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 thirty years, little more than the mere mechanical action of this fluid was discovered, with the existence of some anomalous and permanently elastic vapors, 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.

These 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 Ancient or homogeneous; its common operations to depend merely on its heat, cold, moisture, or dryness; and any effects air, 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 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 analyze with accuracy.

Though, in this view, air seems to be a kind of fink or common sewer, where all the poisonous effluvia arising from putrid and corrupted matters are deposited; manner it yet it has a wonderful facility of purifying itself, and purifies it one way or other of depositing those vapors 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... Of Air in general.

the rivers which run into it. Dr Watson, in his Chemical Essays, has given an account of some experiments made with a view to determine the quantity of 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, no. 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. Sulphurous acid, and metallic 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 housetops where lead and other metals are smelted, have the same pernicious qualities; informing 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 miasma, 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 fluegish nature, incapable of arising in the air, but attaching itself to the walls of houses, bedcloths, 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.

§ 2. Mechanical Properties of the Air.—In common with water, the air we breathe possesses gravity, and gravity 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 to that of water as 1 to 1000; Mercennius, 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 to that of water as 1 to 840, 872, 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 29½ 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 29½ inches, and the thermometer at 50.

By means of its gravity, the air presses with great 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 fats, 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 calendare. Of this power we have numerous proofs. Thus, a blown bladder being squeezed in the hand, we find the included air sensibly raised; 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 nisus 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 a 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. Defaguliers 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 Haukbee 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 acts 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 reaction; 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 atmospherical 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 condensed 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 155 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 depends 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 13,679th part of the space it would possess in vacuo. But if the same air be condensed by art, art, the space it will take up when most dilated, so that it possesses when condensed, will be, according to the same author's experiments, as 550,000 to 1.

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.

The elastic power of the air, then, is the second great source of the effects of this important fluid. Thus it infuses 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 infuses 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 expands and contracts, 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 diffusion of these aliments in the stomach and bowels, of air which is much promoted by it; and, in reality, all in general, 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 powders for dissolving all bodies. It is known that iron and copper readily dissolve and become rusty in air, unless well defended with oil. Boerhaave affirms, 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. Acosta 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 Purbeck 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 various necessary and important agent; the result of particular chemical processes depending on its presence or absence, on its effects of 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 afresh.

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 Falshun 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 bras. 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 ghooft, 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 affirms, 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 affirms, 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 subtile 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 suspected, that the briskness and sparkling of the effluvium 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 Montpelier, 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 fusible 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 fusible in water, but which, &c., 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 milakens; 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, 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.

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.

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 Crans physician to his Ruffian 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 referred, however, for Dr Priestley to 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.

Sect. III. Of Dephlogisticated Air.

§ 1. Discovery and Methods of procuring this Kind of Air.—Dephlogisticated air was first obtained by Dr Priestley on the 1st 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 whence phlogiston. On this supposition he extracted air from first extract-mercurius calcinatus per fæ, by exposing it to the focused of a burning-glass 12 inches in diameter; and, having repeated the experiment with red precipitate and minium, 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 teft 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 teft of nitrous air was applied to it.

This pure kind of air being discovered, the Doctor why named next proceeded to name it dephlogisticated, from his dephlogisticated opinion that common air, in the act of burning, abated, forbed phlogiston; of consequence, he supposed, that which afforded 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, flaked variety of lime, tobacco-pipe clay, flint, Muscovy tales, and even substances. glaas. 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 engaged the dephlogisticated air without applying any more heat than what was generated by the mixture.

While discoveries of this kind engaged Dr Priestley in this kind of England, Mr Scheele was employed in a similar manner of air discovery in Sweden; and had actually obtained the same kind of air, without knowing anything of what Dr Scheele 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 Wartire 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 and violent 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 Air, 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 How de to discover in what manner this fluid, so essentially ne-phlogisticatory to the support of animal life, is naturally produced in quantities sufficient for the great expense of naturally it throughout the whole world, by the breathing of animals, the support of fires, &c. This discovery, in fact, Sect. III.

Dephlogisticated air 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. "When air," says he*, "has been freely 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 environment."

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.

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 to 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."

At the time that Dr Priestley made these experiments, he supposed that the air was meliorated merely 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 some water to keep the plant alive, and the whole apparatus was set over-night in a room. Next morning the air was found to be 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 a half it was so far improved, that by the test of nitrous air it appeared considerably better than common air.

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 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 this method more than by the other. Some poisonous plants, as the hyoscyamus, lauro-cerasus, night-shade, the tobacco-plant, a triplex vulvaria, cicuta aquatica, and sabina, 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 deplogificated 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 deplogificated 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 Ingenhouz's 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 deplogificated 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 deplogificated 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 deplogificated air which the same plant yielded in two hours time in a fair day."—It does not appear, however, that plants yield deplogificated air by any kind of generation of that fluid, but only by filtrating the common air, which all plants absorb through their pores; the deplogificated 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 Ingenhouz's experiments, Sir Benjamin Thompson, in his 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 matter of the water, says he, is evidently of an animal nature; being nothing more than the algaflage of an animal, without any thing resembling tremella, or an animal 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 Ingenhouz 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 deplogificated 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 deplogificated air, without any kind of limitation.

Raw silk possesses a remarkable power of this kind. Deplogificated to determine it, Sir Benjamin introduced 30 grains of air producing this substance, previously washed in water, into a thin ced raw glass globe 4½ inches in diameter, having a cylindrical silk neck ¼ths 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 3½ 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 Dephlogisticated Air.

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 Ingenhouz; 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 incon siderable 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 reverting 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 five 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 peach 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 Ruffian 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 1¾ths 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 housetwort, 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 substances from 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,