AIR

AIR, (Encycl.) The general mass of fluid which goes by the name of air, however simple and homogeneous it may have been thought in former times, 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 analyze 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 these 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 into 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; and calculates, 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 continually flies off into the clouds, thus leaving 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 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 has been shown under AGRICULTURE, n° 5, is probably the food of plants. The phlogistic vapours which ascend along with the aqueous ones, 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, capilly,

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 mosettes, 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 those 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.

Mechanical Properties of the Air.—In common with water, the air we breathe possesses gravity, and consequently will perform every thing 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; Mersennus, as 1 to 1300, or 1 to 1356; Lana, as 1 to 640; and Galileo, only as 1 to 400. But 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, 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 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 surface. M. Pascal has computed the quantity of this 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 length 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 also, 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.

This property of the air was first ascertained by some experiments of lord Bacon, who, upon this principle, constructed his vitrum calendare, the first thermometer. 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.

On this property of elasticity the structure and office of the Air-Pump depend. Every particle of air always exerts this nifus 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 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 included air. So a bladder quite flaccid, containing only the smallest quantity of air, swells in the receiver and appears quite full. The same effect is also found, by 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

Air. with a view to discover, how long air brought to the greatest degree of expansion to which he could reduce it in his air-pump, would retain its spring; and could never 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 latter experiment, that the spring of the air may be so disturbed by a violent pressure, 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, it is obvious, has no dependence on its elasticity; but would be the same whether the air had such a property or not. But the air, being elastic, is necessarily affected by the pressure, which reduces it into such a space, as that the elasticity, which re-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 re-action; i. e. the gravity of the air which tends to compress it, and the elasticity of the air which endeavours to expand it, must be equal. Hence the elasticity increasing, or diminishing universally, as the density increases or diminishes, i. e. as the distance between the particles diminishes or increases, it is no matter whether the air be compressed and retained in such 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, so as to cut off all communication with the external air, the pressure of the inclosed air will be equal to the weight of the atmosphere. 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 which, 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 absolutely 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 9 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 10000, and even at last into 13679 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 13679th part of the space it would possess in vacuo. But if the same air be condensed by 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 550000 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, that air 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."

According to the experiments of this author, and M. de la Hire, a column of air on the surface of the earth 36 fathoms high, is equal in weight to three lines depth of mercury; and it is found, that equal quantities of air possess spaces reciprocally proportional to the weights with which they are pressed: the weight of the air, therefore, which would fill the whole space possessed by the terrestrial globe, would be equal to a cylinder of mercury, whose base is equal to the surface of the earth, and its height containing as many times three lines as the atmospheric space contains orbs equal in weight to 36 fathoms of that wherein the experiment was made.—Hence, taking the densest of all bodies, e. g. gold, whose gravity is about 14630 times greater than that of air in our orb, it is easy to compute that this air would be reduced to the same density as gold, by the pressure of a column of mercury 14630 times 28 inches high, i. e. 409640 inches: since the bulks of air in that case would be in the reciprocal ratio of the weights by which they are pressed. These 409640 inches, therefore, express the height at which the barometer must stand, where the

Air. air would be as heavy as gold, and the number 245528 lines, the thickness to which our column of 36 fathoms of air would be reduced in the same place. Now, we know that 43528 fathoms, which is the depth where the above pressure and consequent reduction take place, are only the 74th part of the semi-diameter of the earth; and therefore, beyond that depth, whatever matter exists, it must be heavier than gold. It is not improbable, therefore, that the remaining sphere of 645528 fathoms diameter may be full of dense air, heavier by many degrees than the heaviest bodies which we know. Hence again, as it is proved, the more air is compressed, the more does the same degree of fire increase the force of its spring, and render it capable of a proportionably greater effect; we may infer, that a degree of heat, which in our orb can only produce a moderate effect, may have a very violent one in such lower orb; and that, as there may be many degrees of heat in nature beyond that of boiling-water, it is probable there may be some whose violence thus assisted by the weight of the air, may be sufficient to tear asunder the solid globe. See the article EARTHQUAKE.

The elastic power of the air is the second great source of the effects of this important fluid. By means of this it insinuates into the pores of bodies possessing this prodigious faculty of expanding, which is so easily excited that 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 reciprocity in several instances, particularly in plants, the trachea or air-vessels of which do the office of lungs; for the contained air alternately expanding and contracting, as the heat increases or diminishes, by turns presses the vessels and eases them again, and thus promotes a circulation of 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; and thus glasses and other vessels frequently crack when their contained liquors are frozen. Thus also, entire columns of marble sometimes cleave in the winter time, from some little bubble of included air's acquiring an increased elasticity. 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

dissolution of these elements in the stomach and bowels, which is much promoted by it.

In reality, all natural corruption and alteration seem to depend on air; and metals, particularly gold, only seem to be durable and incorruptible, in virtue of their not being pervious to air.

Air, effects of the different ingredients of it.—Air not only acts by its common properties of gravity and elasticity, but there are 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 verdigrease 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. Acoffa 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. The reason may be, that sea-salt, which is the only menstruum capable of acting on gold, being very difficult to volatilize, there is but a small proportion of it in the atmosphere. In the laboratories of chemists, where aqua regia is prepared, the air becoming impregnated with an unusual quantity of this salt, gold contracts a rust like other bodies.

Stones also undergo the changes incident to metals. Thus Porbeck 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. Air, likewise, by means of its dissolving power, accelerates evaporation and distillation.

2. Air volatilizes fixed bodies. Thus, sea-salt being first calcined, then fused by the fire, and when fused exposed to the air to liquefy; when liquefied set to dry, and then fused again, repeating the operation; will by degrees be almost wholly evaporated, nothing but a little earth remaining. Helmont mentions it as an arcanum in chemistry, to render fixed salt of tartar volatile; but this is easily effected by air alone; for,

Air. if some of this salt be exposed to the air, in a place replete with acid vapours, the salt draws the acid to itself, and, when saturated with it, is volatile.

3. Air also fixes volatile bodies. Thus, though spirit of nitre or aqua fortis readily evaporate by the fire, yet if there be any putrefied urine near the place, the volatile spirit will be fixed, and fall down in form of aqua secunda.

4. Air brings many quiescent bodies into action; i. e. excites their latent powers. Thus, if an acid vapour be diffused through the air, all the bodies of which that is the proper menstruum, being dissolved by it, are brought into a state proper for action.

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 campanum, 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 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, (Encycl.)

Different kinds of AIR. The late numerous discoveries and improvements relating to this subject 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 those discoveries, 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. Although, therefore, the different kinds of air have already been treated of at some length in various parts of this work, yet to render a subject so useful in itself, and so frequently agitated in conversation, more generally intelligible, a short recapitulation, with necessary additions, shall here be given.

1. Fixed or Fixable AIR, or Aerial Acid, is perhaps the oldest permanently elastic fluid different from common air that has been known to mankind. Its pernicious qualities of suffocating animals and extinguishing flame indeed, are those by which it chiefly, if not entirely manifested itself for a long time, nor have its good properties been discovered till very lately. By the miners, who too frequently feel the fatal effects of it, this kind of air is called the choke-damp. It is found, however, not only in mines, but in various caverns, wells, and cellars, or other places which have not been ventilated for a long time; and it is also found in various mineral waters from whence it is continually escaping; so that at these springs there is generally a superincumbent body of fixed air of a considerable depth. It has often happened, that people who for the sake of heat having gone to sleep near brick-kilns, have been found dead; owing, no doubt, to the fixed air and other noxious vapours exhaling from the bricks during the time of burning. The grotto del Cani in Italy has long been famous for the stratum of fixed air which lies at the bottom of it, which suffocates small animals whose heads cannot reach above it, or even the largest if their heads are held down in it for a very short time. In some places where this kind of air is naturally produced, its generation continues only for a time, and then ceases. In others it is continual; but, in general, in all places where this air is found naturally, it is mixed with a considerable quantity of common air. Hence it sometimes extinguishes flame, where it is not fatal to animals; and frequently some other fluids, as inflammable and phlogisticated air are mixed with it.

Fixed air is artificially produced in three different ways; viz. by fermentation, by heat, and by acids. 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 Holes obtained 639 cubic inches of air in 13 days. From a quantity of sugar 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 dur-

Air. ring 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 vapid 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 vial (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 seems (says Dr Priestley) to depend in some measure upon the time and other circumstances in the dissolution of animal or vegetable substances, whether they yield the proper putrid effluvium, or fixed, or inflammable air."

The elastic fluid produced by putrefying 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 a whole animal, or than the liver, &c. The proportion of inflammable and of fixed air is also various, according to the various parts employed.

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 always considerably diminished. 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 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

Air. 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 thro' water; and that when the experiment was made with vessels standing in silver 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 we have 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 calces 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.

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 vial 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 vial, that may hold about four or five ounces measure. 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 vial, 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 bottle. 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 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

flow

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.

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.

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 incorrupt 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 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."

II. Inflammable Air is produced, like fixed air, both naturally and artificially. It is yielded abundantly by putrid animal and vegetable substances, and in general by those inflammable bodies which part easily with their phlogiston. Hence, in caverns and mines, especially in coal-mines, this fluid is often found. Sometimes, when the miners break a piece of the mineral, thereby probably disclosing some hidden cavity, the inflammable air rushes out in great quantity. Being specifically lighter than common air, it always ascends, and escapes through fissures, except in those places which

which are closely vaulted. Being generally mixed with a considerable quantity of common air, it is less noxious than fixed air; so that the miners breathe it with impunity; though, by reason of its inflammable property, it often produces terrible explosions; from whence the miners have called it the fire-damp.

Inflammable air is often produced in ditches, exhal-
ing from the surface of putrid water; in burying-
places and houses of office, where putrefying animal
or vegetable matters are collected. It may also be ex-
tracted from the waters of moist lakes and rivers where-
in great quantities of fermentable and putrefying mat-
ters are thrown, as in the Thames, Severn, &c. In
warm climates this kind of air is produced in much
greater quantities, as the processes of fermentation and
putrefaction there go on much more rapidly than in
colder countries. So rapid is the production of this
kind of air indeed in hot climates, that if the mud at
the bottom of a pond is well stirred, and a lighted
candle immediately afterwards brought near to the
surface of the water, the inflammable air will take fire,
and a flame instantly spread over the surface of the
pond, affording a very curious spectacle in the night-
time. Inflammable and fixed air, as also phlogisticated
air, are discharged by our aliment in the natural
course of digestion, and afterwards discharged from
the intestines.

There is scarce any substance from which, by one
process or other, inflammable air may not be obtained.
But to the inflammable elastic fluids thus obtained,
perhaps no other properties in common can be as-
cribed than those of being inflammable, and being spe-
cifically lighter than common air. In other respects
they show a material difference between each other,
both in weight, smell, power of burning, of prefer-
ring their properties, and the phenomena attending
their combustion. Hence various species of this fluid
might be enumerated. As yet, however, there have
not been a sufficient number of observations to enable
us to make these distinctions properly. We shall
therefore only remark the difference between a real
inflammable gas, and that which is evidently formed
by mixing a volatile inflammable fluid with the com-
mon atmosphere. A drop of ether put into a quan-
tity of common air, diffuses itself by evaporation
through the whole, taking fire at the approach of a
candle like true inflammable air. In like manner Mr
Cigna observes, that air saturated with volatile alkali
is inflammable; and Dr Priestley has found, that al-
kaline air is itself weakly inflammable.

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 filings, or grossly powdered
zinc, must be used instead of chalk; to which filings
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 filings of iron, small nails, or
small bits of iron-wire, answer equally well.

The inflammable elastic fluid produced by this mix-
ture, has a disagreeable 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 particu-
larly offensive.

When a bottle has been filled with this elastic fluid, Cavallo en Air.
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 con-
tained in the bottle will be immediately inflamed; and
if the capacity of the bottle is nearly equal to four
ounces measure, 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 bot-
tle, in proportion as the inflammable gas is consumed.

In this experiment we see, that inflammable air fol-
lows the general rule of all other combustible sub-
stances, 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
to let it burn instantly, and go off with an explosion,
caused by the sudden rarefaction of the air. In fact,
this effect may be easily observed in the following man-
ner: 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 now is 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 ex-
plodes all at once with a large flame and a consider-
able 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
lead 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 blad-
der, and the bladder is filled with inflammable air,
after the manner described in the preceding experi-
ment (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 in-
flammable 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.

In stagnant waters where inflammable air is naturally produced, it may, especially in the hot months, be caught in great plenty in the following manner. 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 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 may be taken home, in order to examine the contained inflammable fluid at leisure.

Heat alone is capable to extract a good deal of inflammable air out of most inflammable substances, even from some of the metals. Dr Hales obtained inflammable air by simply distilling wax, pitch, pease, amber, coals, and oyster-shells. Mr Fontana says he extracted a considerable quantity of inflammable air from spathose iron, by the action of fire only, applied to a matras. But Dr Priestley has obtained it from a vast many more substances. His method is, to introduce the required substance into a gun-barrel, to the extremity of which a tube of glass, or of a tobacco-pipe, was luted, and a flaccid bladder was tied to the end of this tube, which received the elastic fluid produced, when the gun-barrel was put into the fire. The Doctor observes, that in order to get a considerable quantity of inflammable air from various substances, the heat must be applied suddenly. "For," says he, "notwithstanding the same care be taken in luting, and in every other respect, six 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 about a sheep's bladder full of inflammable air with a brisk heat, when it will only give about two or three ounce measures, if the same heat be applied to it very gradually. To what this difference is owing, I cannot tell. Perhaps the phlogiston, being extricated more slowly, may not be entirely expelled, but form another kind of union with its base." When the Doctor wanted to extract inflammable air from metals by means of heat only, he directed the focus of a lens upon them, whilst they were under the exhausted receiver of the air-pump, or were confined by quicksilver.

From pure filings of iron, and of steel carefully sorted with a magnet, and confined by quicksilver in an

inverted glass-jar, Dr Priestley, by means of the focus of a lens, obtained some permanently elastic fluid, which, as he observes, was weakly inflammable. In sorting the steel-filings for this experiment, the Doctor observes, that care must be taken not to let any other substance be mixed with them, since the least bit of wood, or any vegetable or animal matter, hardly discernible by the eye, will yield more inflammable air than a considerable quantity of iron-filings. "I found (says he) at this time, that eleven grains of steel-filings, from watch-springs, yielded $\backslash frac\{1\}\{2\}$ more inflammable air, than the same weight of iron-filings; which agrees with the hypothesis that steel contains more phlogiston than iron."

By directing the focus of the lens upon filings of zinc or upon brass-dust in vacuo, the Doctor obtained an elastic fluid that was strongly inflammable. But from tin he obtained an elastic fluid that was weakly inflammable. With other metallic substances, or with the calx of any metal whatever, the Doctor had no success.

From a mixture of iron-filings and chalk, the Doctor obtained, by means of the lens, a plentiful mixture of fixed and inflammable air, as might be naturally expected.

The elastic fluid expelled from charcoal by heat only, is fixed air; but the residuum of it Dr Priestley found to be inflammable. Mr Fontana had a very elegant way to extract inflammable air from red-hot charcoal. With a pair of tongs he took a piece of charcoal, when thoroughly ignited, and plunging it in water, brought it instantly under a large-mouthed receiver, which was filled with and inverted in the water, in order to collect the bubbles of inflammable air that proceeded from the charcoal in the act of cooling. After the same manner he treated several pieces of charcoal, or the same piece several successive times, until he got a quantity of inflammable air sufficient for his purpose.

By means of heat in a gun-barrel, Dr Priestley obtained from pipe-clay, first inflammable, and then fixed air. But a quantity of inflammable air is obtained, together with fixed air, in various cases, as we have already remarked.

Pit-coal, by distillation, yields inflammable air, which, when fired in a wide-mouthed phial, burns with a bright lambent flame, without explosion.

By exposing to heat, in a gun-barrel, a mixture of calx of zinc and charcoal, or Prussian blue, Mr de Lassone obtained an inflammable air, which burned without explosion.

Dr Priestley found, that when iron-filings and brimstone, moistened with water, were confined by quicksilver, and suffered to ferment in a warm place, some inflammable air was generated. Filings of zinc and brimstone produced the same effect.

By taking electric sparks in any kind of oil, spirit of wine, ether, or spirit of sal ammoniac, Dr Priestley obtained inflammable air. 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. air should be attributed to the cement which fastened 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 eleven grains of iron yielded 8½ ounces 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 quan-

tity, 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 Ingenhouz) 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.

III. Nitrous Air. This is a permanently elastic fluid, which is never found naturally, like fixed or inflammable air, but is entirely artificial.

This elastic fluid consists principally of nitrous acid and phlogiston; so that it cannot be produced but from that acid, or, which is the same thing, from substances containing the nitrous acid and phlogiston: thus, when the nitrous acid is mixed with metals, or with animal or vegetable substances which contain phlogiston, nitrous air is generally produced; but very little or none of this sort of elastic fluid is yielded when the nitrous acid is mixed with metallic calxes, with pure ashes of vegetable substances, or, in short, with any substance which contains little or nothing of phlogiston. In this case, if the mixture is exposed to a sufficiently strong degree of heat, the nitrous acid is decomposed, and phlogisticated air is produced; though, in some period of the process, a little nitrous air is generally produced, which is owing to the small quantity of phlogiston which exists in most, if not in all metallic calxes, &c.

The variety of elastic fluids obtainable when nitrous acid is employed, may be deduced from observing, that the nitrous acid itself, by being decomposed, may generate dephlogisticated air; that, by joining with the phlogiston of the substance with which it is mixed, may generate nitrous air; that, by the mixture of those two elastic fluids, another elastic fluid, namely, phlogisticated air, may be formed; and that, by its acting as an acid, it may expel one or more other elastic fluids from those substances with which it is mixed.

All metallic substances, when mixed with the nitrous acid, yield nitrous air. Indeed gold, platina, and regulus of antimony, as they are not dissolvable in simple nitrous acid, must be dissolved in aqua regia, in order to obtain nitrous air from them. Although all metallic substances, by means of the nitrous acid, may be made to yield nitrous air; yet they do not produce it in equal quantities, with equal facility, and of equal goodness. The following are the more remarkable particulars relating to this matter:

Either silver, copper, brass, iron, mercury, bismuth,

Air. 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 Prieſley) smoking spirit of nitre into a phial with a ground-stopper and tube, containing $1\backslash frac\{1\}\{2\}$ ounce measures 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 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 produced from it is mostly phlogisticated air. From 4 pennyweights and 17 grains of zinc, dissolved in spirit of nitre diluted with an equal quantity of water, Dr Prieſley obtained about 12 ounce measures of very weak nitrous air. It occasioned a very slight effervescence when mixed with common air (c). 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.

"This experiment also seems to prove, that an earth is the basis of nitrous air; for the same blue spirit of nitre, which gave nitrous air with these flowers of zinc, yielded no air at all when it was treated in the same manner without them."

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 Prieſley's first vo-

lume 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

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-silings; 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 flight 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 vial, 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 vial (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 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 are 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, are 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, and AIR, n° 39. (Encycl.)

IV. 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 some of 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 (D). 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 the first experiments by which this kind of air was produced, Dr Priestley concluded, that the nitrous acid and earth were essential ingredients in its composition †. This theory, however, seems to be overthrown by subsequent experiments, where the pure dephlogisticated air was produced from substances in which no nitrous acid could be supposed to exist. The abbe Fontana having rendered some red precipitate very dry by keeping it in a small degree of heat for several hours, weighed 192 grains of it, and introduced this quantity into a glass vessel proper for expelling the air from it. Applying, then, a sufficient degree of heat under it, he received the air into a glass jar filled with quicksilver, and inverted into a vessel of the same. Thus he procured 26½ cubic inches of very pure dephlogisticated air, the mercury being at the same time revived, and losing 13½ grains of its weight, which is nearly the whole weight of the air emitted by it. By a similar process, red-lead also emits a very pure kind of air; and it is remarkable, that red-lead which has been kept for a long time, yields more dephlogisticated air than such as is new; the reason of which Mr Cavallo supposes to be, that red-lead, when newly made, is not such a pure calx as after it has been kept for some time. From sedative salt also, from manganese, lapis calaminaris, and all the vitriols, pure dephlogisticated air has been obtained; nor was it produced from the native vitriols, which might perhaps have been suspected to contain some nitre; but from iron, copper, and zinc, dissolved directly in pure vitriolic acid. Mercury dissolved in the vitriolic acid, distilled to dryness, and the residuum urged with a strong heat, yields first a quantity of vitriolic acid air mixed with fixed air, and then dephlogisticated air. It is remarkable (says Dr Priestley) that, either by means of oil of vitriol or spirit of nitre, mercury yields a very great quantity of dephlogisticated

Air. air; but with this difference, that in the process with spirit of nitre, almost the whole of it (that is, if the process is conducted with care, with the loss of not more than the 20th part of the mercury) is revived, and therefore may be used again and again; whereas in the process with the oil of vitriol, almost all the mercury is lost." By mixing the same acid with manganese also, dephlogisticated air was obtained. Mr Cavallo mentions one instance where dephlogisticated air was produced by means of the muriatic acid; but in this he seems to have mistaken Dr Priestley, whose authority he quotes. Minium, as already mentioned, yields a quantity of dephlogisticated air by heat, without any addition; and it seems to have been only this air which was set loose by the acid, and not any original production. His words are: "Spirit of salt, I have observed, dissolves a great quantity of minium. In order to discover what became of the air it contains, I distilled a quantity of that solution, which was of a yellow colour, made by the first affusion of the acid. When the solution became hot, it yielded a quantity of fixed air, so as to make lime-water turbid only in the slightest degree. As it boiled, no air at all was procured, nor when it was distilled to dryness. I treated in the same manner a saturated solution of white minium, made so by its colour having been discharged by a previous affusion of the acid. But this solution yielded no air at all from the beginning to the end of the process; nor was the common air in the retort phlogisticated either at the beginning or end."

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. When a lighted candle is introduced into a phial containing dephlogisticated air, its flame not only grows larger, but becomes exceedingly bright. When the dephlogisticated air is very pure, the candle burns with a crackling noise as if the air contained some combustible matter, and wastes the wax or tallow surprisingly fast: the heat of the flame is also very intense. But the best method of observing its intensity is the following: Fill a bladder with dephlogisticated air, then fasten a glass tube to the mouth of the bladder: the outward aperture of this tube must be drawn to a fine point like that of an ordinary blow-pipe; then holding the extremity of the tube near the flame of a large candle or of a lamp, press the bladder so as to force the dephlogisticated air out through the small aperture of the tube, which will drive the flame of the candle into an horizontal direction, augmenting its force prodigiously; so that if small bits of any metal, held upon a piece of charcoal, or upon a piece of broken crucible, are presented to the apex of that flame, they will be presently melted. Even grains of platina may be melted by these means. The heat of other fuel is equally increased by blowing dephlogisticated air into it. "I put (says Dr Priestley) a quantity of Mr Bewley's pyrophorus into one of the small jars which I use for experiments on air in quicksilver, I inverted it in a basin of the same, and threw up dephlogisticated air at different times. It always occasioned a sudden and vehement accession, like the flashing of gunpowder; and the air was greatly diminished, as might have been foreseen." When mixed with in-

flammable air, and the mixture is brought near any flaming substance, or when an electric spark is sent through it, a great explosion ensues. If an ounce glass vial, which for this experiment should be very strong, be filled with a little more than one-third dephlogisticated, and the rest inflammable air, when the flame of a candle is presented to its mouth, it will explode nearly as loud as a small pistol.

It is not, however, only the advantages which might result from such an increase of heat that are 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 the Antediluvians to the great purity of the atmosphere 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 expence 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 deprived, 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

Air. 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, 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 certainly expect nothing from it superior at least 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, (or for 40 hours, as he limits the abbe Fontana's supposition), he 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 belt 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 waxes: 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 quan-

tity 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 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.

V. 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.

This elastic fluid has no action upon quicksilver, but is easily, and in great quantity, absorbed by water; so that if a small quantity of water be introduced into the receiver containing it, and inverted in quicksilver (s), the whole quantity of the acid air disappears, being absorbed by the water; from which it escapes very easily when exposed to the open air. The water thus impregnated acquires the properties of the volatile or sulphureous vitriolic acid.

A piece of camphor introduced into a receiver containing this kind of elastic fluid, is easily dissolved, but by the addition of water the camphor is again recovered.

If a piece of charcoal is admitted to this vitriolic acid gas, it absorbs a good quantity of it, and acquires from it a disagreeable and pungent smell. It is remarkable, that this elastic fluid does not act upon iron; whereas the water combined with it is a powerful solvent of that metal.

VI. Marine Acid Air. is no other than the marine acid itself, which without any addition becomes a permanently elastic fluid. In order to procure it, 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, called marine-acid air, is copiously produced.

A small quantity of water introduced into a receiver, containing this sort of elastic fluid, absorbs instantaneously a prodigious quantity of it, and becomes a very strong marine acid spirit; indeed, much stronger than can be obtained by any other means.

If any metallic, or in general any inflammable substance, capable of furnishing a considerable quantity of phlogiston, as spirit of wine, oil, &c. be introduced into a receiver filled with this elastic fluid, it occasions a remarkable change; viz. the marine acid air, by acting on it, will be changed into an inflammable elastic fluid, that takes fire by the contact of ignited bodies.

When a small quantity, as for instance, about a fourth-part, of marine acid air is mixed with one part of common air, and a lighted candle is introduced into the receiver containing this mixture, the flame of the candle acquires a beautiful green or blueish colour.

VII. Nitrous Acid Air. The elastic fluid, called nitrous acid air, 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 the 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, 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 essential oils, a considerable effervescence and heat is produced, nearly in the same manner as when the nitrous acid itself is poured upon those oils.

VIII. 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 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.

The properties of fluor acid air are nearly the same as those of the vitriolic acid air; hence some excellent philosophers are of opinion, that those two acid airs are essentially the same. The principal, if not the only property by which they are distinguished from each other, is, that when water is admitted into a receiver containing fluor acid air, the water absorbs only a part of it; for a stony crust is formed upon the surface of the water which hinders the farther absorption, and which must be broke before the water absorbs any more of it.

IX. Alkaline Air. Let the usual bottle be about half filled with volatile spirit of salt ammonia; 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.

If a small quantity of water is introduced into the receiver filled with this elastic fluid, the whole quantity of it is readily absorbed by the water, which thereby becomes a strong volatile alkaline spirit. If some of this gas is introduced into a receiver containing either the marine or the vitriolic acid air, a white cloud is instantly formed, and the two invisible elastic fluids, losing at once their elasticity, form a visible substance, namely, concrete ammoniacal salts.

The properties of this air are, that it is permanently elastic and unchangeable when confined in vials perfectly dry; but on the approach of moisture, it is instantly absorbed in great quantity without leaving any residuum. There is, however, a point of saturation betwixt it and water, nor can the latter be made to imbibe more than one-third of its weight of alkaline air. When the barometer is at a mean height, alkaline air is specifically lighter than common air in the proportion of 7 to 15, and its elasticity is greater in the proportion of 475 to 132*.

It destroys animal-life, and extinguishes fire, though in certain cases it has itself a degree of inflammability†. When a burning candle is put into a jar full of this air, it is indeed extinguished; but just before it goes out, the flame is enlarged by the addition of another of a pale yellow colour. Sometimes it will actually take fire when the flame of a candle approaches it, a weak flame being observed to spread considerably around the candle, and even through the whole body of the alkaline air. By the electric spark it is augmented in bulk, sometimes to $\backslash frac\{1\}\{2\}$ more than it was before, and then becomes strongly inflammable air, without the least appearance of volatile alkali. The electric sparks taken in it are red. It dissolves ice almost as readily as a hot iron, even in a very severe cold. It does not act on copper in its dry state, though it does so remarkably when united with water. It readily unites with all the acid airs, forming with them ammoniacal salts according to their different natures. Dr Priestley made two sets of experiments to determine the quantity of these gases necessary to saturate each other; but they vary so much in their result that little dependence can be placed on them. They are as follow:

The last set, however, the Doctor looks upon as less accurate than the foregoing.

X. Hepatic Air. A new kind of elastic fluid discovered by Mr Bergman. He obtained it from a mineral called pseudo-galina nigra Damnemorensis, an ore of zinc; and it has also been obtained from some other ores of this semimetal. One hundred parts of this mineral were found to contain 29 parts of sulphur, one of arsenic, six of water, six of lead, nine of iron, 45 of zinc, and four of siliceous earth. When oil of vitriol is poured upon this mineral, the hepatic smell is immediately perceived, and a small quantity of hepatic air is produced. Marine acid expels a much greater quantity of hepatic air from this mineral than the vitriolic. By means of diluted nitrous acid, nitrous air only was obtained. No other property of this kind of air is known, but that it deposits a quantity of sulphur when mixed with nitrous air.

Circulation of Air in Rooms. To render the circulation of air feasible, let the air of a room be heated by a strong fire, whilst the air of a contiguous room is cold; then let the door between these two rooms be opened, in which case the hot air of one room being lighter, will pass through the upper part of the opening of the door into the cold room; and, on the contrary, the cold air of the other room being heavier, will pass into the former room through the lower part of the opening; accordingly, it will be found, that applying a lighted candle at the top, in the middle, and at the lower part of the opening between the two rooms, a strong current of air will appear to pass from the hot into the cold room near the top; a contrary current of air will appear to pass from the latter into the former room near the lower part of the said opening; whilst in the middle there is little or no motion at all, as may be clearly perceived by the direction of the flame of the candle.

It is for the same reason that when the fire is lighted

in a chimney, a strong current of air is occasioned to enter the room, which may be felt by applying the hand near the key-hole, or other such small openings, if the doors and windows are shut; for the air over the fire being heated, becomes lighter, and ascends into the chimney, consequently other colder air must supply its place, which forces its way through all the small openings it can find. Were a room with a fire in it to be perfectly closed, excepting the chimney, the air in it would soon become unwholesome for respiration, and the fire would be soon extinguished, besides other inconveniences. Hence it appears, that those persons mistake who expect to keep the air of a room sweet and wholesome, especially for convalescents, by accurately stopping all the smallest openings that admit fresh air. When the current of air that enters into a room is on some side of it where it falls immediately upon the persons who sit in the room, then it may be offensive, especially to delicate constitutions. In that case, such opening should be closed: but at the same time another opening should be made for admitting fresh air, in another more convenient part; for a circulation of air, especially in rooms where a fire is kept, is not only salutary and useful, but is absolutely necessary.

In an ingenious publication, intitled A Practical Treatise on Chimneys, there are the following remarks relating to the properest method of admitting air into a room, and of expelling the contaminated air. The author, directing to make a vent-hole near the top of the room, in order to expel the heated and contaminated air, "this," says he, "might be done by means of a small tube opening into the room, either in or near the ceiling; which might either be carried to the top of the building, or be made to communicate with the external air by a small perforation through the wall at the roof of the room; by means of either of which, a proper circulation would be established, and the foul air be carried off.

"For the fire would no sooner have warmed any particles of air within the room, than these would be greatly expanded, and rise immediately upwards, so as to fill the higher parts of the room with rarefied air; and as other particles would be successively heated and rarefied in their turn, by their expansive force they would press upon the sides of the apartment in every place, so as to force the lightest particles through the opening left for that purpose in the top of the room; by which means the foulest air would be gradually drawn off, without descending again into the lower regions to the annoyance of the company."

But in order to admit fresh air into the room, "Let," says he, "another opening be made in the ceiling of the room, having a communication with a small pipe that should lead from thence either to the outside of the wall, or to any other part of the building that might be judged more convenient, where it should be bent, and conducted downwards, till it reached the ground; where it should be left open, to communicate with the external air.—In this situation the cool external air would be forced in at the lower opening of the tube, and made to ascend into the apartment in proportion to the quantity that escaped towards the higher regions by means of the ventilator. And as that weighty air would.

would no sooner enter the room, than it would tend towards the floor by its own natural gravity, it would gradually mix with the heated air in its descent—become, in some measure, warmed by that means, and equally dispersed through the room, so as slowly and imperceptibly to reach the candles and the company in the room, and supply them with a sufficient quantity of fresh and wholesome air, without the inconveniences to which the company are subjected by the usual way of admitting fresh air (G). For if it enters near the floor of the apartment, it is hurried along in a rapid undivided stream towards the fire-place, and striking upon the legs and inferior parts of the body, affects them with a strong sensation of cold. To overcome the effects of this, large fires must be kept; by which other parts of the body are warmed to an extraordinary degree, which is productive of most of those disorders that are pernicious to the young, and often prove fatal to the old, during the winter-season, in these cold regions.

“Thus might our apartments be kept constantly, and moderately, and equally warm, at a moderate expence, without endangering our health on the one hand, by respiring a confined, stagnant, and putrid air, or, on the other hand, by subjecting ourselves to such danger of catching colds, consumptions, and rheumatic complaints, by being exposed to such exceedingly unequal degrees of heat and cold, as are unavoidable where our apartments are so open as to admit a ready passage to the external air during the winter-season.

“The reader will easily perceive, that all that has been here said has a reference only to those apartments in cold climates, and rigorous weather, where fire to warm them becomes necessary. In warmer regions, or during the summer-season, there can be no objection to the wheel-ventilator in the window.—It is a simple contrivance, and a safe and effectual mean of preserving the air in our apartments sweet and wholesome at that season.”

It is a vulgar error among many people, to believe that fire purifies the contaminated air, by destroying the noxious particles mixed with it; and for this reason they think, that the fire kept in a room where the air is tainted, purifies the room, by rendering the air in it again fit for respiration. Indeed, a fire kept in a room or apartment where the air is tainted, as is the case with hospitals, goals, and the like, does certainly purify the apartment, and the practice is very useful; but this effect is only because the fire promotes the circulation of the air, and dries the dampness of rooms, furniture, &c.: so that it is not the infected air that is purified, but is new, fresh, and wholesome air, that by the action of the fire has taken the place of the infected air; which infected air, being rarefied by the heat,

has been expelled from the apartment. Fire and combustion in general is so far from purifying contaminated air, that it actually contaminates a prodigious quantity of it in a short time; so that not only a common fire, but even a lighted candle, when kept in a well-closed room, wherein the external air has not a free access, instead of purifying, renders the air of that room noxious.

Instrument for ascertaining the Wholesomeness of Respirable AIR. See EUDIOMETER.