produce water by its means, 73. Is not affected by alkaline air, 146.
Cotton-wool: Quantity of dephlogisticated air produced by its means from water, 45.
Cretaceous acid: An improper name for fixed air, 107.
D.
Darkness: Its effects on the production of air, 42.
Dephlogisticated air discovered by Dr Priestley, 24. First obtained by means of a burning-glass from precipitate perse, 25. Why called dephlogisticated, 26. Produced from a great variety of substances, ibid. Discovered by Mr Scheele, 28. May be obtained without the use of nitrous acid, 29. Produced in greatest quantities by a sudden and violent heat, 30. Method of procuring it from different substances, 31. How it is produced by nature, 32. Method of obtaining it from water, 36. From the leaves of plants, 37. By means of raw silk, 41. From various other substances, 45. Quantity of it produced from water, 46. Of the cause of its production, 47. At what times it is produced of the best quality, 48. Found in sea-water, 53. How to preserve it in large quantity, 54. It produces intense heat, 55.

Explodes violently with inflammable air, 56. Burns violently with pyrophorus, 57. Is diminished by combustion, 59,—and by nitrous air, 60, 154. In what manner it may be contaminated, 61. Does not support vegetation, 62. Of its component parts, 63. Does not contain earth, 65. Whether it contains any nitrous acid, 66. Imbibed by calces of metals, 67. By iron, 68. Mr Cavendish's experiments on its composition, 75. Nitrous acid produced from a mixture of it and inflammable air, 77. Supposed to be one of the component parts of water, 81, 82, 83. Effects of the electric spark on it when inclosed between different liquors, 105. Dr Priestley's experiments on the production of fixed air from it, 110.
Dephlogisticated nitrous air, how procured, 160. Its component parts, 161. Best method of procuring it, 163. Made to approach to the nature of atmospheric air, 164.
Diminution of air, supposed to be owing to phlogiston emitted into it, 89.
E.
Earth is not a component part of dephlogisticated air, 65.
Effervescence between acids and alkalis occasioned by fixed air in the latter, 21.
Eider-down: Dephlogisticated air produced by its means from water, 45.
Electric spark: Its effects on dephlogisticated air inclosed between

tween different liquors, 105. On fixed air, 113. On nitrous air, 159. F.

Fermentation: Why it will not go on in vacuo, 12.

Fermented liquors restored from a vapid state by adding fixed air to them, 180.

Finery-cinder, the same with scales of iron, consists of the metal united with dephlogisticated air, 124.

Fire supposed to be the cause of the air's elasticity, 11.

Fixed air contained in absorbent earths and alkaline salts, 21. Its proportion in these substances, 22. Effervescence of these substances with acids occasioned by fixed air, 21. Increases the weight of metallic precipitates, 21. Supposed to be the principle of union in terrestrial bodies, ibid. Separated from fermenting and putrefying substances, 21. Dissolves earths and metals, 22. Formed by the union of phlogiston with dephlogisticated air, 67. Found in a great variety of substances, 106. Specific gravity, and other properties of this kind of air, 107, 108. Its constituent principles, 109. Dr Priestley's experiments on its composition, 110. Proportion of it produced from dephlogisticated air, 112. Effects of the electric spark on it, 113. Of a strong heat on it, 115. Quantity of it expelled from different substances, 116. Generated in the decomposition of inflammable air, 135. Convertible into inflammable air, 136. Great quantities produced by fermenting substances, 180. Proportions contained in different kinds of wines, 181. Emitted by putrefying matters, 182.

Fontana, Abbé: Effects of his breathing inflammable air, 141.

French philosophers, their experiments on the composition of water, 82.

Fur of a Russian hare produces dephlogisticated air with water, 45.

G.

Gold: A peculiar kind of air produced from its solution,

175. A beautiful experiment with it, ibid. Green matter observed by Dr Priestley in glass jars producing dephlogisticated air, proved to be of an animal nature, 40.

H.

Hales, Dr, his discoveries, 18, 19.

Heat: Its effects on fixed air, 115.

Hepatic air, produced from an ore of zinc, 176.

Beef obtained from liver of sulphur, 177. Its properties, ibid.

Hot climates: Great quantity of inflammable air produced in them, 118.

Human hair produces dephlogisticated air with water, 45.

I.

Ice dissolved very fast by alkaline air, 146. And by marine acid air, 171.

Incondensable vapour arising from water, 86. Priestley's conjectures concerning it, 87. Attempts to collect it, 88.

Inflammable air: Method of burning it in the dephlogisticated kind, 59. Water produced from a mixture of inflammable and dephlogisticated air, 77. Quantity of it necessary to phlogisticate common air, 78. This kind of air produced in mines, from putrid waters, &c. 117. Great quantities generated in hot climates, 118. Mr Cavallo's method of collecting it from ponds, 119. Meteorologists thought to be produced by it, 120. Different kinds of inflammable air, 121. Extracted from various substances by heat, 122. More air procured by a sudden and violent than by a gradual heat, 123. How procured from water and other fluid and solid substances, 124. Proportions of inflammable air procured from iron by means of steam, 125. Of the constituent parts of inflammable air, 126. No acid contained in it, 127. Water necessary to its production according to Dr Priestley, 128. Denied by Mr Kirwan, 138. Charcoal totally convertible into it, 129. Experiment showing the neces-

sity of water for the production of inflammable air, 131. Is not pure phlogiston, 133. Priestley's analysis of different kinds of it, 134. Fixed air generated in its decomposition, 135. Fixed air convertible into it, 136. Has a great propensity to unite with water, 137. Dr Priestley's conclusion with regard to its component parts, 139. Its absorption by water, 140. Its effects on vegetation and animal life, 141. Has littlerefractive power, 142. Schemes to employ it for various purposes, 143.

Ingenbeuse, Dr, his experiments in the melioration of air by vegetation, 35. Produces dephlogisticated air from water by means of the leaves of plants, 37. Conclusions from his experiments, 38. His theory disputed, 51.

Iron sometimes dissolved by the air, 13. Yields dephlogisticated air with oil of vitriol, 31. Imbibes dephlogisticated air, 68. Takes it from the atmosphere, 69. May be made to imbibe dephlogisticated air as often as we please, 74. Properties of the inflammable air obtained from it by means of steam, 125.

K.

Kirwan's conclusion concerning the artificial production of water, 83. Observes the propensity of inflammable air to unite with water, 137. His opinion concerning the constituent principles of inflammable air, 138.

L.

Lavoisier corrects a process of Dr Priestley, 31. His experiments on the diminution of air by burning, 58, 59. Differences betwixt him and Dr Priestley, 64. Denies the existence of phlogiston, 91. His experiments on the calcination of metals and respiration, 92, 93, 94.

Lead: Proportions of it revived in alkaline air, 147.

Leaves of plants separate dephlogisticated air from water, 37. Resume this property after they seem to have lost it, 52.

Light: Effects of it in the pro-

duction of dephlogisticated air, 36. Effects of light without heat, 43. Of artificial light, 44.

Lint produces dephlogisticated air, 45.

Litmus, its solution decomposed by taking the electric spark in dephlogisticated air confined over it, 105.

Liver of sulphur absorbs dephlogisticated air, 95. Yields hepatic air in plenty, 177.

M.

Manganese: Sulphurated inflammable air first produced from it, 144.

Marble, why it sometimes bursts with frost, 5.

Marine acid air, how procured, 170. Its properties, 171. Changed into inflammable air, 172.

Mediterranean sea: Quantity of water evaporated from its surface, 4.

Metallic vapours, their poisonous qualities, 5.

Metallic calces imbibe dephlogisticated air, 67.

Mercury yields dephlogisticated air either with nitrous or vitriolic acid, 31.

Mineral waters contain air, 19, 20.

Mint restores noxious air to a state of salubrity by its vegetation, 32, 33.

Mosets, their nature, 5.

Mustard, its effects on air, 35.

N.

Nitre yields a great quantity of dephlogisticated air, 28.

Nitrous air diminishes dephlogisticated air, 60, 154. Yields nitrous acid when decomposed, 76. How procured, 150. Why strong nitrous acid yields none, 151. Properties of it, 152. Extremely fatal to vegetable and animal life, 153. Has a strong antiseptic power, 155. Its specific gravity, 156. Its component parts, 157. Composed of phlogisticated nitrous acid and water, 158. Effects of the electric spark on it, 159.

Nitrous acid, whether or not it enters the composition of nitrous air, 66. Produced from dephlogisticated and inflammable air, 77.

Nitrous acid air, how procured, 166. Cannot be preserved

served by means of any fluid, 167. Assumes a red colour by being heated, 168. Its effects on red lead, 169.

Noxious air, how purified by agitation in water, 97.

Oils and Salts, why they separate in vacuo, 7.

Olive oil, with whiting, yields inflammable air, 124.

P.

Phlogisticated air, its properties, 99. Nitrous acid procured by means of it, 100. Mr Cavendish's opinions on its nature, 103.

Phlogistication of air, whether it produces any vitriolic acid, 76. Explained, 89.

Phlogiston, too great powers attributed to it, 90. Its existence denied by the foreign chemists, 91. Whether inflammable air is pure phlogiston or not, 133, 138. Contained in alkaline air, 147.

Plants purify air by their vegetation, 38.

Populus nigra, dephlogisticated air plentifully produced from water by means of its cotton-like substance, 46, 47.

Precipitate per se, yields no water on being revived into a metal, 73.

Priefley, Dr, discovers dephlogisticated air, 24. His first hypothesis concerning the component parts of dephlogisticated air, 63. Difference betwixt some of his experiments and those of Lavoisier, 64. His opinion con-

cerning the non-existence of nitrous acid in dephlogisticated air, 66. Difficulties arising from some of his experiments concerning the generation of water in dephlogisticated and inflammable air, 85. His conjectures concerning the inconsiderable vapour of water, 87. His experiments on the composition of fixed air, 110. His opinion concerning the composition of phlogisticated air, 111. Experiment in favour of his hypothesis concerning phlogisticated air, 114.

Putrefying substances emit fixed air, 182.

R.

Raw-silk produces dephlogisticated air by means of water, 41. Various substances substituted for it, 45. Comparison between its surface and that of the cotton-like substance of the Populus nigra, 47.

Red-lead yields no dephlogisticated air when first prepared, and but little for some time after, 29. Gives a greater quantity by a sudden than a slow heat, 30.

Respiration, Mr Lavoisier's experiments on it, 91.

Retorts with long necks proper for distilling dephlogisticated air, 31.

S.

Scales of iron the same with finery-cinder, 124.

Sebeele discovers dephlogisticated air, 28. His experiments

on its diminution by combustion, 59. On the component parts of the atmosphere, 24.

Sea-water contains pure air, 53.

Seltzer-water imitated by Mr Vene, 20.

Sheep's wool separates dephlogisticated air from water, 45.

Soot yields pure air by distillation, 87.

Sponge imbibes a great quantity of alkaline air, 146.

Span-glass, unsuccessful attempt to procure dephlogisticated air from water by its means, 49.

Steam, proportions of inflammable air obtained by its means from different substances, 125. Its influence on the production of inflammable air from charcoal, 132.

Stones sometimes dissolved by the air, 14.

Sulphureous vapours, their pernicious effects, 5.

Sulphur yields inflammable air with steam, 124.

Sulphurated inflammable air procured from manganese, 144. and from iron melted in vitriolic acid air, 145.

T.

Thomson, Sir Benjamin, his experiments on the production of dephlogisticated air, 39 et seq.

Turpentine oil yields inflammable air, 124.

V.

Vegetation will not go on in vacuo, and why, 12. Produces dephlogisticated air, 32. Improves noxious air, 33.

Experiments seemingly contradictory, 34. Dr Ingenhousz's experiments on this subject, 35.

Van Helmont's discoveries, 16.

Vegetable acid air phlogisticates common air, 174.

Vitriolic acid air, 165.

Volatile alkali produced from nitrous acid and iron, 149.

W.

Water, quantity of it evaporated from the Mediterranean, 4. From an acre of ground, ibid. Why it boils violently in vacuo, 7. Produces dephlogisticated air, 36. Quantity of air yielded by it, with the mixture of various substances, 46. By water alone, 50. Formed by the desiccation of inflammable and dephlogisticated air, 71. Quantity produced in this manner, 72, 73. Cavendish's experiments on this subject, 75. Dr Priefley's experiments on the same, 80, 81. Experiments of the French philosophers and Mr Kirwan, 82, 83. Water pervious to air, 98. Method of procuring inflammable air by its means, 124. Always necessary to the production of this kind of air, 128, 131. Attraction betwixt it and burning charcoal or iron, 132. Great propensity of inflammable air to unite with it, 137, 140. Its effects on nitrous air, 162.

Wines, proportions of fixed air in different kinds of them, 182.

AEROMANCY, a species of divination performed by means of air, wind, &c. See DIVINATION, n^o 5.

AEROMETRY, the science of measuring the air. It comprehends not only the doctrine of the air itself, considered as a fluid body; but also its pressure, elasticity, rarefaction, and condensation. But the term is at present not much in use, this branch of natural philosophy being more frequently called PNEUMATICS.

AERONAUT, a person who attends and guides an air-balloon. See AEROSTATION and AIR-BALLOON.

AERONAUTICA, from aer, and autonome, derived from aer, ship; the art of sailing in a vessel or machine

through the atmosphere, sustained as a ship in the sea. See AEROSTATION.

AEROPHYLLACEA, a term used by naturalists for caverns or reservoirs of air, supposed to exist in the bowels of the earth. Kircher speaks much of aerophylles, or huge caverns, replete with air, disposed under ground; from whence, through numerous occult passages, that element is conveyed either to subterraneous receptacles of water, which, according to him, are hereby raised into springs or rivers, or into the funds of subterraneous fire, which are hereby fed and kept alive for the reparation of metals, minerals, and the like.

Aerophyl-
lacea.

IS a science newly introduced into the Encyclopædia. The word, in its primitive sense, denotes the science of suspending weights in the air; but in its modern acceptance, it signifies aerial navigation, or the art of navigating through the atmosphere. Hence also the machines which are employed for this purpose are called aerostats, or aerostatic machines; and from their globular shape, air-balloons.

The romances of almost every nation have recorded instances of persons being carried through the air, both by the agency of spirits and by mechanical inventions; but till the time of the celebrated Lord Bacon, no rational principle appears ever to have been thought of by which this might be accomplished. Before that time, indeed, Friar Bacon had written upon the subject; and many had been of opinion, that, by means of artificial wings, fixed to the arms or legs, a man might fly as well as a bird: but these opinions were thoroughly refuted by Borelli in his treatise De Motu Animalium, where, from a comparison between the power of the muscles which move the wings of a bird, and those which move the arms of a man, he demonstrates that the latter are utterly insufficient to strike the air with such force as to raise him from the ground. It cannot be denied, however, that wings of this kind, if properly constructed, and dexterously managed, might be sufficient to break the fall of a human body from an high place, so that some adventurers in this way might possibly come off with safety; though by far the greatest number of those who have rashly adopted such schemes, have either lost their lives or limbs in the attempt.

In the year 1672, Bishop Wilkins published a treatise, intitled, The Discovery of the New World; in which he mentions, though in a very indistinct and confused manner, the true principle on which the air is navigable; quoting, from Albertus de Saxonia and Francis Mendoca, "that the air is in some part of it navigable: and upon this static principle, any brass or iron vessel (suppose a kettle), whose substance is much heavier than that of water, yet being filled with the lighter air, it will swim upon it and not sink. So suppose a cup or wooden vessel upon the outward borders of this elementary air, the capacity of it being filled with fire, or rather ethereal air, it must necessarily, upon the same ground, remain swimming there, and of itself can no more fall than an empty ship can sink." This idea, however, he did not by any means pursue, but rested his hopes entirely upon mechanical motions, to be accomplished by the mere strength of a man, or by springs, &c. and which have been demonstrated incapable of answering any useful purpose.

The only person who brought his scheme of flying to any kind of rational principle was the Jesuit Francis Lana, contemporary with Bishop Wilkins. He, being acquainted with the real weight of the atmosphere, justly concluded, that if a globular vessel were exhausted of air, it would weigh less than before; and con-

dering that the solid contents of vessels increase in much greater proportion than their surfaces; he supposed that a metalline vessel might be made so large, that, when emptied of its air, it would be able not only to raise itself in the atmosphere, but to carry up passengers along with it; and he made a number of calculations necessary for putting the project in execution. But though the theory was here unexceptionable, the means proposed were certainly insufficient to accomplish the end: for a vessel of copper, made so thin as was necessary to make it float in the atmosphere, would be utterly unable to resist the external pressure; which being demonstrated by those skilled in mechanics, no attempt was made on that principle.

In the year 1709, however, as we were informed by a letter published in France in 1784, a Portuguese proposal of projector, Friar Gusman, applied to the king for encouragement to his invention of a flying machine. The principle on which this was constructed, if indeed it had any principle, seems to have been that of the paper kite. The machine was constructed in form of a bird, and contained several tubes through which the wind was to pass, in order to fill a kind of sails, which were to elevate it; and when the wind was deficient, the same effect was to be performed by means of bellows concealed within the body of the machine. The ascent was also to be promoted by the electric attraction of pieces of amber placed in the top, and by two spheres inclosing magnets in the same situation.

These childish inventions show the low state of science at that time in Portugal, especially as the king, in order to encourage him to farther exertions in such an useful invention, granted him the first vacant place in his college of Barcelos or Santarem, with the first professorship in the University of Coimbra, and an annual pension of 600,000 reis during his life. Of this De Gusman, it is also related, that in the year 1736, he made a wicker basket of about seven or eight feet diameter, and covered with paper, which raised itself about 200 feet in the air, and the effect was generally attributed to witchcraft.

In the year 1766, Mr Henry Cavendish ascertained the weight and other properties of inflammable air, determining it to be at least seven times lighter than common air. Soon after which, it occurred to Dr Black, that perhaps a thin bag filled with inflammable air might be buoyed up by the common atmosphere; and he thought of having the allantois of a calf prepared for this purpose: but his other avocations prevented him from prosecuting the experiment. The same thought occurred some years afterwards to Mr Cavallo; and he has the honour of being the first who made experiments on the subject. He first tried bladders; but the thinness of these, however well scraped and prepared, were found too heavy. He then tried Chinese paper; but that proved so permeable, that the vapour passed through it like water through a sieve. His experiments, therefore, made in the year 1782, proceeded

ed no farther than blowing up soap-bubbles with inflammable air, which ascended rapidly to the ceiling, and broke against it.

⁷ Aerostation discovered by Montgolfier. But while the discovery of the art of aerostation seemed thus on the point of being made in Britain, it was all at once announced in France, and that from a quarter whence nothing of the kind was to have been expected. Two brothers, Stephen and John Montgolfier, natives of Annonay, and masters of a considerable paper-manufacture there, had turned their thoughts towards this project as early as the middle of the year 1782. The idea was first suggested by the natural ascent of the smoke and clouds in the atmosphere; and their design was to form an artificial cloud, by inclosing the smoke in a bag, and making it carry up the covering along with it. Towards the middle of November of that year, the experiment was made at Avignon with a fine silk bag of a parallelopiped shape. By applying burning paper to the lower aperture, the air was rarefied, and the bag ascended in the atmosphere, and struck rapidly against the ceiling. On repeating the experiment in the open air, it rose to the height of about 70 feet.

⁸ Account of his experi- An experiment on a more enlarged scale was now projected; and a new machine, containing about 650 cubic feet, was made, which broke the cords that confined it, and rose to the height of about 600 feet. Another of 35 feet in diameter rose about 1000 feet high, and fell to the ground three quarters of a mile from the place where it ascended. A public exhibition was next made on the 5th of June 1783, at Annonay, where a vast number of spectators assembled. An immense bag of linen, lined with paper, and containing upwards of 23,000 cubic feet, was found to have a power of lifting about 500 pounds, including its own weight. The operation was begun by burning chopped straw and wool under the aperture of the machine, which immediately began to swell; and after being set at liberty, ascended into the atmosphere. In ten minutes it had ascended 6000 feet; and when its force was exhausted, it fell to the ground at the distance of 7668 feet from the place from whence it set out.

Soon after this, one of the brothers arrived at Paris, where he was invited by the Academy of Sciences to repeat his experiments at their expence. In consequence of this invitation, he constructed, in a garden in the Fauxbourg of St Germain, a large balloon of an elliptical form. In a preliminary experiment, this machine lifted up from the ground eight persons who held it, and would have carried them all off if more had not quickly come to their assistance. Next day the experiment was repeated in presence of the members of the academy; the machine was filled by the combustion of 50 pounds of straw made up in small bundles, upon which about 12 pounds of chopped wool were thrown at intervals. The usual success attended this exhibition: The machine soon swelled; endeavoured to ascend; and immediately after sustained itself in the air, together with the charge of between 4 and 500 pounds weight. It was evident that it would have ascended to a great height; but as it was designed to repeat the experiment before the king and royal family at Versailles, the cords by which it was tied down were not cut. But in consequence of a violent rain and wind which happened at this time, the machine was

so far damaged, that it became necessary to prepare a new one for the time that it had been determined to honour the experiment with the royal presence; and such expedition was used, that this vast machine, of near 60 feet in height and 43 in diameter, was made, painted with water-colours both within and without, and finely decorated, in no more than four days and four nights. Along with this machine was sent a wicker cage, containing a sheep, a cock, and a duck, which were the first animals ever sent through the atmosphere. The full success of the experiment was prevented by a violent gust of wind which tore the cloth in two places near the top before it ascended: However, it rose to the height of 1440 feet; and, after remaining in the air about eight minutes, fell to the ground at the distance of 10,200 feet from the place of its setting out. The animals were not in the least hurt.

⁹ Some animals safely sent thro' the air. The great power of these aerostatic machines, and Mr Pilatre de Rozier their very gradual descent in falling to the ground, had originally showed that they were capable of transporting people through the air with all imaginable safety; and this was further confirmed by the experiment already mentioned. As Mr Montgolfier, therefore, proposed to make a new aerostatic machine of a firmer and better construction than the former, Mr Pilatre de Rozier offered himself to be the first aerial adventurer.

This new machine was constructed in a garden in the Fauxbourg of St Antoine. It was of an oval shape, about 48 feet in diameter and 74 in height; elegantly painted on the outside with the signs of the zodiac, ciphers of the king's name, and other ornaments. A proper gallery, grate, &c. were appended in the manner afterwards described; so that it was easy for the person who ascended to supply the fire with fuel, and thus keep up the machine as long as he pleased. The weight of the whole apparatus was upwards of 1600 pounds. The experiment was performed on the 15th of October 1783. Mr Pilatre having placed himself in the gallery, the machine was inflated, and permitted to ascend to the height of 84 feet, where he kept it afloat for about four minutes and a half; after which it descended very gently: and such was its tendency to ascend, that it rebounded to a considerable height after touching the ground. Two days after, he repeated the experiment with the same success as before; but the wind being strong, the machine did not sustain itself so well as formerly. On repeating the experiment in calmer weather, he ascended to the height of 210 feet. His next ascent was 262 feet; and in the descent, a gust of wind having blown the machine over some large trees of an adjoining garden, Mr Pilatre suddenly extricated himself from so dangerous a situation, by throwing some straw and chopped wool on the fire, which raised him at once to a sufficient height. On descending again, he once more raised himself to a proper height by throwing straw on the fire. Some time after, he ascended in company with Mr Girond de Villette to the height of 330 feet; hovering over Paris at least nine minutes in sight of all the inhabitants, and the machine keeping all the while perfectly steady.

These experiments had shown, that the aerostatic machines might be raised or lowered at the pleasure of the

the persons who ascended: they had likewise discovered, that the keeping them fast with ropes was no advantage; but, on the contrary, that this was attended with inconvenience and hazard. On the 21st of November 1783, therefore, M. Pilatre determined to undertake an aerial voyage in which the machine should be fully set at liberty. Every thing being got in readiness, the balloon was filled in a few minutes; and M. Pilatre placed himself in the gallery, counterpoised by the Marquis d'Arlandes, who occupied the other side. It was intended to make some preliminary experiments on the ascending power of the machine: but the violence of the wind prevented this from being done, and even damaged the balloon essentially; so that it would have been entirely destroyed had not timely assistance been given. The extraordinary exertions of the workmen, however, repaired it again in two hours, and the adventurers set out. They met with no inconvenience during their voyage, which lasted about 25 minutes; during which time they had passed over a space of above five miles.—From the account given by the Marquis d'Arlandes, it appears that they met with several different currents of air; the effect of which was, to give a very sensible shock to the machine, and the direction of the motion seemed to be from the upper part downwards. It appears also that they were in some danger of having the balloon burnt altogether; as the Marquis observed several round holes made by the fire in the lower part of it, which alarmed him considerably, and indeed not without reason. However, the progress of the fire was easily stopped by the application of a wet sponge, and all appearance of danger ceased in a very short time.

This voyage of M. Pilatre and the Marquis d'Arlandes may be said to conclude the history of those aerostatic machines which are elevated by means of fire; for though many other attempts have been made upon the same principle, most of them have either proved unsuccessful or were of little consequence. They have therefore given place to the other kind, filled with inflammable air; which, by reason of its smaller specific gravity, is both more manageable, and capable of performing voyages of greater length, as it does not require to be supplied with fuel like the others. This was invented a very short time after the discovery had been made by M. Montgollier. This gentleman had indeed designed to keep his method in some degree a secret from the world; but as it could not be concealed, that a bag filled with any kind of fluid lighter than the common atmosphere would rise in it, inflammable air was naturally thought of as a proper succedaneum for the rarefied air of M. Montgollier. The first experiment was made by two brothers Messrs Roberts, and M. Charles a professor of experimental philosophy. The bag which contained the gas was composed of lutestring, varnished over with a solution of the elastic gum called caoutchouc; and that with which they made their first essay was only about 13 English feet in diameter. Many difficulties occurred in filling it with the inflammable air, chiefly owing to their ignorance of the proper apparatus; insomuch, that, after a whole day's labour from nine in the morning, they had got the balloon only one third part full. Next morning they were surprised to find that it had

N^o 5.

fully inflated of itself during the night: but upon inquiry, it was found, that they had inadvertently left open a stop-cock connected with the balloon, by which the common air gaining access, had mixed itself with the inflammable air; forming a compound still lighter than the common atmosphere, but not sufficiently light to answer the purposes of aerostation. Thus they were obliged to renew their operation; and, by six o'clock in the evening of next day, they found the machine considerably lighter than the common air; and, in an hour after, it made a considerable effort to ascend. The public exhibition, however, had been announced only for the third day after; so that the balloon was allowed to remain in an inflated state for a whole day; during which they found it had lost a power of ascent equal to about three pounds, being one seventh part of the whole. When it was at last set at liberty, after having been well filled with inflammable air, it was 35 pounds lighter than an equal bulk of common air. It remained in the atmosphere only three quarters of an hour, during which it had traversed 15 miles. Its sudden descent was supposed to have been owing to a rupture which had taken place when it ascended into the higher regions of the atmosphere.

The success of this experiment, and the aerial voyage made by Messrs Rozier and Arlandes, naturally suggested the idea of undertaking something of the same kind with a balloon filled with inflammable air. The machine used on this occasion was formed of gorea of silk, covered over with a varnish made of caoutchouc, of a spherical figure, and measuring 27½ feet in diameter. A net was spread over the upper hemisphere, and was fastened to an hoop which passed round the middle of the balloon. To this a sort of car, or rather boat, was suspended by ropes, in such a manner as to hang a few feet below the lower part of the balloon; and, in order to prevent the bursting of the machine, a valve was placed in it; by opening of which some of the inflammable air might be occasionally let out. A long silken pipe communicated with the balloon, by means of which it was filled. The boat was made of basket-work, covered with painted linen, and beautifully ornamented; being 8 feet long, 4 broad, and 3½ deep; its weight 130 pounds. At this time, however, as at the former, they met with great difficulties in filling the machine with inflammable air, owing to their ignorance of the most proper apparatus. But at last, all obstacles being removed, the two adventurers took their seats at three quarters after one in the afternoon of the 1st of December 1783. Persons skilled in mathematics were conveniently stationed with proper instruments to calculate the height, velocity, &c. of the balloon. The weight of the whole apparatus, including that of the two adventurers, was found to be 604½ pounds, and the power of ascent when they set out was 20 pounds; so that the whole difference betwixt the weight of this balloon and an equal bulk of common air was 624 pounds. But the weight of common atmosphere displaced by the inflammable gas was calculated to be 771 pounds, so that there remains 147 for the weight of the latter; and this calculation makes it only 5½ times lighter than common air.

At the time the balloon left the ground, the thermometer stood at 9° of Fahrenheit's scale, and the quicksilver in the barometer at 30.18 inches; and, by means

Montgolfiers Balloon.
Franckourgeff & Antoine?

AEROSTATION.

Blanchard's Balloon.

Verfailles
B.

Charles' & Roberts' B.
Champ de Mars.

Montgolfier's B.
Franckourgeff & St. Germain?

Albatt sculpt.

means of the power of ascent with which they left the ground, the balloon rose till the mercury fell to 27 inches, from which they calculated their height to be about 600 yards. By throwing out ballast occasionally as they found the machine descending by the escape of some of the inflammable air, they found it practicable to keep at pretty near the same distance from the earth during the rest of their voyage; the quicksilver fluctuating between 27 and 27.65 inches, and the thermometer between 53° and 57°, the whole time. They continued in the air for the space of an hour and three quarters, when they alighted at the distance of 27 miles from Paris; having suffered no inconvenience during their voyage, nor experienced any contrary currents of air, as had been felt by Messrs Pilatre and Arlandes. As the balloon still retained a great quantity of inflammable gas, Mr Charles determined to take another voyage by himself. Mr Robert accordingly got out of the boat, which was thus lightened by 130 pounds, and of consequence the aerostatic machine now had nearly as much power of ascent. Thus he was carried up with such velocity, that in twenty minutes he was almost 9000 feet high, and entirely out of sight of terrestrial objects. At the moment of his parting with the ground, the globe had been rather flaccid; but it soon began to swell, and the inflammable air escaped from it in great quantity through the silken tube. He also frequently drew the valve that it might be the more freely emitted, and the balloon effectually prevented from bursting. The inflammable gas being considerably warmer than the external air, diffused itself all round, and was felt like a warm atmosphere; but in ten minutes the thermometer indicated a variation of temperature as great as that between the warmth of spring and the ordinary cold of winter. His fingers were benumbed by the cold, and he felt a violent pain in his right ear and jaw, which he ascribed to the dilatation of the air in these organs as well as to the external cold. The beauty of the prospect which he now enjoyed, however, made amends for these inconveniences. At his departure the sun was set on the valleys; but the height to which Mr Charles was got in the atmosphere, rendered him again visible, tho' only for a short time. He saw, for a few seconds, vapours rising from the valleys and rivers. The clouds seemed to ascend from the earth, and collect one upon the other, still preserving their usual form; only their colour was grey and monotonous for want of sufficient light in the atmosphere. By the light of the moon, he perceived that the machine was turning round with him in the air; and he observed that there were contrary currents which brought him back again. He observed also, with surprise, the effects of the wind, and that the streamers of his banners pointed upwards; which, he says, could not be the effect either of his ascent or descent, as he was moving horizontally at the time. At last, recollecting his promise of returning to his friends in half an hour, he pulled the valve, and accelerated his descent. When within 200 feet of the earth, he threw out two or three pounds of ballast, which rendered the balloon again stationary: but, in a little time afterwards, he gently alighted in a field about three miles distant from the place whence he set out; though, by making allowance for all the turn-

ings and windings of the voyage, he supposes that he had gone through nine miles at least. By the calculations of M. de Maunier, he rose at this time not less than 10,500 feet high; a height somewhat greater than that of Mount Aetna. A small balloon, which had been sent off before the two brothers set out on their voyage, took a direction opposite to that of the large one, having met with an opposite current of air, probably at a much greater height.

The subsequent aerial voyages differ so little from that just now related, that any particular description of them seems to be superfluous. It had occurred to Mr Charles, however, in his last flight, that there might be a possibility of directing the machine in the atmosphere; and this was soon attempted by Mr Jean-Pierre Blanchard, a gentleman who had, for several years before, amused himself with endeavours to fly by mechanical means, though he had never succeeded in the undertaking. As soon as the discovery of the aerostatic machines was announced, however, he resolved to add the wings of his former machine to a balloon, and made no doubt that it would then be in his power to direct himself through the air at pleasure. In his first attempt he was frustrated by the impetuosity of a young gentleman, who insisted, right or wrong, on ascending along with him. In the scuffle which ensued on this occasion, the wings and other apparatus were entirely destroyed; so that Mr Blanchard was obliged to commit himself to the direction of the wind; and in another attempt it was found, that all the strength he could apply to the wings was scarce sufficient to counteract the impression of the wind in any degree. In his voyage, he found his balloon, at a certain period, acted upon by two contrary winds; but, on throwing out four pounds of ballast, he ascended to a place where he met with the same current he had at setting out from the earth. His account of the sensations he felt during this voyage, was somewhat different from that of Mr Charles; having, in one part of it, found the atmosphere very warm, in another cold; and having once found himself very hungry, and at another time almost overcome by a propensity to sleep. The height to which he arose, as measured by several observations with mathematical instruments, was thought to be very little less than 10,000 feet; and he remained in the atmosphere an hour and a quarter.

The attempts of Mr Blanchard to direct his machine through the atmosphere, were repeated in the month of April 1784 by Messrs Morveau and Bertrand, at Dijon, who raised themselves with an inflammable air-balloon to the height, as it was thought, of 13,000 feet; passing through a space of 18 miles in an hour and 25 minutes. Mr Morveau had prepared a kind of oars for directing the machine through the air; but they were damaged by a gulf of wind, so that only two of them remained serviceable; by working these, however, they were able to produce a sensible effect on the motion of the machine. In a third aerial voyage performed by Mr Blanchard, he seemed to produce some effect by the agitation of his wings, both in ascending, descending, moving sidewise, and even in some measure against the wind; however, this is supposed, with some probability, to have been a mistake, as, in all his succeeding voyages, the effects of his machinery could not be perceived.

The success of Messrs Charles and Robert in their former experiments, encouraged them soon to repeat them, with the addition of some machinery to direct their course. Having enlarged their former balloon to the size of an oblong spheroid 46½ feet long and 27½ in diameter, they made it to float with its longest part parallel to the horizon. The wings were made in the shape of an umbrella without the handle, to the top of which a stick was fastened parallel to the aperture of the umbrella. Five of these were disposed round the boat, which was near 17 feet in length. The balloon was filled in three hours, and, with the addition of 450 pounds of ballast, remained in equilibrio with the atmosphere. About noon, on the 19th of September 1784, they began to ascend very gently in consequence of throwing out 24 pounds of ballast, but were soon obliged to throw out eight pounds more in order to

avoid running against some trees. Thus they rose to the height of 1400 feet, when they perceived some thunder-clouds near the horizon. On this they ascended and descended, to avoid the danger, as the wind blew directly towards the threatening clouds; but, from the height of 600 feet to that of 4200 above the surface of the earth, the current was quite uniform and in one direction. During their voyage they lost one of their oars; but found, that by means of those which remained, they considerably accelerated their course. From the account of their voyage, it would seem that they had passed safely through the thunder-clouds; as we are informed, that, about 40 minutes after three, they heard a loud clap of thunder; and, three minutes after, another much louder; at which time the thermometer sunk from 77 to 59 degrees. This sudden cold, occasioned by the approach of the clouds, condensed the inflammable air so that the balloon descended very low, and they were obliged to throw out 40 pounds of ballast; yet on examining the heat of the air

within the balloon, they found it to be 104°, when that of the external atmosphere was only 63. When they had got so high that the mercury in the barometer stood only at 23.94 inches, they found themselves becalmed; so that the machine did not go even at the rate of two feet in a second, though it had before gone at the rate of 24 feet in a second. On this they determined to try the effect of their oars to the utmost; and, by working them for 35 minutes, and marking the shadow of the balloon on the ground, they found, in that time, that they had described the segment of an ellipse whose longest diameter was 6000 feet. After having travelled about 150 miles, they descended, only on account of the approach of night, having still 200 pounds of ballast left.

Their conclusion, with regard to the effect of their wings, is as follows: "Those experiments show, that far from going against the wind, as is said by some persons to be possible in a certain manner, and some aeronauts pretend to have actually done, we only obtained, by means of two oars, a deviation of 22 degrees: it is certain, however, that if we could have used our four oars, we might have deviated about 40 degrees from the direction of the wind, and as our machine would have been capable of carrying seven persons, it would have been easy for five persons to have gone, and to have put in action eight oars, by means of

which a deviation of about 80 degrees would have been obtained.

"We had already observed (say they), that if we did not deviate more than 22 degrees, it was because the wind carried us at the rate of 24 miles an hour; and it is natural to judge, that, if the wind had been twice as strong as it was, we should not have deviated more than one-half of what we actually did; and, on the contrary, if the wind had been only half as strong, our deviation would have been proportionably greater."

Having thus related all that has been done with regard to the conducting of aerostatic machines through the atmosphere, we shall now relate the attempts that have been made to lessen their expence, by falling upon some contrivance to ascend without throwing out ballast, and to descend without losing any of the inflammable air.

The first attempt of this kind was made Voyage of by the Duke de Chartres; who, on the 15th of July 1784, ascended with the two brothers, Charles and Robert, from the Park of St Cloud. The balloon was of an oblong form, made to ascend with its longest diameter horizontally, and measured 55 feet in length and 24 in breadth. It contained within it a smaller balloon filled with common air; by blowing into which with a pair of bellows, and thus throwing in a considerable quantity of common air, it was supposed that the machine would become sufficiently heavy to descend, especially as, by the inflation of the internal bag, the inflammable air in the external one would be condensed into a smaller space, and thus become specifically heavier. The voyage, however, was attended with such circumstances as rendered it impossible to know what would have been the event of the scheme. The power of ascent with which they set out, seems to have been very great; as, in three minutes after parting with the ground, they were lost in the clouds, and involved in such a dense vapour that they could see neither the sky nor the earth. In this situation they seemed to be attacked by a whirlwind, which, besides turning the balloon three times round from right to left, shocked, and beat it so about, that they were rendered incapable of using any of the means proposed for directing their course, and the silk stuff of which the helm had been composed was even torn away. No scene can be conceived more terrible than that in which they were now involved. An immense ocean of shapeless clouds rolled one upon another below them, and seemed to prevent any return to the earth, which still continued invisible, while the agitation of the balloon became greater every moment. In this extremity they cut the cords which held the interior balloon, and of consequence it fell down upon the aperture of the tube that came from the large balloon into the boat, and stopped it up. They were then driven upwards by a gust of wind from below, which carried them to the top of that stormy vapour in which they had been involved. They now saw the sun without a cloud; but the heat of his rays, with the diminished density of the atmosphere, had such an effect on the inflammable air, that the balloon seemed every moment ready to burst. To prevent this they introduced a stick through the tube, in order to push away the inner balloon from its aperture; but the expansion of the inflammable air pushed it so close, that

all attempts of this kind proved ineffectual. It was now, however, become absolutely necessary to give vent to a very considerable quantity of the inflammable air; for which purpose the Duke de Chartres himself bored two holes in the balloon, which tore open for the length of seven or eight feet. On this they descended with great rapidity; and would have fallen into a lake, had they not hastily thrown out 60 pounds of ballast, which enabled them just to reach the water's edge.

The success of the scheme for raising or lowering aerostatic machines by means of bags filled with common air being thus rendered dubious, another method was thought of. This was to put a small aerostatic machine with rarefied air under an inflammable air-balloon, but at such a distance that the inflammable air of the latter might be perfectly out of the reach of the fire used for inflating the former; and thus, by increasing or diminishing the fire in the small machine, the absolute weight of the whole would be considerably diminished or augmented. This scheme was unhappily put in execution by the celebrated Mr Pilatre de Rozier, and another gentleman named Mr Romaine. Their inflammable-air balloon was about 37 feet in diameter, and the power of the rarefied-air one was equivalent to about 60 pounds. They ascended without any appearance of danger or similar accident; but had not been long in the atmosphere when the inflammable-air balloon was seen to swell very considerably, at the same time that the aeronauts were observed, by means of telescopes, very anxious to get down, and busied in pulling the valve and opening the appendages to the balloon, in order to facilitate the escape of as much inflammable air as possible. A short time after this the whole machine was on fire, when they had then attained the height of about three quarters of a mile from the ground. No explosion was heard; and the silk which composed the air-balloon continued expanded, and seemed to resist the atmosphere for about a minute; after which it collapsed, and the remains of the apparatus descended along with the two unfortunate travellers so rapidly, that both of them were killed. Mr Pilatre seemed to have been dead before he came to the ground; but Mr Romaine was alive when some persons came up to the place where he lay, though he expired immediately after.

These are the most remarkable attempts that have been made to improve the science of aerostation; tho' a great number of other expeditions through the atmosphere have taken place. But of all the voyages which had been hitherto projected or put in execution, the most daring was that of Mr Blanchard and Dr Jeffries across the straits of Dover which separate Britain from France. This took place on the 7th of January 1785, being a clear frosty morning, with a wind, barely perceptible, at N. N. W. The operation of filling the balloon began at 10 o'clock, and, at three quarters after twelve, every thing was ready for their departure. At one o'clock Mr Blanchard desired the boat to be pushed off, which now stood only two feet distant from that precipice so finely described by Shakespeare in his tragedy of King

Lear. As the balloon was scarcely sufficient to carry two, they were obliged to throw out all their ballast except three bags of 10 pounds each; when they at last rose gently, though making very little way on account of there being so little wind. At a quarter after one o'clock, the barometer, which on the cliff stood at 29.7 inches, was now fallen to 27.3, and the weather proved fine and warm. They had now a most beautiful prospect of the south coast of England, and were able to count 37 villages upon it. After passing over several vessels, they found that the balloon, at 50 minutes after one, was descending, on which they threw out a sack and an half of ballast; but as they saw that it still descended, and that with much greater velocity than before, they now threw out all the ballast. This still proving ineffectual, they next threw out a parcel of books they carried along with them, which made the balloon ascend, when they were about midway betwixt France and England. At a quarter past two, finding themselves again descending, they threw away the remainder of their books, and, ten minutes after, they had a most enchanting prospect of the French coast. Still, however, the machine descended; and as they had now no more ballast, they were fain to throw away their provisions for eating, the wings of their boat, and every other moveable they could easily spare. "We threw away, says Dr Jeffries, our only bottle, which, in its descent, cast out a steam like smoke, with a rushing noise; and when it struck the water, we heard and felt the shock very perceptibly on our ear and balloon." All this proving insufficient to stop the descent of the balloon, they next threw out their anchors and cords, and at last stripped off their clothes, fastening themselves to certain slings, and intending to cut away the boat as their last resource. They had now the satisfaction, however, to find that they were rising; and as they passed over the high lands between Cape Blanc and Calais, the machine rose very fast, and carried them to a greater height than they had been at any former part of their voyage. They descended safely among some trees in the forest of Guennes, where there was just opening enough to admit them.

It would be tedious as well as unnecessary to recount all the other aerial voyages that have been performed in our own or other countries: It appeared sufficient for the purpose of this article to notice those which were most remarkable and interesting; and therefore an account of the ingenious Mr Baldwin's excursion from Chester, alluded to above, shall now close our enumeration.

On the 8th of September 1785, at forty minutes past one P. M. Mr Baldwin ascended from Chester in Mr Lunardi's (A) balloon. After traversing in a variety of different directions, he first alighted, at 28 minutes after three, about twelve miles from Chester, in the neighbourhood of Frodsham; then reascending and pursuing his excursion, he finally landed at Rixton moor, five miles N. N. E. of Wavington, and 25 miles from Chester. Mr Baldwin has published his Observations and Remarks made during his voyage, and taken from minutes. Our limits will not admit of relating many

(A) Of this gentleman's adventurous excursions most people have been witnesses; and therefore it appeared unnecessary to take up room with an account of them in this article.

34
Unfortunate voyage and death of Messrs Rozier and Romaine.

35
Voyage of Messrs Blanchard and Jeffries across the Straits of Dover.

many of his observations; but the few following are some of the most important and curious. "The sensation of ascending is compared to that of a strong pressure from the bottom of the car upwards against the soles of his feet. At the distance of what appeared to him seven miles from the earth, though by the barometer scarcely a mile and a half, he had a grand and most enchanting view of the city of Chester and its adjacent places below. The river Dee appeared of a red colour; the city very diminutive; and the town entirely blue. The whole appeared a perfect plain, the highest building having no apparent height, but reduced all to the same level, and the whole terrestrial prospect appeared like a coloured map. Just after his first ascent, being in a well-watered and maritime part of the country, he observed a remarkable and regular tendency of the balloon towards the sea; but shortly after rising into another current of air, he escaped the danger: this upper current, he says, was visible to him at the time of his ascent, by a lofty found stratum of clouds flying in a safe direction. The perspective appearance of things to him was very remarkable. The lowest bed of vapour that first appeared as cloud was pure white, in detached fleeces, increasing as they rose: they presently coalesced, and formed, as he expresses it, a sea of cotton, tufting here and there by the action of the air in the undisturbed part of the clouds. The whole became an extended white floor of cloud, the upper surface being smooth and even. Above this white floor he observed, at great and unequal distances, a vast assemblage of thunder-clouds, each parcel consisting of whole acres in the densest form: he compares their form and appearance to the smoke of pieces of ordnance, which had consolidated as it were into masses of snow, and penetrated through the upper surface or white floor of common clouds, there remaining visible and at rest. Some clouds had motions in flow and various directions, forming an appearance truly stupendous and majestic. He endeavours to convey some idea of the scene by a figure; (and from which fig. 13. of 2d Plate II. is copied). A represents a circular view he had from the car of the balloon, himself being over the centre of the view, looking down on the white floor of clouds and seeing the city of Chester through an opening, which discovered the landscape below, limited by surrounding vapour, to less than two miles in diameter. The breadth of the outer margin defines his apparent height in the balloon (viz. 4 miles) above the white floor of clouds. Mr Baldwin also gives a curious description of his tracing the shadow of the balloon over tops of volumes of clouds. At first it was small, in size and shape like an egg; but soon increased to the magnitude of the sun's disc, still growing larger, and attended with a most captivating appearance of an iris encircling the whole shadow at some distance round it, the colours of which were remarkably brilliant. The regions did not feel colder, but rather warmer, than below. The sun was hottest to him when the balloon was stationary. The discharge of a cannon when the balloon was at a considerable height, was distinctly heard by the aeronaut; and a discharge from the same piece, when at the height of 30 yards, so disturbed him as to oblige him for safety to lay hold firmly of the cords of the balloon. At a considerable

height he poured down a pint-bottle full of water; and as the air did not oppose a resistance sufficient to break the steam into small drops, it mostly fell down in large drops. In the course of the balloon's tract it was found much affected by the water (a circumstance observed in former aerial voyages). At one time the direction of the balloon kept continually over the water, going directly towards the sea, so much as to endanger the aeronaut; the mouth of the balloon was opened, and he in two minutes descended into an under current blowing from the sea: he kept descending, and landed at Bellair farm in Rinsley, 12 miles from Chester. Here he lightened his car by 31 pounds, and instantly reascending, was carried into the interior part of the country, performing a number of different manoeuvres. At his greatest altitude he found his respiration free and easy. Several bladders which he had along with him crackled and expanded very considerably. Clouds and land, as before, appeared on the same level. By way of experiment, he tried the upper valve two or three times, the neck of the balloon being close; and remarked, that the escape of the gas was attended with a growling noise like millstones, but not near so loud. Again, round the shadow of the balloon, on the clouds he observed the iris. A variety of other circumstances and appearances he met with, is fancifully described; and at 53 minutes past three he finally landed.

The frequency of aerial voyages, accompanied with particular details of trifling and uninteresting circumstances, and apparently made with a view to promote the interest of particular persons, regardless of any advancement in knowledge, have now sunk the science of aerostation so low in the opinion of most people, that before giving any account of the most proper method of constructing these machines, it may seem necessary to premise something concerning the uses to which they may possibly be applied. These, according to Mr Cavallo, are the following.

"The small balloons, especially those made of paper, and raised by means of spirit of wine, may serve to explore the direction of the winds in the upper regions of the atmosphere, particularly when there is a calm below: they may serve for signals in various circumstances, in which no other means can be used; and letters or other small things may be easily sent by them, as for instance from ships that cannot safely land on account of storms, from besieged places, islands, or the like. The larger aerostatic machines may answer all the abovementioned purposes in a better manner; and they may, besides, be used as a help to a person who wants to ascend a mountain, a precipice, or to cross a river; and perhaps one of those machines tied to a boat by a long rope, may be, in some cases, a better sort of sail than any that is used at present. The largest sort of machines, which can take up one or more men, may evidently be subservient to various economical and philosophical purposes. Their conveying people from place to place with great swiftness, and without trouble, may be of essential use, even if the art of guiding them in a direction different from that of the wind should never be discovered. By means of those machines the shape of certain seas and lands may be better ascertained; men may ascend to the tops of mountains they never visited before; they may be carried over marshy and

and dangerous grounds; they may by that means come out of a besieged place, or an island; and they may, in hot climates, ascend to a cold region of the atmosphere, either to refresh themselves, or to observe the ice, which is never seen below; and, in short, they may be thus taken to several places, to which human art hitherto knew of no conveyance.

"The philosophical uses, to which these machines may be subservient, are numerous indeed; and it may be sufficient to say, that hardly any thing which passes in the atmosphere is known with precision, and that principally for want of a method of ascending into it. The formation of rain, of thunder-storms, of vapours, hail, snow, and meteors in general, require to be attentively examined and ascertained. The action of the barometer, the refraction and temperature of the air in various regions, the descent of bodies, the propagation of sound, &c. are subjects which all require a series of observations and experiments, the performance of which could never have been properly expected before the discovery of aerostatic machines."

To those uses we may add the gratification of curiosity and pleasure as a very strong inducement to the practice of an art, in which, with any tolerable degree of caution, there appears not to be the smallest danger. Every one who has tried the experiment testifies, that the beauty of the prospect afforded by an ascent, or the pleasure of being conveyed through the atmosphere, cannot be exceeded. No one has felt the least of that giddiness consequent upon looking from the top of a very high building or of a precipice, nor have they any of the sickness arising from the motion of a vessel at sea. Many have been carried by balloons at the rate of 30, 40, or even 50 miles an hour, without feeling the least inconvenience, or even agitation of the wind; the reason of which is, that as the machine moves with nearly the velocity of the wind itself, they are always in a calm, and without uneasiness. Some have apprehended danger from the electricity of the atmosphere; and have thought that a stroke of lightning, or the smallest electric spark, happening near a balloon, might set fire to the inflammable air, and destroy both the machine and the adventurers. Mr Cavallo has suggested several considerations for diminishing apprehensions of this kind. Balloons have been already raised in every season of the year, and even when thunder has been heard, without injury. In case of danger, the aeronauts may either descend to the earth, or ascend above the region of the clouds and thunder-storms. Besides, as balloons are formed of materials that are not conductors of electricity, they are not likely to receive strokes, especially as by being encompassed with air they stand insulated. Moreover, inflammable air by itself, or unmixed with a certain quantity of common air, will not burn; so that if an electric spark should happen to pass through the balloon, it would not set fire to the inflammable air, unless a hole was made in the covering.

The general principles of acrolation are so little different from those of hydrostatics, that it may seem superfluous to insist much upon them. It is a fact universally known, that when a body is immersed in any fluid, if its weight be less than an equal bulk of that fluid, it will rise to the surface; but if heavier, it will sink; and if equal, it will remain in the place where it

is left. For this reason smoke ascends into the atmosphere, and heated air in that which is colder. The ascent of the latter is shown in a very easy and satisfactory manner by bringing a red-hot iron under one of the scales of a balance, by which the latter is instantly made to ascend; for, as soon as the red-hot iron is brought under the scale, the hot air being lighter than that which is colder, ascends, and strikes the bottom, which is thus impelled upwards, and the opposite scale descends, as if a weight had been put into it.

Upon this simple principle depends the whole theory of aerostation; for it is the same thing whether we render the air lighter by introducing a quantity of heat into it, or inclosing a quantity of gas specifically lighter than the common atmosphere in a certain space; both will ascend, and for the same reason. A cubic foot of air, by the most accurate experiments, has been found to weigh about 554 grains, and to be expanded by every degree of heat, marked on Fahrenheit's thermometer, about $\backslash frac\{1\}\{257\}$ part of the whole. By heating a quantity of air, therefore, to 500 degrees of Fahrenheit, we will just double its bulk when the thermometer stands at 54 in the open air, and in the same proportion we will diminish its weight; and if such a quantity of this hot air be inclosed in a bag, that the excess of the weight of an equal bulk of common air weighs more than the bag with the air contained in it, both the bag and air will rise into the atmosphere, and continue to do so until they arrive at a place where the external air is naturally so much rarefied that the weight becomes equal; and here the whole will float.

The power of hot air in raising weights, or rather that by which it is itself impelled upwards, may be shown in the following manner: Roll up a sheet of paper into a conical form, and, by thrusting a pin into it near the apex, prevent it from unrolling. Fasten it then, by its apex, under one of the scales of a balance by means of a thread, and, having properly counterpoised it by weights, put it into the opposite scale; apply the flame of a candle underneath, you will instantly perceive the cone to arise, and it will not be brought into equilibrium with the other but by a much greater weight than those who have never seen the experiment would believe. If we try this experiment with more accuracy, by getting proper receptacles made which contain determinate quantities of air, we will find that the power of the heat depends much more on the capacity of the bag which contains it than could well be supposed. Thus, let a cubical receptacle be made of a small wooden frame covered with paper capable of containing one foot of air, and let the power of a candle be tried with this as above directed for the paper cone. It will then be found that a certain weight may be raised; but a much greater one will be raised by having a receptacle of the same kind which contains two cubic feet; a still greater by one of three feet; a yet greater by one of four feet, &c. and this even though the very same candle be made use of; nor is it known to what extent even the power of this small flame might be carried.

From these experiments it appears, that in the aerostatic machines constructed on Montgolfier's plan, it must be an advantage to have them as large as possible, because possible,

because a smaller quantity of fire will then have a greater effect in raising them, and the danger from that element, which in this kind of machines is chiefly to be dreaded, will be in a great measure avoided. On this subject it may be remarked, that as the cubical contents of a globe, or any other figure of which balloons are made, increase much more rapidly than their surface, there must ultimately be a degree of magnitude at which the smallest imaginable heat would raise any weight whatever. Thus, supposing any aerostatic machine capable of containing 500 cubic feet, and the air within it to be only one degree hotter than the external atmosphere; the tendency of this machine to rise, even without the application of artificial heat, would be near an ounce. Let its capacity be increased 16 times; and the tendency to arise will be equivalent to a pound, though this may be done without making the machine 16 times heavier than before. It is certain, however, that all aerostatic machines have a tendency to produce or preserve heat within them, which would by no means be imagined by those who have not made the experiment. When Messrs Charles and Roberts made their longest aerial voyage of 150 miles, they had the curiosity to try the temperature of the air within their balloon, in comparison with that of the external atmosphere; and at this time they found, that, when the external atmosphere was 63°, the thermometer within the balloon stood at 104°. Such a difference of temperature must have given a machine of the magnitude which carried them a considerable ascending power independent of any other cause, as it amounted to 41 grains on every cubic foot; and therefore in a machine containing 50,000 such feet would have been almost 200 pounds. Hence we may easily account for what happened at Dijon, and is recorded by Mr Morreau. "A balloon, intended to be filled with inflammable air, being completed, was, by way of trial, filled with common air, and in that state exposed to the atmosphere. Now it was observed, and indeed a similar observation had been made before, that the air within the balloon was much hotter than the circumambient air: the thermometer in the former stood at 120°; whereas in the latter, even when the sun shone upon it, the thermometer stood at 84°. This showed a considerable degree of rarefaction within the balloon; and consequently it was suspected, that, by means of this rarefaction alone, especially if it were to increase a little, the balloon might ascend. On the 30th of May, about noon, the wind being rather strong, agitated the balloon so that two men were employed to take care of it; but, notwithstanding all their endeavours, it escaped from its confinement, and, lifting up about 65 pounds weight of cords, equatorial circle, &c. rose many feet high, and, passing over some houses, went to the distance of 250 yards, where at length it was properly secured."

This difference between the external and internal heat being so very considerable, must have a great influence upon aerostatic machines, and will undoubtedly influence those filled with inflammable air as well as the other kind. Nor is it unlikely, that the short time which many aerial voyagers have been able to continue in the atmosphere, may have been owing to the want of a method of preserving this internal heat. It may naturally be supposed, and indeed it has always been

found, that balloons, in passing through the higher regions of the atmosphere, acquire a very considerable quantity of moisture, not only from the rain or snow they sometimes meet with, but even from the dew and vapour which condenses upon them. On this an evaporation will instantly take place; and, as it is the property of this operation to produce a very violent cold, the internal heat of the balloon must be soon exhausted in such a manner as to make it become specifically heavier than the common atmosphere, and consequently descend in a much shorter time than it would have done by the mere loss of air. To this, in all probability, we are to ascribe the descent of the balloon which carried Messrs Blanchard and Jeffries; and which seemed so extraordinary to many people, that they were obliged to have recourse to an imaginary attraction in order to solve the phenomenon. This supposition is rejected by Mr Cavallo; who explains the matter, by remarking, that in two former voyages made with the same machine, it could not long support two men in the atmosphere; so that we had no occasion to wonder at its weakness on this occasion. "As for its rising higher (says he), just when it got over the land, that may be easily accounted for. In the first place, the two travellers threw out their clothes just about that time; secondly, in consequence of the wind's then increasing, the balloon travelled at a much greater rate than it had done whilst over the sea; which increase of velocity lessened its tendency to descend: besides which, the vicissitudes of heat and cold may produce a very considerable effect; for if we suppose, that the air over the land was colder than that over the sea, the balloon coming into the latter from the former, continued to be hotter than the circumambient air for some time after; and consequently, it was comparatively much lighter when in the cold air over the land, than when in the hotter air over the sea; hence it floated easier in the former than in the latter case."

It seems indeed very probable, that there was something uncommon in the case of Mr Blanchard's balloon while passing over the sea; for, as it rose higher after reaching the land than in any former period of the voyage, and likewise carried them to the distance over land more than half of that which they had passed over water, we can scarcely avoid supposing, that it had a tendency to descend when over the water more than when over land, independent of any loss of air. Now, it does not appear that the air over the sea is at all warmer than that above land; on the contrary, there is every reason to believe, that the superior reflective power of the land renders the atmosphere above it warmer than the sea can do: but it is very natural to suppose, that the air above the sea is more moist than that above land; and consequently, by letting fall its moisture upon the balloon, must have occasioned an evaporation that would deprive the machine of its internal heat, which it would partly recover after it entered the warmer and drier atmosphere over land.

We shall now proceed to the construction of aerostatic machines; of which the smaller are only for amusement, or some slight experiments, and are very easily made. As in all of them, however, it is of the utmost consequence to have the weight as little as possible, the shape becomes an object of great consideration.

45
Of their
shape.

tion. For this purpose a spherical figure has been mathematically demonstrated to be the best; as capable of containing a greater quantity under a smaller surface than any other. Thus a perfect sphere contains less surface in proportion to its solidity than a spheroid; a spheroid less than a cylinder; the latter less than a cube; and a cube still less than a parallelopiped. In all cases, therefore, where we can fill the whole capacity of the balloon with air equally light, the spherical figure is undoubtedly to be preferred; and this holds good with regard to all inflammable air-balloons, whether their size be great or small; but in the rarefied air ones, where the under part must necessarily be much colder than the upper, the globular shape seems not so proper. An inverted cone, or truncated pyramid, with the smaller part underneath, seems then to be most proper, as it allows the heated air (which has a great tendency to expand as well as to ascend) to collect in the wide part at the top, while the useless surface in the lower part, and which, in any other figure, would contain only the colder and heavier air, is thus thrown aside. In fact it has been found, that aerostatic machines, raised by means of rarefied air, when made of the shape of a parallelopiped, or even one deviating still more from the shape of a globe, have answered the purpose as well as they could have been supposed to do, had ever so much care been taken in forming them exactly to that shape. The very first machine made by Mr. Montgolfier was in form of a parallelopiped; and though it contained only 40 cubic feet, showed a very considerable power of ascent. A very large one, 74 feet high, which Mr. Montgolfier had designed to exhibit before the royal family, had the middle part of it prismatic for about the height of 25 feet; its top was a pyramid of 29 feet; and its lower part was a truncated cone of near 20 feet. It weighed 1000 pounds; and, notwithstanding its shape, in a very short time manifested a power of ascent equal to 500 pounds. Another aerostatic machine of a small size, but of the figure of a parallelopiped, being suffered to ascend with 30 sheets of oiled paper fixed in a wire frame, and set on fire, rose to a great height, and in 22 minutes could not be seen. It seems therefore, that, with regard to the shape of these machines, it is by no means necessary to adhere rigidly to that of a sphere; but that any oblong form answers very well.

46
Materials.

For experimental purposes, both the inflammable and rarefied air-balloons may be made of paper; the former being made of that kind called thin-pess, varnished over with linseed-oil; the latter either of that or any other kind, without varnish. In order to avoid the danger of burning, however, it has been proposed to impregnate the paper of which these small rarefied air-balloons are made with solution of sal-ammoniac, alum or some other salt; but this does not seem to be necessary. Those filled with inflammable air have been made of gold-heater skin or peeled bladders; but the cheaper material of paper is undoubtedly preferable.

47
Best varnish
for inflamm-
able-air
balloon, ac-
cording to
Mr de St
Fond.

For aerostatic machines of a larger size, the material universally employed is varnished silk; and for those of the rarefied-air kind, linen painted over with some fine colour, or lined with paper. The best varnish for an inflammable air-balloon is that made with bird-lime, and recommended by Mr Faujas de Saint-Fond, in a treatise published on the subject. The following is his

method of preparing it: "Take one pound of bird-lime, put it into a new proper earthen pot that can resist the fire, and let it boil gently for about one hour, viz. till it ceases to crackle; or, which is the same thing, till it is so far boiled, as that a drop of it being let fall upon the fire will burn: then pour upon it a pound of spirits of turpentine, stirring it at the same time with a wooden spatula, and keeping the pot at a good distance from the flame, lest the vapour of this essential oil should take fire. After this, let it boil for about six minutes longer; then pour upon the whole three pounds of boiling oil of nuts, linseed, or poppy, rendered drying by means of litharge; stir it well, let it boil for a quarter of an hour longer, and the varnish is made. After it has rested for 24 hours, and the sediment has gone to the bottom, decant it into another pot; and when you want to use it, warm, and apply it with a flat brush upon the silk stuff, whilst that is kept well stretched. One coat of it may be sufficient; but if two are necessary, it will be proper to give one on each side of the silk, and to let them dry in the open air while the silk remains extended."

48
Mr Cavallo gives the following method of preparing Mr Cavallo's method.
this varnish, which he prefers to that of M. de St Fond.—"In order to render linseed-oil drying, boil it with two ounces of saccharum faturni and three ounces of litharge, for every pint of oil, till the oil has dissolved them, which will be accomplished in half an hour; then put a pound of birdlime and half a pint of the drying oil into a pot (iron or copper pots are the safest for this purpose), the capacity of which may be equal to about one gallon, and let it boil very gently over a slow charcoal fire till the birdlime ceases to crackle, which will be in about half or three quarters of an hour; then pour upon it two pints and a half more of drying oil, and let it boil for one hour longer, stirring it very frequently with an iron or wooden spatula. As the varnish, whilst boiling, and especially when it is nearly done, swells very much, care should be had to remove, in those cases, the pot from the fire, and to replace it when the varnish subsides, otherwise it will boil over. Whilst the stuff is boiling, the operator should, from time to time, examine whether the varnish has boiled enough; which is thus known:—Take some of it upon the blade of a knife, and then, after rubbing the blade of another knife upon it, separate the knives; and when, on this separation, the varnish begins to form threads between the two, you may conclude that it is done; and, without losing time, it must be removed from the fire. When it is almost, though not quite, cold, add about an equal quantity of spirit of turpentine: mix it well together, and let it rest till the next day; when, having warmed it a little, strain and bottle it. If it is too thick, add some more spirit of turpentine. When this varnish is laid upon the silk, the stuff should be made perfectly dry, and stretched; so that the varnish, which ought to be used lukewarm, may fill up the pores of the stuff. The varnish should be laid once very thin upon one side of the stuff; and, about 12 hours after, two other coats of it should be laid on, one on each side; and, 24 hours after, the silk may be used, though, in cold weather, it may be left to dry some time longer."

Much has been said in France of their elastic gum-varnish,

varnish, and its composition kept a secret; but Mr Baldwin, after many expensive trials, declares to the world what he considers as the secret; and it is merely this: "Take any quantity of caoutchouc, as two ounces averdupois; cut it into small bits with a pair of scissors; put a strong iron ladle (like that used by plumbers) over a common pitcoal or other fire. The fire must be gentle, glowing, and without smoke. When the ladle is hot, much below a red heat, put a single bit into the ladle. If black smoke issues, it will presently flame and disappear, or it will evaporate without flame: the ladle is then too hot. When the ladle is less hot, put in a second bit, which will produce a white smoke. This white smoke will continue during the operation, and evaporate the caoutchouc: therefore no time is to be lost; but little bits are to be put in, a few at a time, till the whole are melted. It should be continually and gently stirred with an iron or brass spoon. Two pounds or one quart of the best drying oil (or of raw linseed-oil, which, together with a few drops of neats-foot oil, has stood a month, or not so long, on a lump of quicklime, to make it more or less drying), is to be put into the melted caoutchouc, and stirred till hot, and the whole poured into a glazed vessel, through a coarse gauze or fine sieve. When settled and clear, which will be in a few minutes, it will be fit for use either hot or cold." Mr Baldwin is not at liberty, he observes, to publish the art of laying on the varnish; but says, that it consists in making no intensive motion in the varnish, which would create minute bubbles; that therefore brushes are improper. Mr Blanchard's method of making elastic gum varnish for the silk of a balloon, is the following. "Dissolve elastic gum (caoutchouc) cut small in five times its weight of spirit of turpentine, by keeping them some days together; then boil one ounce of this solution in eight ounces of drying linseed-oil for a few minutes; lastly, strain it. It must be used warm." The pieces of silk for the balloon must be cut out of a proper size, according to the dimensions, after the varnish is sufficiently dry. They may be joined by laying about half an inch of the edge of one piece over the edge of the other, and sewing them by a double stitching. Mr Blanchard uses expeditiously the following method. He lays about half an inch of the edge of one piece flat over the edge of the other, and passes a hot iron over it; in doing which a piece of paper ought to be laid both under and over the silk. The joining may be rendered more secure by running it with a silk thread, and sticking a ribband over it. The ribbands laid over seams may be stuck with common glue, provided the varnish of the silk is properly dried. When the glue is quite dry, the ribbands should be varnished over, to prevent their being unglued by the rain.

The best method of cutting the pieces of silk that are to form a balloon, is to describe a pattern of wood or stiff card-paper, and then to cut the silk upon it. As the edges of such a pattern are not perfect circles, they cannot be described by a pair of compasses: but the method of drawing them is as follows. First, draw, on a flat surface two right lines AE and BC, perpendicular to each other. Secondly, find the circumference answering to the given diameter of the balloon in feet and decimals of a foot; and make AD and DE

each equal to a quarter of the circumference, so that the whole length AE of the pattern may be equal to half the circumference. Thirdly, divide AD into 18 equal parts; and to the points of division apply the lines fg, hi, kl, &c. parallel to each other, and perpendicular to AD. Fourthly, divide the whole circumference in twice the given number of pieces, and make DC and BB each equal to the quotient of this division; so that the whole, BC, is equal to the greatest breadth of one of these pieces. Fifthly, multiply the above-mentioned quotient by the decimals annexed to fg, viz. 0.99619, and then the product expresses the length of fg; again multiply the same length of DE by the decimals annexed to hi, and the product expresses the length of hi; and, in short, the product arising from the multiplication of the length of DC by the decimals annexed to each of the parallel lines, gives the length of that line. Lastly, having found the lengths of all these lines, draw by hand a curve-line passing through all the extremities of the said lines, and that is the edge of one quarter of the pattern. The other quarters may be easily described, by applying to them a piece of paper cut according to that already found.—Suppose, for example, that the diameter of the balloon to be constructed is 20 feet, and that it is required to make it of 12 pieces: then, in order to draw the pattern for those pieces, find the circumference of the balloon, which is 62.83 feet, and, dividing it by four, the quotient is 15.7 feet; make therefore AD equal to 15.7 feet, and DE likewise of the same length. Divide the circumference 62.83 by 24, which is double the number of pieces that are to form the balloon, and the quotient, 2.618 feet, is the length of DC and likewise of BD; so that BC is equal to 5.236 feet. Then, having divided the line AD into 18 equal parts, and having drawn the parallel lines from those points of division, find the length of each of those lines by multiplying 2.618 by the decimals annexed to that line. Thus, 2.618, multiplied by 0.99619, gives 2.608 feet for the length of fg; and again, multiplying 2.618 by 0.98481, gives 2.578 feet for the length of hi; and so of the rest.—In cutting the pieces after such a pattern, care should be taken to leave them about three quarters of an inch all round larger than the pattern, which will be taken up by the seams.

To the upper part of the balloon there should be adapted, and well fitted in, a valve opening inwards; to which should be fastened a string passing through a hole made in a small piece of round wood fixed in the lowest part of the balloon opposite to the valve, the end of this string fastened in the car below, so that the aeronaut may open the valve when occasion requires. The action of this valve may be understood from fig. 3. A round brass plate AB has a round hole CD, about two or three inches diameter, covered on both sides with strong smooth leather. On the inside there is a shutter E, also of brass, covered with leather, which is to close the hole CD; being about two inches larger in diameter than the hole. It is fastened to the leather of the plate AB; and by a spring, which need not be very strong, it is kept against the hole. The elasticity of the gas itself will help to keep it shut. To this shutter the string is fastened, by which it is occasionally opened for the escape of gas. A small firing

ACOUSTICS.

Fig. 1.

Fig. 3.

Fig. 2.

Fig. 4.

Fig. 12. Fig. 6.

AEROSTATION.

Fig. 13.

A view from a Balloon above the clouds, seen by Mr. Balloon.

Fig. 5.

Fig. 11.

Fig. 8.

Fig. 9.

Fig. 10.

Fig. 7.

A Ball of the same size.

firing or other security should be fixed to the shutter and the plate, so as not to admit the shutter to be opened beyond a certain safe distance. To the lower part of the balloon two pipes should be fixed, made of the same stuff as the envelope; 6 inches diameter for a balloon of 30 feet, and proportionally larger for balloons of a greater capacity. They must be long enough for the car. For balloons of 18 feet and less diameter, one neck or pipe will be sufficient. These pipes are the apertures through which the inflammable gas is introduced into the balloon.

The car or boat is best made of wicker-work, covered with leather, and well painted or varnished over; and the proper method of suspending it, is by ropes proceeding from the net which goes over the balloon. This net should be formed to the shape of the balloon, and fall down to the middle of it, with various cords proceeding from it to the circumference of a circle about two feet below the balloon; and from that circle other ropes should go to the edge of the boat. This circle may be made of wood, or of several pieces of slender cane bound together. The meshes of the net may be small at top, against which part of the balloon the inflammable air exerts the greatest force; and increase in size as they recede from the top. A hoop has sometimes been applied round the middle of the balloon to fasten the net. This, though not absolutely necessary, is best made of pieces of cane bound together, and covered with leather.

With regard to the rarefied-air machines, Mr Cavallo recommends first to soak the cloth in a solution of sal ammoniac and common size, using one pound of each to every gallon of water; and when the cloth is quite dry, to paint it over in the inside with some earthy colour, and strong size or glue. When this paint has dried perfectly, it will then be proper to varnish it with oily varnish, which might dry before it could penetrate quite through the cloth. Simple drying linseed oil will answer the purpose as well as any, provided it be not very fluid.

It now only remains to give some account of the method by which aerostatic machines may be filled with their proper gas, in order to give them their power of ascending into the atmosphere; and here we are enabled to determine with much greater precision concerning the inflammable-air balloons than the others. With regard to them, a primary consideration is, the most proper method of procuring the inflammable air. It may be obtained in various ways, as has been shown under the article AEROLGY: But the most advantageous methods are, by applying acids to certain metals; by exposing animal, vegetable, and some mineral substances, in a close vessel to a strong fire; or by transmitting the vapour of certain fluids through red-hot tubes.

1. In the first of these methods, iron, zinc, and vitriolic acid, are the materials most generally used. The vitriolic acid must be diluted by five or six parts of water. Iron may be expected to yield in the common way 1700 times its own bulk of gas; or one cubic foot of inflammable air to be produced by 4¹/₂ ounces of iron, the like weight of oil of vitriol, and 22¹/₂ ounces of water. Six ounces of zinc, an equal weight of oil of vitriol, and 30 ounces of water, are necessary for producing the same quantity of gas. It is more

proper to use the turnings or chippings of great pieces of iron, as of cannon, &c. than the filings of that metal, because the heat attending the effervescence will be diminished; and the diluted acid will pass more readily through the interstices of the turnings when they are heaped together, than through the filings, which stick closer to one another. The weight of the inflammable air thus obtained by means of acid of vitriol, is, in the common way of procuring it, generally one seventh part of the weight of common air; but with the necessary precautions for philosophical experiments, less than one-tenth of the weight of common air. Two other sorts of elastic fluids are sometimes generated with the inflammable air. These may be separated from it by passing the inflammable air through water in which quicklime has been dissolved. The water will absorb these fluids, cool the inflammable air, and prevent its over-heating the balloon when introduced into it.

Fig. 7. of 2d Plate II. represents an apparatus described by Mr Cavallo as proper for filling balloons of the size of two or three feet in diameter with inflammable air, after passing it through water.—A is the bottle with the ingredients; BCD a tube fastened in the neck at B, and passing through C, the cork of the other bottle, in which there is another hole made to receive the tube on which the balloon is tied. Thus it is plain, that the inflammable air coming out of the tube D will pass first through the water of the bottle E and then into the balloon. Two small casks may be used instead of the bottles A and E.

2. Inflammable air may be obtained at a much cheaper rate by the action of fire on various substances; but the gas which these yield is not so light as that produced by the effervescence of acids and metals. The substances proper to be used in this way are, pit-coal, asphaltum, amber, rock-oil, and other minerals; wood, and especially oak, camphor-oil, spirits of wine, ether, and animal substances, which yield air in different degrees, and of various specific gravities; but pit-coal is the preferable substance. A pound of this exposed to a red heat, yields about three cubic feet of inflammable air, which, whether it be passed through water or not, weighs about one-fourth of the weight of common air. Dr Priestley found, as we have elsewhere noticed, that animal or vegetable substances will yield six or seven times more inflammable air when the fire is suddenly increased than when it is gently raised, though it be afterwards made very strong. Mr Cavallo observes, that the various substances above enumerated generally yield all their inflammable air in about one hour's time. The general method is, to inclose the substances in iron or earthen vessels, and thus expose them to a strong fire sufficient to make the vessels red-hot: the inflammable air proceeding from the aperture of the vessel is received into a tube or refrigeratory, and, passing through the tube or worm, is at last collected in a balloon or other vessel. A gun-barrel has often been used for effays of this kind. The substance is put into it so as to fill six or eight inches of its lowest part, the remainder filled with dry sand; a tube, adapted to the mouth of the barrel, is brought into a basin of water under an inverted receiver; and the part of the barrel containing the substance being put into the fire and made red-hot, the inflammable air is collected

lected in the inverted receiver. As the gun-barrel cannot serve for producing a large quantity of inflammable air, Mr Cavallo recommends, as the most advantageous shape, the following contrivance:—Let the vessel be made of clay, or rather of iron, in the shape of a Florence flask, somewhat larger, and whose neck is longer and larger (See ABC, fig. 8.) Put the substance to be used into this vessel, so as to fill about four-fifths or less of its cavity AB. If the substance is of such a nature as to swell much by the action of the fire, lute a tube of brass, or first a brass and then a leaden tube, to the neck C of the vessel; and let the end D of the tube be shaped as in the figure, so that going into the interior of a tube HI, it may terminate under a sort of inverted vessel EF, to the upper aperture of which the balloon G is adapted. Things thus prepared, if the part AB of the vessel is put into the fire, and made red-hot, the inflammable air produced will come out of the tube CD, and passing through the water will at last enter into the balloon G. Previous to the operation, as a considerable quantity of common air remains in the inverted vessel EF, which it is more proper to expel, the vessel EF should have a stop-cock K, through which the common air may be sucked out, and the water ascend as high as the stop-cock. The dimensions of such an apparatus Mr Cavallo gives thus: Diameter of largest part of the vessel ABC seven inches, length of whole vessel 16 inches; diameter of its aperture one inch, diameter of the cavity of tube CD three-fourths of an inch; lower aperture of the vessel EF six inches, least height of the vessel EF 24 inches; its aperture F about two inches. The aperture of the vessel EF should be at least one foot below the surface of the water in HI. Care must be taken that the fire used in this process be at a sufficient distance, otherwise it may happen to fire the inflammable air which may escape out of the vessel EF.

3. The last method of obtaining inflammable air was lately discovered by Mr Lavoisier, and also by Dr Priestley. Mr Lavoisier made the steam of boiling water pass through the barrel of a gun, kept red-hot by burning coals. Dr Priestley uses, instead of the gun-barrel, a tube of red-hot brass, upon which the steam of water has no effect, and which he fills with the pieces of iron which are separated in the boring of cannon. By this method he obtains an inflammable air, the specific gravity of which is to that of common air as 1 to 13. In this method, not yet indeed reduced to general practice, a tube, about three quarters of an inch in diameter, and about three feet long, is filled with iron turnings; then the neck of a retort, or close boiler, is luted to one of its ends, and the worm of a refrigerator is adapted to its other extremity. The middle part of the tube is then surrounded with burning coals, so as to keep about one foot in length of it red-hot, and a fire is always made under the retort or boiler sufficient to make the water boil with vehemence. In this process a considerable quantity of inflammable air comes out of the worm of the refrigerator. It is said that iron yields one half more air by this means than by the action of vitriolic acid.

For filling large balloons, a greater apparatus is necessary; and the only materials that can, with any certainty of success, be employed for producing the proper gas, are, oil of vitriol, and iron filings or turnings.

It has indeed been recommended to use zinc instead of iron filings, because white vitriol, the salt produced by the union of the vitriolic acid and zinc, is much more valuable than the green sort produced by the union of the same acid with iron. But though this is undoubtedly the case, it will as certainly be found, upon trial, that the superior price of the zinc will be more than an equivalent for all the advantage that can be derived from the additional price of the white vitriol. For a balloon of 30 feet diameter, Mr Cavallo recommends 3900 pounds of iron turnings, as much oil of vitriol, and 19,500 pounds of water. These proportions, however, appear too great with respect to the acid and metal, and too little with respect to the water. Oil of vitriol will not exert its power upon iron unless it be diluted with five or six times its quantity of water; in which case, a much smaller quantity of both acid and metal will serve. Mr Lunardi, who from the number of his voyages had certainly much practical knowledge in aeronautics, filled his balloon at Edinburgh and Glasgow with about 2000 pounds of iron (the borings of cannon procured from Carron), as much vitriolic acid, and 12,000 pounds of water. The iron was placed in his vessels in layers, with straw between them, in order to increase the surface. His apparatus was not materially different from that of Mr Cavallo, represented bottom of Plate I. fig. 2. where AA are two tubes, about three feet in diameter and nearly two feet deep, inverted in large tubs BB filled with water. In the bottom of each of the inverted tubs a hole is made, and a tube E of tin adapted, which is about seven inches in diameter, and seven or eight long. To these tubes the silken ones of the balloon are to be tied. Round each of the tubs B, five, six, or more strong casks are placed; in the top of each two holes are made, and to one of these holes a tin tube is adapted, and so shaped, that, passing over the edge of the tub B, and through the water, it may terminate with its aperture under the inverted tub A. The other hole of these casks serves for the introduction of materials, and is stopped with a wooden plug. When the balloon is to be filled, put the net over it, and let it be suspended as shown by CDE; and having expelled all the common air from it, let the silken tubes be fastened round the tin ones EE; and the materials being put into the casks, the inflammable air, passing into the balloon, will soon distend, and render it capable of supporting itself; after which the rope GH may be slipped off. As the balloon continues to be filled, the net is adjusted properly round it; the cords that surround it are fastened to the hoop MN; then the boat IK being placed between the two sets of casks, is fastened to the hoop MN, and every thing that is required to be sent up, as ballast, instruments, &c. is placed in it. At last, when the balloon is little more than three quarters full, the silken tubes are separated from the tin ones of the inverted tubs, and their extremities being tied up, are placed in the boat. Lastly, the aeronauts being seated in the boat, the lateral ropes are slipped off, and the machine is abandoned to the air. (See Blanchard's balloon, Plate II.) This apparatus was at last reduced by Mr Lunardi to its utmost simplicity, by using only two large casks, and suffering the vapour to go into the balloon without passing through water. Thus his balloon was filled

in less than half an hour, when, before, it had required two hours at least. The sinking of his cases in the ground was also an additional convenience, as it created no confusion, and rendered the materials much more easily conveyed into them.

With regard to the rarefied-air balloons, the method of filling them is as follows. A scaffold ABCD, the breadth of which is at least two-thirds of the diameter of the machine, is elevated about six or eight feet above the ground. From the middle of it descends a well E, rising about two or three feet above it, and reaching to the ground, furnished with a door or two, through which the fire in the well is supplied with fuel. The well should be constructed of brick or of plastered wood, and its diameter should be somewhat less than that of the machine. On each side of the scaffold are erected two masts HI, KL, each of which has a pulley at the top, and rendered firm by means of ropes KG, KP, HP, HG. The machine to be filled is to be placed on the scaffold, with its neck round the aperture of the well. The rope passing over the pulleys of the two masts, serves, by pulling its two ends, to lift the balloon about 15 feet or more above the scaffold; and the rest of the machine is represented by the dotted lines in the figure MNO. The machine is kept steady, and held down, whilst filling, by ropes passing through loops or holes about its equator; and these ropes may easily be disengaged from the machine, by slipping them through the loops when it is able to sustain itself. The proper combustibles to be lighted in the well, are those which burn quick and clear, rather than such as produce much smoke; because it is hot air, and not smoke, that is required to be introduced into the machine. Small wood and straw have been found to be very fit for this purpose. Mr Cavallo observes, as the result of many experiments with small machines, that spirits of wine are upon the whole the best combustible; but its price may prevent its being used for large machines. As the current of hot air ascends, the machine will soon dilate, and lift itself above the scaffold and gallery which was covered by it. The passengers, fuel, instruments, &c. are then placed in the gallery. When the machine makes efforts to ascend, its aperture must be brought, by means of the ropes annexed to it, towards the side of the well a little above the scaffold; the fire-place is then suspended in it, the fire lighted in the grate, and the lateral ropes being slipped off the machine is abandoned to the air. (See Montgolfer's balloon, Plate II.) It has been determined by accurate experiments, that only one-third of the common air can be expelled from these large machines; and therefore the ascending power of the rarefied air in them can be estimated as only equal to half an ounce averdupoise for every cubic foot.

The conduct of balloons, when constructed, filled, and actually ascending in the atmosphere, is an object of great importance in the practice of aerostation. The method generally used for elevating or lowering the balloons with rarefied air, has been the increase or diminution of the fire; and this is entirely at the command of the aeronaut, as long as he has any fuel in the gallery. The inflammable-air balloons have been generally raised or lowered by diminishing the weight in the boat, or by letting out some of the gas through the valve: but the alternate escape of the air in de-

scending, and discharge of the ballast for ascending, will by degrees render the machine incapable of floating; for in the air it is impossible to supply the loss of ballast, and very difficult to supply that of inflammable air. These balloons will also rise or fall by means of the rarefaction or condensation of the enclosed air, occasioned by heat and cold. It has been proposed to aid a balloon in its alternate motion of ascent and descent, by annexing to it a vessel of common air, which might be condensed for lowering the machine, and rarefied again, by expelling part of it, for raising the machine: But a vessel adapted to this purpose must be very strong; and, after all, the assistance afforded by it would not be very considerable. M. Meunier, in order to attain this end, proposes to inclose one balloon filled with common air in another filled with inflammable air: as the balloon ascends, the inflammable air is dilated, and of course compresses the internal balloon containing the common air; and by diminishing its quantity, lessens its weight. If it should be necessary to supply this loss, he says it may be easily done by a pair of bellows fixed in the gallery. Others have proposed to annex a small machine with rarefied air to an inflammable-air balloon by ropes, at such a distance that the fire of the former might not affect the inflammable air of the latter: the whole apparatus, thus combined, of balloons formed on the two principles of heated and inflammable air, might be raised or lowered by merely increasing or diminishing the fire in the lower balloon.

Wings or oars are the only means of this sort that have been used with some success; and, as Mr Cavallo observes, they seem to be capable of considerable improvement. Although great effects are not to be expected from them, when the machine goes at a great rate, the best methods of moving those wings are by the human strength applied similarly to the oars of a waterman. They may be made in general of silk stretched between wires, tubes, or sticks; and when used, must be turned edgewise when they are moved in the direction in which the machine is intended to be impelled, but flat in the opposite direction. Fig. 9. 2d Plate II. is the representation of one of Mr Blanchard's wings. Fig. 10. is one of those used by Mr Lunardi, which consists of many silk shutters or valves, ABCD, DECF, &c. every one of which opens on one side only, viz. ADBC opens upon the line AB, DECF opens upon the line DC, &c. In consequence of this construction, this sort of oars do not need being turned edgewise. Fig. 11. represents one of the wings used by the brothers Roberts in the aerial voyage of the 19th September 1784; and fig. 12. represents one of the wings constructed by Count Zambeccari, which consists of a piece of silk stretched between two tin tubes set at an angle; but these wings are so contrived as to turn edgewise by themselves when they go on one direction. Other contrivances have been made to direct aerostatic machines, but they have mostly been invented to effect a power upon them as upon a ship. It appears, however, that they can have no effect when a machine is only moved by the wind alone, because the circumambient air is at rest in respect to the machine. The case is quite different with a vessel at sea, because the water on which it floats stands still whilst the vessel goes on; but it must be time and experience that can realize the expectations suggested by these contrivances.