(Dr Henry), an eminent nonconformist divine, who, about the year 1590, distinguished himself among the Brownists; who drew upon him such troubles, that he was obliged to retire to Holland, and became minister of a church at Amsterdam. His skill in the Hebrew language, and his excellent Annotations on the Holy Scriptures, which are still highly esteemed, gained him great reputation. He also wrote several pieces in defence of the Brownists, and several other works.

(Robert), born at Woodvale in Lancashire in 1666, was master of a boarding-school at Bethnal-green, from whence he removed to Hackney, and to other places in the neighbourhood of London. After acquiring a moderate fortune, he retired, and lived privately till the time of his death, which happened in 1743. We are indebted to him for the best Latin and English Dictionary extant: he published it in quarto 1736; and in 1752, the fourth edition, under the care of Doctor Ward of Gresham College, and the Rev. William Younger, was enlarged to two vols folio.

AIR is that invisible fluid which everywhere surrounds the globe; and on which depends the life not only of every kind of animals, but of vegetables also; and which seems, in short, to be one of the great agents employed by nature in carrying on her operations throughout the whole world.

For many ages the air was considered as an absolutely simple fluid, the component parts of which were beyond the reach of man's wisdom to discover. Its common operations were thought to be performed, either by its heat or cold, its moisture or dryness; and if any effects were discovered which could not be explained by these, (such as the appearance of pestilential diseases,) they were reckoned to be entirely supernatural, and the immediate effect of Divine power.

In the beginning of the last century, Lord Bacon discovered some of what may be called the mechanical powers of the air. The former, from experiments, ascertained its elasticity; and the latter, its weight. The pressure of the atmosphere, however, was more fully discovered by Torricelli, the disciple of Galileo, and inventor of the barometer, as Lord Bacon had been of the thermometer. Pascal observed, that this pressure was not always the same; but diminished according to the height to which the barometer was carried. Otto de Guerick soon after invented the air-pump. pump; which was much improved by Mr Boyle and Doctor Hoock, two members of the Royal Society. The complete knowledge of the mechanical properties of the air; however, must be ascribed to the labours of Doctor Halley and Sir Isaac Newton; who have, by mathematical demonstration, established its rarefaction, and the proportion in which it is rarified, according to its distance from the earth, &c.

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

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

Doctor Hales was the first person who attempted to determine the quantity of air produced from different bodies; and, for this purpose, he made experiments on almost every known substance in nature, examining them by distillation, fermentation, combustion, combinations, &c. Of the vegetable substances which he examined, crude tartar seems to have yielded the greatest quantity of air, and essential oils the least. From a cubic-inch of the former he obtained 904 cubic-inches of air; and from a like quantity of oil of aniseeds, only 22. Of the animal-substances, the greatest quantity of air was obtained from the human calculus, or stone extracted from the bladder; three quarters of a cubic-inch of this substance yielding, on distillation, no less than 516 cubic-inches of air; while a cubic-inch of tallow yielded only 18 inches. In the mineral kingdom, pit-coal gave out the greatest quantity of this fluid, 360 inches of air being obtained from one inch of it, or nearly one-third of its whole weight. From the same quantity of antimony, only 28 inches were obtained. By fermentation, 639 cubic inches of air were obtained from 42 inches of small-beer in seven days; and from 26 inches of bruised apples, 968 inches of air were obtained in thirteen days.

In examining the quantities of air produced from combinations of different bodies, very strange phenomena appeared; the very combinations which produced air one day, would absorb all they had produced, and sometimes much more, the next. Half a cubic inch of sal-ammoniac, with one cubic-inch of oil of vitriol, produced five or six cubic inches of air the first day; and the next, absorbed 15. In a few hours, six inches of oyster-shells, and as much vinegar, produced 29 inches of air; but, in nine days, 21 inches were absorbed, and the remainder disappeared upon pouring water into the vessel. A quarter of an inch of iron-fillings, and one cubic inch of sulphur, instead of producing, absorbed 19 inches of air. A cubic inch of aquafortis, with an equal quantity of marcasite, absorbed 85 inches; but the same quantity of aquafortis and coal, absorbed 18 inches in three days; after which, instead of absorbing, they generated 12 inches. Two cubic inches of lime, with four of vinegar, absorbed 22 inches of air; but two inches of lime, with an equal quantity of sal-ammoniac, absorbed 115 inches.

By examining flaming substances, it appeared that all of them, nitre alone excepted, absorbed or consumed air. A lighted candle, three-fifths of an English inch in diameter, consumed 78 inches of air; linen-rags, dipped in melted brimstone, and burnt in a large vessel, consumed 198 inches; in a smaller one, 150. Two grains of Kunkel's phosphorus absorbed 28 inches of air; after which it had only lost half a grain in weight; and in a short time gained a whole grain. A rat, confined in a large receiver, consumed 78 inches before it died; and 73 inches of air breathed by a man till he was almost suffocated, were reduced to 20.

Doctor Hales also first suspected, that the briskness of and sparkling of the waters, improperly called acidulous, were owing to the air they contained. But notwithstanding all his discoveries concerning the quantity of elastic fluid obtained from different bodies, he did not imagine there was any essential difference between this fluid and the air we breathe, only that it was loaded with noxious vapours, foreign to its nature. He therefore endeavoured to restore air which had been depraved by the respiration of animals, or by burning bodies, to its original purity. This he attempted, by filtering it through flannel which had been steeped in a solution of salt of tartar; and by this means the air was perfectly restored. A candle, likewise placed under a receiver, lined with flannel dipped in a solution of this salt, burned considerably longer than it would otherwise have done. The flannels, however, through which the air was filtered, were sensibly increased in weight.

What doctor Hales only suspected, concerning the impregnation of some kinds of waters with air, was confirmed by M. Venel, professor of chemistry at Montpelier. pelier, in a memoir read before the Royal Academy of Sciences in 1750. This gentleman proceeded so far as to disengage the air from the Seltzer waters, and to measure its quantity; which he constantly found to amount to about one-fifth of its bulk. When the water was deprived of this air, it became flat, and ceased to sparkle; the only difference then between the Seltzer water, deprived of its air, and common water, was, that the former contained a small quantity of sea-salt. Upon these principles he attempted to recompose Seltzer water, by dissolving in a pint of common water two drachms of felspar alkali, and then adding an equal quantity of marine acid. The quantity of sea-salt produced by the union of these two, he knew would prove equal to that contained in a pint of Seltzer water; and the effervescence produced by the action of the acid and alkali upon each other, he imagined, would produce air sufficient for the impregnation of the water. In this he was not deceived; the water thus produced was not only analogous to Seltzer, but much more strongly impregnated with air.

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

In the mean time, the count de Saluces, at Turin, was employing himself in making experiments on the elastic fluid discharged from gun-powder.—He found, that, when at liberty, this species of air occupied two hundred times the space of that taken up by the gunpowder itself. He was able to reduce it to the same state with common air, by filtering through alkaline solutions, or by exposing it for twelve hours to the degree of cold in which water freezes. The air detached from pulverulent substances he found to be much less in quantity than that from gun-powder, notwithstanding the explosion of the former is much greater. He also observed, that air disengaged from effervescing bodies extinguishes flame; but that what was separated from volatile alkali and vinegar, was an exception to this rule. He was, however, of opinion, that all these different fluids were only common air loaded with heterogeneous particles.

Mr Haller first inferred, from Doctor Hales's experiments, that air is the real cement of bodies; which, by fixing itself in the solids and fluids, unites them to each other, and serves as a bond by which they are kept from dissolution. In 1764, Dr Macbride of Dublin published a number of experiments in support of this doctrine. From his work it appears, that fixed air is separated, not only from all substances in fermentation, but also from all animal substances as they begin to putrefy; and that this air is capable of uniting itself to all calcareous earths, as well as alkalies both fixed and volatile, and restoring to them the property of effervescing with acids when they have by any means been deprived of it.—The conclusions drawn by him from his numerous experiments were, that fixed air is an elastic fluid, very different from the common air we breathe; that it is possessed of a strong antiseptic quality, and may be introduced with safety into the intestinal canal, and other parts of the animal economy, where common air would have fatal effects; but is mortal if breathed into the lungs, &c.

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

About the same time that Dr Macbride published his experiments, a treatise appeared, written in German by Mr Meyer, apothecary at Osnabruck, wherein he opposes Doctor Black's theory concerning fixed air being the cause of effervescence in calcareous earths and alkaline salts. The loss of weight these substances suffer by calcination, he attributes to the quantity of water expelled by the vehement heat; and their not effervescing afterwards, he attributes to their having been neutralized, while in the fire, by a peculiar kind of acid, which he calls acidum pingue. The existence of such an acid in lime he proves from the precipitation of lime-water by alkaline solutions. From this he concludes, that the acidum pingue surpasses the earth, which it before kept in a dissolved state, to unite with the alkali. This acid he also affirms to be what escapes from charcoal in burning; what unites with metals in their calcination; and what gives the causticity to volatile and fixed alkalies, as being the very acid, caustic, or power of fire itself.

A strong objection lies against this theory, from a fact discovered by Doctor Black; namely, that pure calcareous earth dissolved in the nitrous acid, may be precipitated either in the form of lime, or of chalk, according as we make use of the caustic or the mild alcali. The reason given by Dr Black for this phenomenon is, that, in the dissolution of the earth by the acid, all its fixed air is expelled. In the precipitation, if a mild fixed alkali is made use of, the fixed air is expelled from it by its union with the acid, and the calcareous earth has liberty again to combine with the fixed air expelled from the alkali; in which case, the earth appears in its natural mild state: but if an alkali is made use of, which contains no fixed air, the calcareous earth has none to combine with, and therefore appears in the state of lime.

This formidable objection Mr Meyer easily solves by his new hypothesis of the acidum pingue. "When we mix (says he) a solution of calcareous earth in the nitrous acid with a caustic fixed alkali in a fluid state, we mix solutions of two neutral salts together; the one of calcareous nitre, the other of alkali saturated with acidum pingue. In this case, according to the known laws of affinities, a double decomposition ought to take place; and we see it actually does so. The weaker acidum pingue is expelled from its basis by the nitrous acid, which forfeits the earth to unite with the alkali. The acidum pingue, having now nothing else to combine with, unites with the earth which the nitrous acid hath left, neutralizes, and forms it into lime. The case is different when the mild alkali is employed: for this having no acidum pingue joined with it, can communicate none; and therefore the precipitate falls as a calcareous earth."

To this new system of Mr Meyer's, Mr Jacquin, botanical professor at Vienna, published an answer in 1769.—He first attempts to prove, that calcareous earth is not converted into quicklime merely by the loss of its humidity. To ascertain this, he distilled 38 ounces of limestone in a stone-retort, fitted with a large tubulated receiver, with a fire gradually increased to the highest degree; and obtained only two ounces of water, which had some slight traces of volatile alkali. This came over with a moderate fire; and after the aqueous vapours ceased, an elastic vapour began to separate very plentifully, and continued for an hour and a half to fly off through the tube of the receiver with an hissing noise. The lime which was left in the retort weighed only 17 ounces.—Here was therefore a deficiency of 19 ounces, which Mr Jacquin attributed to the air; and, according to him, limestone contains five or seven hundred times its bulk of air.

Mr Jacquin afterwards examines the action of water upon lime; and finds, that it is by no means the absence or presence of moisture in any degree, which constitutes lime; seeing it can be preserved under water for any length of time as lime, provided we keep the surface of the water from contact with the air.

About the time that Mr Jacquin's performance made its appearance, Mr Meyer died; but Mr Crans, physician to his Prussian majesty, published a reply to Mr Jacquin at Leipzig. He eludes the force of Mr Jacquin's experiment with limestone distilled in a retort, by attributing to water, reduced to a state of vapour, or in a great degree of expansion, the elastic separation during the continuance of the distillation. But of this assertion he hath not brought any decisive proof.

Mr Crans denies that lime is deprived of the power of effervescing with acids; and corroborates his assertion, both from experiments made by himself, and by the united testimonies of Messrs Duhamel, Geoffroy, Homberg, and Pott.—On this occasion he objects, that if lime differs from calcareous earth only in being deprived of its air, it ought, by a short exposure to the open air, to imbibe all that it has lost; but to far from this, he affirms, that, after being exposed a considerable time, it even acquires greater causticity. In favour of Mr Meyer's hypothesis, he likewise observes, that the sudden swelling and heat, observed in the flaking of lime, is a natural consequence of his system, whereas it is absolutely inexplicable on Dr Black's hypothesis; which also can give no reason why calcareous earth dissolves with very little heat in the nitrous acid, while the dissolution of lime in the same acid produces a degree of heat superior to that of boiling water; and affords the particles of fixed air to be utterly unable to explain many phenomena, which upon Mr Meyer's plan are perfectly intelligible.

This author further observes, that lime-water dissolves sulphur, camphor, and resins, nearly in the same manner that spirit of wine does. If Dr Black's disciples then reason consistently, they ought to say that it renders those substances soluble by attracting their air from them; but thus they will be obliged to affirm the same of spirit of wine, which, he says, would lead them into a labyrinth of difficulties, if not of absurdities.

With regard to effervescence, Mr Crans observes, that in the dissolution of a calcareous earth, we may have an effervescence or not, just as we please, by employing a strong or a weak acid; whereas, on Dr Black's plan, there ought to be an effervescence whether the acid employed is strong or weak.—He afterwards shows, that a brisk effervescence may be obtained by a mixture of caustic lixivium with an acid, though, according to Dr Black and Mr Jacquin, neither of these substances contain any air. Mr Crans's method is to pour some caustic lixivium into a solution of calcareous earth. The alkali trickles down the sides of the bottle, and reaches the bottom. If the two liquors are afterwards suddenly agitated, a brisk effervescence ensues, and the precipitation is formed in an instant.

The experiments adduced by Dr Black and Mr Jacquin for the support of their system, from the precipitation of calcareous earths in the form of lime by caustic alkalies, are absolutely denied by Mr Crans; who affirms, that, with whatever alkali he precipitated the earth, the precipitate always effervesced with acids. The only difference he could perceive, was, that it had some degree of solubility in water, and turned syrup of violets green.

A strong argument in Dr Black's favour is, that calcareous earths, when dissolved in acids, suffer a loss of weight equal to what they would have done, had experiments been reduced by calcination into quicklime.—Here Mr Crans opposes experiments made by solutions of calcareous stones in the nitrous acid, compared with solutions of lime. In these processes, he always observed a considerable diminution of weight, but without any rule; sometimes the lime appeared more diminished than the calcareous earth, at other times the calcareous earth appeared to receive an augmentation in weight during its dissolution. These experiments, however, do not appear to have been made with sufficient accuracy; both as Mr Crans employed too shallow vessels, and likewise operated upon such small quantities, that... an error in the scales might occasion most of the inequalities he has remarked.

Lastly, Mr Crans proceeds to Dr M'Bride's experiments concerning the restoration of the effervescing power to alcalies by means of fixed air. To this trial he submitted the caustic lixivium made after Mr Meyer's method. The air detached from an effervescing mixture precipitated from the lixivium a white sediment, which collected at the bottom of the bottle. The liquor also acquired, after some time, the property of effervescing with acids; but he observed, that it did so, nearly, in as short a time when exposed to the open air. He also remarked, that this property was much sooner recovered if the lixivium was placed over a moderate fire; and that it was recovered at the instant when the fumes began to arise. Hence Mr Crans concludes, that it acquires the effervescing power only in proportion to the evaporation of the caustic principle, or acidum pingue, to which the alcali was united.

The same thing was observed with respect to the caustic volatile alcali obtained from sal ammoniac. Mr Crans placed one portion of it in a flame; another on hot cinders; and exposed the third to the vapours of an effervescing mixture. At the end of eight hours all the three effervesced. The reason he gives, is, the evaporation of the acidum pingue; so that, according to him, the fixed air had no other effect than what might have naturally taken place in the open air.

Upon the whole, Mr Crans agrees that fixed air combines with alkaline liquors; but he affirms that these liquors are impregnated in the same manner with common water, and denies that there is any real combination, or that such a combination is owing the mild state of alkaline salts. This he constantly attributes to the evaporation of the acidum pingue.

While Mr de Craus thus attacked Dr Black's doctrine at Leipzig, Mr de Smeth did the same at Utrecht. This gentleman begins with asserting that we have no knowledge of common air, except by some of its physical effects; of its internal nature and composition we know nothing; and therefore we ought not to call any substance air, merely because it has elasticity, and gravity, while it wants the other essential properties of air. He affirms, that elasticity is a very equivocal characteristic of air; and that we may at this rate affirm water reduced into vapours to be atmospheric fluid. He is of opinion, that the elastic vapours which arise either from fermenting or effervescing liquors, are very different from atmospheric air; and he particularly observes, that the vapour of fermentation is much more subtle than common air, as passing through bodies which would be an unsurmountable obstacle to the latter. This vapour he found incapable of being retained by lutes; a moistened bladder, tied over the mouth of the vessel, was not at all inflated, though he was certain, from other experiments, that a great quantity of this vapour had escaped. Nay, so far is he from thinking it a particular element, or simple, in the sense which chemists give to that word, that he is very positive it did not originally exist in the bodies from which it is extracted by art, but is only a misnomer formed by the collision of solid and fluid parts; that it is therefore never produced, but in cases where the bodies suffer violent intestine motion, in consequence of which their parts are altered, broken, and attenuated, so as to exhibit the phenomena of fixed air.

The antiseptic virtue of astringents, according to Dr M'Bride, consists in the power they have of contracting the pores of animal substances, and thus preventing the escape of their fixed air. This argument Mr de Smeth pays no regard to; and affirms that we know too little of the manner in which astringents act, to be able to form the least induction from thence. Indeed, from the following experiments mentioned by Mr Henry, F.R.S., it would seem that the sweetening properties of fixed air may possibly depend on an affinity between this fluid and the septic particles arising from putrid bodies.—"A piece of putrid beef, fastened by a string to a cork, was confined in three pints of fixed air for 13 hours, during which time it was considerably, though not entirely, sweetened; but the air in the bottle seemed to have acquired all the putrid smell of which the flesh had been deprived; so that the septic effluvium did not seem to be destroyed, but only to change its place. Slips of linen cloth also, dipped in very rancid oil, were much sweetened by being exposed to a stream of fixed air from an effervescing mixture; but a pint-bottle of the same oil, though it absorbed much of this air, so as to become entirely saturated with it, was not sweetened in the least."

Mr de Smeth endeavours to overthrow Dr Black's theory by a number of experiments, most of which are evidently inconclusive. The principal, indeed the only one, which deserves attention, is the following:—Having observed that Homberg's pyrophorus gained weight considerably by being exposed to the air, he was induced to make the same experiment with regard to quicklime. Twelve ounces of this substance, being exposed to the air in a balance, augmented almost visibly in weight during the first month. After this period, its attractive power diminished considerably; and at the end of a year, or thirteen months, was absolutely lost. In this time it had acquired an augmentation in weight of four ounces, three drachms, and forty grains; was reduced to a fine powder, and no longer separated the volatile alcali but in a concrete form. After a space of thirteen months, then, the whole weight of this lime was sixteen ounces, three drachms, and forty grains. Mr de Smeth weighed, separately, twelve ounces, three drachms, and forty grains; which, by calculation, he found ought to contain three ounces, two drachms, fifty-four grains and an half, of matter attracted from the atmosphere. This matter he thought would be easily dissipated by fire; and to ascertain himself of this, he put the abovementioned quantity into an earthen retort, and exposed it to a very strong fire for two hours. During the operation, there passed into the receiver, one ounce, four drachms, and forty grains of pure phlegm, in which no saline matter could be discovered. The residuum, weighing ten ounces five drachms, proved a quicklime, notwithstanding there was only two drachms of weight lost upon the whole. If there had been a separation of air then, during the operation, it could by no means have been so considerable, as according to Dr Black's theory it ought to have been.—From this experiment it also appears, that quicklime, by being exposed to the air, gains something from it which cannot afterwards be separated by fire. He afterwards repeated the same operation in open vessels, with the same success. Having put the remaining remaining four ounces of lime in a wind furnace, and urged it with a very strong fire, it retained one drachm eleven grains of matter, attracted from the atmosphere. Being again exposed to the air, it regained in weight, 4 drachms, 28 grains. The same thing has been observed by Mr du Hamel; who relates, that lime, flaked in the air, retained an increase of weight, amounting to about four and a half drachms per pound, and which could not be driven off by the strongest fire he could employ.

During this controversy among the learned, concerning the existence or non-existence of fixed air, as such, in terrestrial bodies, none of the contending parties seem to have apprehended, that this fluid might possibly be one of the component parts of our atmosphere; and, tho' pernicious when separated from the others, might nevertheless be absolutely necessary, in a certain degree, to preserve that life which its suffocating properties, when collected by itself, would seem calculated rather than to destroy.—To decompose the subtle invisible fluid we daily breathe; to be able to recompose it again, and produce air either salutary or noxious as we please; seems to be one of the highest discoveries ever made by man.—This, however, hath been accomplished by Dr Priestley, whose discoveries we now begin to relate.

The Doctor began his experiments much about the same time with Mr de Smeth. He begins with observing, that the term fixed air may be equally applied to every species of air hitherto discovered; being inflammable, and other kinds of air, are fixed in terrestrial bodies as well as this. As the term, however, has come into such general use, he chuses to retain it, and distinguishes by that name the fluid which issues from fermenting liquors, and from the effervescence of acids with calcareous earths. It may be obtained in its greatest purity from a mixture of oil of vitriol and chalk. From fermenting liquors also, if the quantity is considerable, it may be obtained tolerably pure; and in this way Dr Priestley himself used frequently to procure it, when living in the neighbourhood of a large brewery.

One general property of this air is to be imbibed by water with great avidity. By agitation, the water may be impregnated very quickly with a great quantity of it; but as agitation will also make water part with its fixed air, so great a quantity cannot be imbibed by this means as when the water is left to take up the air leisurely by being at rest.—The air thus taken up is discharged by boiling, or by freezing, the water which contains it.

Dr Priestley agrees with Dr Black, that the concrete form of volatile alkaline salts, as well as the effervescing power of both kinds of alkalies, and calcareous earths, depends upon the presence of fixed air. He also owns it to be of an acid nature, though weak, and of a peculiar kind. This was demonstrated by Mr Bewly, in some letters to Dr Priestley, wherein he gives an account of his having both changed the blue juices of vegetables red with this acid, and likewise formed perfectly neutral salts, both from fixed and volatile alkalies, by means of it; and in the last volume of his observations, Doctor Priestley hath given very strong reasons for thinking that fixed air is a modification of the nitrous acid. He found also, that it possessed an exhilarating quality; and, when combined with fixed alkali in such quantity as to neutralize it, could not be expelled by a boiling heat, unless the liquor was exposed to the open air; in which case it was impossible to retain it. The Doctor hath also observed, that water held long in fixed air discharged from fermenting liquor, acquires a very disagreeable taste; once he observed it like tar-water; but could not satisfy himself whence this arose, for fear of hurting the liquor; having once injured a large quantity of beer, by holding over it a quantity of ether in a glass.

By agitating pure fixed air in a glass, with water, a part of it always remained, which the water could not imbibe; and in this residuum the Doctor found that animals could live, though flame was extinguished. By a mixture of iron-filings and brimstone, about one fifth of the air was imbibed, and the remainder was not so noxious as before.

In making experiments on common air made noxious by the burning of candles, brimstone, &c. he found, that lime-water became turbid by being placed in the vessel where the candle was burning. This made him suspect, that the manner in which this change happens to the air, is by its depositing its heaviest part, or that which commonly goes by the name of fixed air. This he was afterwards assured of, by finding air considerably diminished by the electric spark; and that, in consequence of this, blue juices of vegetables were turned red, and lime-water was precipitated exactly as by fixed air.—The Count de Saluces, at Turin, had imagined, that air which had been rendered incapable of supporting flame, could be restored merely by being exposed to a considerable degree of cold, and also by being compressed in bladders. Dr Priestley repeated his experiments; but found them not to succeed, unless the air was compressed in bladders only, which he attributes to the porosity of the bladders; and with great reason, having constantly found, that however he compressed it, or to whatever degree of cold he exposed it, in glass-vessels, the air underwent no change. Vegetation alone he found effectual for this purpose; which was generally accomplished in five or six days; after which time candles would burn in it perfectly well; while another portion of the same air, after being kept for many months, without any vegetation, would extinguish candles equally as at first.

The restoration of the air depended entirely upon the vegetation of the plant made use of; for a great number of fresh leaves of mint were unsuccessfully used for a long time, in endeavouring to restore a small quantity of air in which candles had burnt out. Though mint was the first plant made use of by the doctor in this experiment, he found all others to answer equally well, as well aromatics, as those which had no smell; and even poisonous plants, as well as others. The plant he found most efficacious for this purpose was spinach.

One caution the doctor gives in making experiments of this kind, viz. that it is absolutely necessary to remove all the dead or rotten leaves of the plant; for they will deprive air in such a manner as to render it incapable of supporting flame. A fresh cabbage-leaf, put under a glass vessel for one night, so affected the air in it, that it extinguished a candle next morning; and this without any appearance of putrefaction in the leaf.

After candles cease to burn, animals feel little or no inconvenience. inconvenience from breathing the same air. It is impossible, however, for them to breathe air of this or any other kind for any length of time without suffocation.

The reason of their death, according to Dr Priestley, is not the want of the "pabulum vitae," supposed to be contained in the air; but to the air being impregnated with something stimulating to the lungs. The noxious effluvium with which the air, in this case, is loaded, cannot be absorbed by standing, without agitation, in fresh or salt water. Growing vegetables, however, restored air deprived by animal respiration, superficially as that in which candles had burned out. The same effect was produced by agitating this air with water; and in some degree, also, by a mixture of fixed air.

Notwithstanding that this kind of air, (which the Doctor distinguishes by the name of putrid air), proves to be very noxious to most animals; yet vegetables thrive in it to a surprising degree. It is also impossible for them to be kept clean from swarms of insects; which Dr Priestley was frequently obliged to brush off the sprigs of mint on which he made his experiments.

Inflammable air was first observed by Mr Cavendish. He obtained it from a solution of iron, zinc, or tin, in the marine acid. Doctor Priestley hath found, that this air may be procured from every inflammable substance, either animal, mineral, or vegetable, by combustion alone. From these substances he extracted it, by heating them in a gun-barrel, to the orifice of which a glass-tube or tobacco-pipe was fitted, and to this was tied a flaccid bladder, in order to catch the generated air: but, in order to get a great quantity of air, it was necessary to apply the heat as suddenly, and as vehemently, as possible. By this treatment, a bit of dry oak, weighing twelve grams, will yield a sheep's bladder full of air, while only two or three ounce measures of it can be obtained if the heat is slowly applied.

Inflammable air, when made by a quick process, has a strong offensive smell, from whatever substance it is extracted. It differs, however, according to the substance from which it is obtained; and is most fetid when procured from animal bodies. If a quantity of this kind of air is contained in a glass vessel standing inverted in water, it will even smell through the water; which will soon become covered with a thin film, assuming all the different colours. If the air has been generated from iron, the film will be a red ochre; if from zinc, it is a whitish substance, probably the calx of that metal; it likewise settles to the bottom; and, when the water is stirred, has very much the appearance of wool. When water is once impregnated in this manner, it continues to yield this scum for a considerable time after the air is removed.

This kind of air is no less noxious to animals than the fixed or putrid kinds. It was generally thought to be immiscible with water: but Dr Priestley hath observed four instances of its entirely losing the inflammable property, and being reduced to half its bulk, by long standing in a bottle inverted in water. In this state it extinguished candles much more speedily than that air in which they had formerly burnt out, and instantly killed animals that were put into it.

If inflammable air, contained in a vial, be mixed with an equal quantity of common air, it will instantly explode on the approach of flame. If less than an equal quantity of common air is introduced, a number of explosions may be produced from the same quantity of inflammable air; only taking care to stop the mouth of the vial immediately after every explosion, otherwise the inflammable air will continue burning, though invisibly in the day-time, till the whole is consumed. A small mixture of the fumes of smoking spirit of nitre, makes it go off at once, as if mixed with an equal quantity of common air. This kind of air Dr Priestley could not kindle without bringing it into contact with a substance actually flaming; but Mr Volta, inventor of the electrophorus or perpetual electrifying machine, hath succeeded in firing it by the simple electric spark; even when the electricity is very moderate, by a well lighted coal without any flame, by a red hot iron, and even by a flint and steel.

Upon trial, with fixed air, the inflammable kind seemed incapable of mixing with it. Even after equal quantities of the two had been confined together in a vial for three years, they did not seem to have at all united, or affected each other; the fixed air being absorbed by water, and the inflammable air exploding as usual.

Vegetables continued to grow in this kind of air, but without making it lose its inflammability, or become fit for respiration. This could be accomplished only by agitation in water. By agitating a large quantity of inflammable air in water, one fourth of it disappeared in ten minutes, and a mouse lived 20 minutes in 24 ounce measures of the remainder; which is as long as that creature can live in the same quantity of common air. The air was yet, however, inflammable, though very weakly so. By a continuance of the agitation, this air admitted a candle to burn in it; and at last came to extinguish it like that in which a candle had burned out. The degree of diminution suffered by this kind of air when it lost its inflammability, was about one half. Distilled water imbibed about one fourteenth of its bulk of inflammable air; but the taste was not sensibly altered.

A mixture of iron-filings and brimstone, made into a paste with water, diminished the air in which it stood, between one fourth and one fifth of its whole quantity; which then became rather lighter than common air. In this state it is highly noxious; has a very pungent and offensive smell; nor is it meliorated by standing in water.—The diminution in this, as well as in other cases, Dr Priestley concludes to arise from a deposition of the fixed air, owing to a superabundant quantity of phlogiston being introduced.

All the acids have been reduced by Dr Priestley Nitrous air, into the form of air. He begins with the nitrous, which is obtained from a solution of any kind of metallic substance in that acid. From gold, and the regulus of antimony, it is obtained by means of aqua regia. He hath even found that it may be obtained in great plenty from common water. See Water.

One of the most conspicuous properties of this air is Diminishes the great diminution of any quantity of common air common air, with which it is mixed, attended with a turbid red or deep orange colour, exactly like that which appears on untopping a bottle containing smoking spirit of nitre, which the air itself very much resembles in smell. This diminution is attended with a considerable degree of heat. If one ounce measure of nitrous be put to double the quantity of common air, in a few minutes the mixture will want one ninth of the original quantity; and if both kinds of air be very pure, the diminution will still go on very slowly, till the whole, in a day or two, is reduced to one fifth less than the original quantity of common air. After this saturation of common with nitrous air, a fresh quantity of the latter makes an addition equal to its own bulk, without producing the least redness, or other visible effect.—The diminution in this mixture, was found to arise from a precipitation of the fixed part of the common air, and the condensation of the nitrous air into the acid, called spirit of nitre. The precipitation of fixed air appeared, when the process was conducted in lime-water, by its becoming turbid, though a small quantity of this water put into the vessel was not affected by it. The condensation was evident by the acid taste communicated to water in which this process had been conducted; and Mr Bewley has observed, that, without a mixture of common air, the condensation of nitrous air will not take place.

It is also very remarkable, that the effervescence with a test of the nitrous air is peculiar to common air; or that fit for respiration; and this exactly in proportion to its goodness; that is, the more pure, or fit for respiration, any quantity of air is, the greater degree of redness will be communicated to it on the admixture of nitrous air, and vice versa. Thus the Doctor was furnished with a most accurate method of measuring the degree of goodness of any kind of air he had occasion to try.—This test is equally applicable to air, on whatever account it is rendered unfit for respiration; nor the least effervescence being made between the nitrous and fixed, inflammable, putrid, or any species of noxious air. By this test he was able to discover, that air in which candles had burned out, was thereby rendered about one third worse than common air.

Inflammable air, mixed with nitrous, burns with a green flame. Equal proportions of oil of vitriol and spirit of nitre produced nitrous air; but with a less proportion of the nitrous acid, an inflammable kind, burning with a green flame, was produced.

By a mixture of iron filings and brimstone, made into a paste with water, nitrous air is remarkably diminished; no more than one fourth of the original quantity being left in one hour after the effervescence of the iron and brimstone has begun; which generally takes place in about five or six hours after the mixture has been made. The glass in which this mixture was made, usually acquired such a degree of heat, that it could not be touched.

Nitrous air, thus diminished, has not so strong a smell as at first, but smells exactly like common air diminished by the same mixture. It is not then capable of being further diminished by a fresh mixture of iron and brimstone. Nor is common air, saturated with nitrous, any farther diminished by a mixture of iron and brimstone; though the mixture ferments with great heat, and swells very much in it.

This kind of air, as well as common air saturated with nitrous, proves fatal both to vegetable and animal life. Neither of these differ in specific gravity from the common atmospheric air.

Distilled water absorbs nitrous air with great avidity, and acquires from it a remarkably acid and astringent taste, with a peculiarly pungent smell. A filmy kind of substance is also precipitated by the union of this kind of air with water. The Doctor supposes it to be a calx of the metal employed in producing the nitrous air.

The most remarkable, and, as Dr Priestley observes, probably the most useful, property of this kind of air, is its power of preserving animal substances from putrefaction, and restoring those that are already putrid; which it possesses in a degree far superior to fixed air. In the months of July and August, 1772, the Doctor put two mice, one of them just killed, the other soft and putrid, into the same jar of nitrous air; and after 25 days, having observed little or no change in the quantity of the air, he took them out; when both were found perfectly sweet; that which had been put into the jar when just dead, was quite firm; the other continued soft, but perfectly sweet.—A mouse inclosed for a month in fixed air, became insufferably offensive.

Though this kind of air may be obtained from all metallic substances, yet it is got with difficulty from some metals, and the proportion yielded by them is very different. Iron yields the greatest quantity, fifteen ounce measures of air being obtained from 20 grains of this metal; next to iron, copper, or brass, yield the most; after them silver, quicksilver, &c. In attempting to get nitrous air from zinc, the following phenomena occurred.

Four penny-weights, and seventeen grains of zinc being dissolved in spirit of nitre diluted with an equal quantity of water, yielded twelve ounce measures of air, which was in some degree nitrous. The solution being boiled in a sand heat, some air came from it, which appeared to be the same with nitrous air diminished about \( \frac{1}{3} \) or \( \frac{1}{4} \), by washing in water. Upon the evaporation of the fluid, there remained a brown fixed substance, which, on an increase of heat, gave out very dense red fumes; and the air was considerably diminished within the receiver. This substance, therefore, the Doctor concludes, must have contained the principle on which the properties of nitrous air depend.

Although the air, however, within the receiver was diminished \( \frac{1}{3} \) by this process, it was as much affected by nitrous air, as common air itself is, and a candle burned in it perfectly well.

The Doctor next proceeds to an investigation of the air produced from the fumes of burning charcoal; and he finds, that in this case, as well as in others, a considerable diminution of air is occasioned, and, by the precipitation of lime-water contained in the vessel, there appeared to be a deposition of fixed air. At first he concluded, that the fixed air in this case came from the charcoal; but, considering the intense heat requisite for making charcoal, he thought it more probable it came from the air, as the great heat requisite to calcine the charcoal would have expelled all the air out of it. This, however, was determined in the following manner.

Having suspected, from the experiments with charcoal, that the diminution of air in all cases was owing to the deposition of its fixed part, in consequence of its having more than the usual quantity of phlogiston; the calculation of metals, which are supposed to contain nothing else than a particular kind of earth united to phlogiston, phlogiston, appeared to be the most certain method of determining this point. Pieces of lead and tin were accordingly suspended in given quantities of air, and had the focus of a burning mirror thrown upon them, so as to make them fume copiously. A great diminution of the air immediately took place; it became in the highest degree noxious, made no effervescence with nitrous air, nor was farther diminished by a mixture of iron-fillings and brimstone.

The water over which metals have been calcined, acquires a yellowish tinge, and an exceedingly pungent smell and taste, much like that over which brimstone has been frequently burned. A thin whitish pellicle, also, covered the surface of the water, and the sides of the vial in which the calcination was made. Mr La Voisier has proved, by some experiments, that the calcination of metals depends entirely on the absorption of fixed air; that, exactly in proportion to the increase of their weight, the air in the receiver which contains them is diminished; and that when all the fixed part of the air has been deposited, the calcination cannot proceed farther, until fresh air is admitted. Dr Priestley also has observed, that lime-water is not precipitated by having metals calcined over it; but it always acquires the peculiar smell and taste above-mentioned. The reason why none of the lime is precipitated in this case, is, that the metallic calx has a greater affinity with fixed air than lime has, and consequently absorbs it preferably to the lime.

From all these experiments, and many more than what can be mentioned here, the Doctor concludes, that in all cases the diminution of the air is owing to the deposition of its fixed part; which happens in consequence of a saturation with phlogiston: that the inflammable principle, having a greater affinity with some of the constituent parts of the air than its fixed part, unites with them in preference to the other; which immediately joins itself to whatever has a tendency to absorb it. When an animal or vegetable putrefies, the phlogistic matter, together with all its other constituent parts, is set loose, which he supposes to be the cause of the diminution of the air in that case. When iron ferments with brimstone and water, there is an evident escape of phlogiston, by the metal's being reduced to calx. The same must necessarily happen upon the ignition of charcoal; and as spirit of nitre has a very strong affinity with phlogiston, it is highly probable that nitrous air diminishes common air, by imparting phlogiston to it; while the acid of the nitrous air, uniting with the aqueous part of the atmosphere, condenses into a liquor.

As for the Doctor's experiments on the other kinds of acid and alkaline air, as they come more properly under Chemistry, we shall here only mention, that from the fume of the marine acid he always obtained inflammable air, by putting it to spirit of wine, oil of olives, oil of turpentine, charcoal, phosphorus, bees wax, and even sulphur. This made him suspect, that the common air we breathe, was no other than some kind of acid united with phlogiston; and that it really was so, he discovered by the following experiments.

Having exposed mercurius calcinatus per se to the focus of a burning glass twelve inches diameter, he obtained air from it very plentifully. This air, he found, was not absorbed by water; a candle burned with a very vigorous and greatly enlarged flame; a piece of red-hot wood sparkled in it like paper dipped in a solution of nitre, and consumed very fast.—The same properties he observed in air drawn from red precipitate. From muriatic acid, he extracted air of the very same kind. One third of this air, indeed, was readily absorbed by water; but in the remainder, a candle burned very strongly, and with a crackling noise.

After some time, it occurred to him to apply the test of nitrous air to that which he had newly procured; and, upon doing so, he found that it was fully as much diminished as common air. From hence he concluded, that this air was respirable. Accordingly, he put a mouse into two ounce measures of air, obtained from mercurius calcinatus per se. Had it been common air, he knew that this creature would have lived a quarter of an hour in such a quantity. In the dephlogisticated air, however, as Dr Priestley calls it, the mouse lived a full half hour; nor did it, when taken out, show signs of being injured any otherwise than by cold, as it presently revived upon being held to the fire. The remainder of the air which had been so long breathed by the mouse, and which, had it been common air, would have been in the highest degree noxious, was still found to be much better than common air, being reduced by nitrous air to almost one half of its original quantity.

From this quality of taking more phlogiston from nitrous air, than common air was capable of doing, he concluded, that it must originally contain less of that principle than common air. In his experiments to know why this kind of air comes to be so much dephlogisticated, he at last hit upon a method of producing very pure air readily, and in considerable quantity. Having moistened half an ounce of red lead with spirit of nitre, and then dried the mass, he obtained from it not quite a pint of dephlogisticated air, exceedingly pure, in which a candle burned very briskly; and which seemed to be about five times as pure as common air. From this experiment, the Doctor concluded, that the nitrous acid was that which gave the minimum power to emit this dephlogisticated air. The vitriolic and marine acids were tried without effect. No air of any kind was produced by treating them in the same manner. The minimum effervesced violently with all the acids.

For the same purpose, the Doctor tried, with success, flowers of zinc, chalk, quicklime, flaked lime, tobacco-pipe-clay, flint, Muley's talc, and even glass itself; from which he draws the general conclusion, "That the air we breathe consists of the nitrous acid and earth, with as much phlogiston as is necessary to its elasticity; and likewise as much more as is necessary to bring it from its state of perfect purity, to the mean condition in which we find it." The residuum of his distillation, he found equally fit with fresh earth for the production of more air, upon being again moistened with the spirit of nitre. In his third volume, published in 1777, the Doctor acquaints us, that very pure dephlogisticated air is produced by simply distilling a solution of any metal in the nitrous acid; and Mr Bewly found even that trouble unnecessary; nothing more being requisite, than to moisten red lead with the spirit of nitre, and then pour upon it oil of vitriol; when the dephlogisticated air would immediately be expelled without any more heat being required than what was generated by the mixture. The Doctor hath also endeavoured to determine the proportions tions of earth and nitrous acid, required to produce this kind of air; but hitherto without success. Air, he finds, will take up a great deal of earth when hot, which it deposits when cold. See Earth.

We shall conclude this subject with some observations which the Doctor has made on the use of the blood in animals, and on respiration. They are to be found at length in the Philosophical Transactions for the year 1776, and in his third volume on air published in 1777.

In his treatise on putrid air, or that infected by animal respiration, he had shewn, that respiration was a phlogistic process; and that by means of it a putrid effluvium was carried off from the body, without which he imagined that a living body might perhaps putrefy as soon as a dead one. In this paper he proves, that the blood is the principal agent in carrying off the superabundant phlogiston; that when the whole mass of blood is successively brought almost into contact with the air in the lungs, it discharges phlogiston into it; and that the blood receives its red florid colour from the air, he proves by the following experiments.

Pieces of the nearly black-coloured crafamentum of a black coloured blood sheep's blood, inclosed in nets of open gauze, or wire, having been introduced thro' water or quicksilver into inverted receivers containing common air, received from it a florid red colour, at the same time that the air was considerably deprived.—The brightest red blood became black in phlogisticated or any otherwise depraved air; and resumed its colour again upon being exposed to the fresh air, parting, in this last situation, with the phlogiston it had acquired in the preceding.

That pure air is deprived by the presence of blood, while the colour of it is changed from black to red, the Doctor proved by his very pure deplogisticated air being considerably vitiated by successively introducing fresh pieces of crafamentum to the same portion of it; and this without any tendency to putrefaction in the blood employed.

In the course of his experiments on blood, he made the following remarkable discovery concerning the nature of serum, viz. that a covering of serum several inches deep was no impediment to the action of the air upon the crafamentum of the blood, as it acquired the red colour as easily on being exposed to pure air with this thick coat of serum, as without it; whereas the slightest covering of water, or saliva, effectually prevented any change of colour. On reversing the experiment, he found that phlogisticated air would act upon crafamentum, so as to turn it black, through a covering of serum two inches deep.—From these experiments he concluded, that the ferous part of the blood was particularly organized for the purpose of transmitting air through it.

It now remains that we give some account of the apparatus requisite for making experiments on air: and for this purpose it will be sufficient to give an idea of that made use of by Dr Priestley; both as being most easily understood, and likewise, if we may judge from the discoveries he hath made by the use of it, as being the most efficacious of any that hath hitherto been invented.

For experiments in which air will bear to be confined in water, he made use of an oblong wooden trough fig. 1, two feet long, 11 inches deep, and 18 inches wide; with a shelf, fig. 2, about an inch lower than the top, for the convenience of placing the jars upon it. The several kinds of air are kept in cylindrical jars fig. 3, about 10 inches long, and 2½ wide; though it is necessary, for particular experiments, to have vessels of different forms and sizes. When he has occasion to transfer air from one jar to another in quicksilver, a small oblong trough is absolutely necessary; but, on other occasions, a basin is more convenient for holding the quicksilver.

When vessels of air are to be removed from the large trough, they are placed in pots or dishes fig. 4, of different sizes, to hold more or less water as there is occasion. For the purpose of merely removing a jar of air from one place to another, where it is to stand only a few days, common tea-dishes may be used; unless the air be in a state of diminution, when vessels of a larger size must be made use of.

When an experiment is to be tried how long a small animal, a mouse for instance, will live in a certain species of air, a tall beer-glass, such as is represented by fig. 5, which contains between two and three ounce measures of air, will answer the purpose. In this quantity of common air a mouse will live 20 minutes, or half an hour.—On this occasion the Doctor observes, that mice must be kept in a pretty exact temperature, as they are unable to bear either much heat or much cold. He was also surprized to find that they lived entirely without water; and he had an instance of one mouse tearing another almost in pieces, though there was plenty of provisions at the time for both. The method of putting these creatures into the quantity of air designed for the experiment, is to pass them through the water into the cavity of the glass, into which something must be put for them to sit conveniently out of the reach of the water.—The same method may be used when a plant is to be conveyed into any given quantity of air. If the plant is of such a nature that it will grow in water only, there will be no occasion to set it in a pot of earth, which otherwise will be necessary.

For opening the mouth of a vial, in any quantity of air, without admitting the water into the vial, it is necessary to have a cork cut tapering, with a strong wire thrust through it, fig. 6, by which means it may be introduced into the mouth of an inverted jar, and the cork withdrawn by means of the wire, which afterwards can be replaced by the same means, if there is occasion.—For supporting a gallipot at a considerable height within a jar, it is convenient to have such wire-stands as are represented fig. 7. They answer better than any other, as they take up but little room, and easily bend to any form.

When air is to be poured from a wide-necked, into a very narrow-necked vessel, a glass funnel fig. 8 must be used, by which means the operation is rendered exceedingly easy; first filling the vessel into which the air is to be conveyed with water, and unstoppering the other containing the air under the funnel, which is inserted into the mouth of the narrow vessel, and immersed in water. The air immediately ascends through the neck of the funnel, makes the water descend, and take its place.

To expel air from solid substances by means of heat, a gun-barrel may be sometimes used, which is filled up with dry sand that has been well burned; so that no air can come from it. To the open end is fitted the stem of a tobacco-pipe, or a small glass tube. Having put the closed end of the barrel containing the materials into the fire, the generated air, rising through the tube, may be received in a vessel of quicksilver having its mouth inverted into a basin of the same, suspended all together by wires, in the manner represented in the figure.

But the most accurate method of extracting air from several substances, by means of heat, is to put them, if they will bear it, into vials full of quicksilver, with the mouths immersed in the same; and then throwing the focus of a burning mirror, or convex lens, upon them. The vials used for this purpose should have their bottoms round and very thin, so that they may not break with a sudden application of heat; for which Florence flasks seem very proper.

If air is to be expelled from any liquid, a vial is nearly filled with it. To the vial is fitted a perforated cork, having a glass tube inserted in it, bent as in fig. 1, and secured with cement. The vial is then to be put in a kettle of boiling water, in order to expel the air; or it may be heated by means of a candle, or red-hot poker. But where the air is readily imbibed by water, quicksilver ought always to be used; or if a sufficient quantity of it cannot be procured, oil will in some measure answer the purpose.

When air is to be transferred from a jar standing in the trough of water to any other vessel, the contrivance in fig. 12, is made use of. It consists of a bladder furnished at one end with a small tube of glass bent, having at the other a cork perforated, so as just to admit the small end of a funnel. When the common air is pressed out of this bladder, and the funnel thrust tightly into the cork, it may be filled with any kind of air as easily as a glass jar. A string being then tied above the cork in which the funnel is inserted, and the orifice in the other cork closed by pressing the bladder against it, it may be carried to any place; and if the tube be carefully wiped, the air may be conveyed quite free from moisture through a body of quicksilver, or anything else.

To impregnate fluids with air of any kind, as water with fixed air, a vial is filled with the fluid, as in fig. 5. It is then inverted in a bowl b, containing a quantity of the same fluid; and the bladder c being filled with the air, as much of it as is thought proper may be thrown into the vial; and, to accelerate the impregnation, the vial may be shaken as much as possible. The same apparatus serves very conveniently for conveying air immediately as it is generated from an effervescing mixture, into any other species of fluid; and that the vial may be more conveniently shaken, in order to make the effervescence occasionally more brisk, a flexible leather pipe may be sometimes used, instead of the inflexible glass one.

When any kind of air is to be tried with regard to its capacity for sustaining flame, a cylindrical glass vessel (fig. 4) is made use of, with a bit of wax candle, fastened to the end of a wire, and turned up in such a manner as to be let down into the vessel with the flame upwards. The vessel should be kept carefully covered, till the moment the candle is admitted; and by this means the Doctor has extinguished a candle more than 20 times successively; although it is impossible to dip the candle in it without giving the external air an opportunity of mixing more or less with that in the vessel. The candle at the other end of the wire is very convenient for being held under a jar standing in water, in order to burn as long as the inclosed air can supply it; for, the moment it is extinguished, it may be drawn through the water, before any smoke can have mixed with the air.

In order to draw air out of a vessel which has its mouth immersed in water, and thereby to raise the water to any height, it is convenient to use a glass syphon, fig. 6, putting one of the legs up into the vessel, and drawing the air out of the other by the mouth. If the air is of a noxious quality, it may be necessary to have a syringe fastened to the syphon; or if a very small hole is made in the upper part of a glass vessel, it may be filled to any height, by holding it under water, while the air is discharged at the hole, which may be afterwards closed with cement.

When a particular kind of air is to be admitted to any thing that will not bear wetting, especially if it is a powder, and must be placed on a stand, as in those experiments in which the focus of a burning mirror is to be thrown upon it, a receiver is first exhausted, in which it is previously placed; and having a glass tube fitted for the purpose, as in fig. 7, it is to be screwed to the stem of a transfer of the air-pump on which the receiver had been exhausted; and introducing it into a jar of that kind of air with which the receiver is desired to be filled, the purpose is gained, by only turning the cock.

To take the electric spark in any kind of air, the quantity of which must be very small, to produce a sensible effect upon it in a short time, a piece of wire is put into the end of a small tube, and fastened with hot cement, as in fig. 8, and having got the air delivered into the tube, by means of the apparatus already described, it is placed inverted in a basin containing quicksilver or any other fluid substance. By the help of the air-pump, then, as much of the air is driven out as is thought proper; and putting a brass ball on the end of the wire, the sparks, or shocks, are communicated by its means, thro' the air contained in the tube, to the fluid.—If air is generated very fast by this process, a glass is used, fig. 10, which is narrow above, and grows wider below, that the quicksilver may not too soon recede beyond the striking distance.

Besides this general apparatus, which hitherto may be considered as merely experimental, and a matter of curiosity only, it will be proper to mention that for impregnating water with fixed air; as water impregnated air with this kind of air hath been found exceedingly salutary in putrid diseases, particularly in the fea-fever. For this reason, a method of impregnating large quantities of water with fixed air has become an object worthy of public attention. A proposal for doing so was laid before the board of Admiralty, and was accepted of; and the captains of two ships that were just failing for the south seas, had orders to make trial of the impregnated water; for which purpose Dr Priestley drew out his directions in writing, and sent a drawing of his apparatus.

The apparatus recommended by Dr Priestley for impregnating water, is not in the least different from that represented fig. 5, where a represents a glass vessel, vessel, with a pretty narrow neck, but so formed that it will stand upright with its mouth downwards. Having filled it with water, lay a slip of clean paper, or thin pasteboard, upon the mouth; then, if they be pressed close together, the vessel may be turned upside down, without danger of admitting common air into it; and when thus inverted, it must be placed into another vessel, in the form of a bowl or bason, &c., with a little water in it, so much as to permit the slip of paper or pasteboard to be withdrawn, and the end of the crooked pipe to be introduced. One end of this pipe is inserted into a bladder, which is tied round it; and the other communicates, by means of a perforated cork, with a vial which contains the effervescing mixture, from whence the fixed air is to be detached.

On some occasions it may be convenient to have this pipe flexible; when it will be best made of leather sewed with a waxed thread, in the manner used by shoemakers. When this pipe is flexible, a piece of quill must be thrust into each end of it, to keep them open, while one of them is introduced into the vessel of water, and the other into the bladder, the opposite end of which must be tied round a cork perforated, and the hole kept open by a quill. The cork must fit the vial containing the effervescing mixture, two-thirds of which must be filled with chalk, just covered with oil of vitriol. The Doctor, however, finds it most convenient to use a glass tube; and, for the advantage of agitating the vial, to have two bladders, communicating by a perforated cork, to which they are both tied.

Things being thus prepared, and the vial containing the chalk and water being detached from the bladder, and the pipe from the vessel of water, pour a little oil of vitriol upon the chalk and water; and having carefully pressed all the common air out of the bladder, put the cork into the bottle presently after the effervescence has begun. Also, press the bladder once more, after a little of the newly generated air has got into it, in order the more effectually to clear it of all remains of common air; and then introduce the end of the pipe into the mouth of the vessel of water, as in the drawing, and begin to agitate the chalk and water briskly. This will presently produce a considerable quantity of fixed air, which will distend the bladder; and this being pressed, the air will force its way through the pipe, and ascend into the vessel of water, the water at the same time descending and coming into the bason.

When about one half of the water is forced out, let the operator lay his hand upon the uppermost part of the vessel, &c., and shake it as briskly as he can, not to throw the water out of the bason; and, in a few minutes, the water will absorb the air; and, taking its place, will nearly fill the vessel as at first. Then shake the vial containing the chalk and water again, and force more air into the vessel, till upon the whole an equal bulk of air has been thrown into it. Also shake the water as before, till no more of the air can be imbibed. As soon as this is perceived to be the case, the water is ready for use; and if it is not to be used immediately, should be put, as soon as possible, into a bottle well corked and cemented. It will, however, keep very well, if the bottle be only well corked, and kept with the mouth downwards. A little more than a tea-spoonful of oil of vitriol will be sufficient to impregnate three pints of water with fixed air.

By this process may fixed air be given to wine, beer, and almost any liquor whatever; and when beer is become flat or dead, it will be revived by this means; but the delicate agreeable flavour, or acidulous taste, communicated by the fixed air, and which is manifest in water, will hardly be perceived in wine, or other liquors, which have much taste of their own.

By the same means also may be prepared water having all the medicinal virtues of Pyrmont water, or any other mineral water similar to it; especially if a few iron-fillings be added, to render it a chalybeate like genuine Pyrmont water; which it may be made to resemble exactly, by putting eight or ten drops of tinctura martis cum spiritu salis to every pint.

The first hint of the uses to which fixed air may be applied, was given by Sir John Pringle; who discovered that putrefaction was checked by fermentation. Doctor Macbride found this to be an effect of the fixed air produced in the process; upon which principle he recommended the use of wort to sailors, as a substitute to fresh vegetables, by supplying a quantity of fixed air from its fermentation in the stomach; which conjecture is now confirmed by experience. Dr Black discovered the existence of fixed air in calcareous substances; Dr Brownrigg claims the discovery of it in Pyrmont, and other mineral waters; and Dr Priestley, that of an easy method of impregnating water with it in large quantities. He also conjectured, that, if applied by way of clyster, it might be of service in putrid fevers; which is likewise verified by experience. The fixed air may be injected into the intestinal canal, by the same apparatus employed for injecting the smoke of tobacco.

The use of bladders in this apparatus was objected to by Dr Nooth; who asserted, that they were apt apparatus, to communicate an urinous flavour to the water. This he attributed to the action of the solvent power of the air upon the bladder; and he gave a particular kind of apparatus of his own invention, in which, the vessels being entirely made of glass, no inconvenience of this sort could be apprehended.—To Dr Nooth's objections Dr Priestley replied, that he had been conversant with bladders, and fixed air contained in bladders, as much as any man, and never found any such flavour arising from the use of them as Dr Nooth had experienced. He suspected, therefore, that the taste complained of had arisen from the carelessness of the servant, and that urine had really been mixed with the water made use of. He owned, however, that the apparatus recommended by Dr Nooth, and improved by Mr Parker, had in some respects the advantage of his own, particularly in being more cleanly to the operator, and requiring less attendance; though it was more inconvenient, where large quantities of water were to be impregnated, on account of its being much slower.

This apparatus is represented, fig. 3. In the lowest Plate VII., vessel, the chalk, or pounded marble, (which last is preferred by Dr Priestley), and the water acidulated with oil of vitriol, is to be put; in the middle vessel is the water to be impregnated, the defecation of which is prevented by the ascent of the fixed air. During the effervescence, the fixed air rises into the middle vessel, displaces part of the water in it, thro' the bent tube into the upper vessel, the common air going out through a channel in the flapper. When this bent tube... tube is of a proper length, the process requires no attention; and if the production of air be copious, the water will generally be sufficiently impregnated in five or six hours. At least, all the attention that needs be given to it is to raise the uppermost vessel once or twice, to let out that part of the fixed air which is not readily absorbed by water. If the operator chooses to accelerate the process by agitating the mixture, he must separate the two uppermost vessels from the lowest, or the air will be too copiously produced, and he will also be in danger of throwing the liquor contained in the lowest vessel, in contact with the stopple which separates it from the middle vessel, by which means some of the oil of vitriol might get into the water.

**Flour-acid Air**

**Marine-acid Air**

**Vegetable-acid Air**

**Vitriolic-acid Air**