Dephlogisticated Air superior, though exposed only for three hours and an half; but the next day, it being cold and cloudy, only \( \frac{3}{4} \)ths of an inch of air were produced, and these manifestly inferior to the foregoing. No more air could afterwards be procured, excepting one quarter of a cubic inch; so that from 296 inches of this water, 33.96 of air were obtained.

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

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

In reflecting on the experiments above related, it occurred to Sir Benjamin, that the cotton-like substance produced by the populus nigra, a species of poplar tree, might be a proper substitute for the raw silk; especially as he recollected, that on rendering it very dry for some other purpose, some parcels of it had quitted the plate on which they were laid, and mounted up to the top of the room. An hundred and twenty grains of this substance were therefore put into the large globe containing 296 inches; but after exposure to the sun for some hours, the air produced, in quantity about \( \frac{1}{4} \)ths of a cubic inch, was found to be little better than phlogisticated air. In three days after, only one cubic inch was formed; and this appeared to be completely phlogisticated. Next day, only a few inconsiderable air-bubbles appeared; but, the day following, the water suddenly changed to a greenish colour, and began all at once to give good air, and in great abundance. This day 104.42 cubic inches were produced, and the next 144.34. The same water continued to furnish air for four days longer; the whole quantity amounting to 445 cubic inches, the quality of which was superior to that of the air produced in former experiments.

In speculating on the cause of this production of air, it occurred to our author, that perhaps the quantity of it might be in proportion to the surfaces of both. In order to ascertain this, he viewed an hair of silk, and another of poplar-cotton, through a good microscope, when the former appeared twice the diameter of the latter. The specific gravity of the cotton was found to be nearly equivalent to that of water; and, by a Dephlogisticated Air comparative view of the two through a microscope, the surfaces appeared to be as 1000 to 3468. By proceeding in this calculation, it appeared that the surface of 30 grains of the cotton could not be less than 6600 square inches, while that of a like quantity of the silk amounted to no more than 476. Hence it evidently appeared, that the produce of air from the two substances was neither in proportion to their weights nor their surfaces. It appeared also, that the quality of the air produced at first was considerably inferior to that yielded some time afterwards. In order to ascertain the times at which air of the best quality was produced, &c. the following experiments were made: 1. A globe containing 46 cubic inches, being filled with water, and 30 grains of raw silk, well washed, and freed from the remains of former experiments, put into it, yielded in a cold and cloudy day only \( \frac{1}{4} \)th of a cubic inch of air: the two following days it yielded 3\( \frac{1}{2} \) cubic inches, the quality of which was superior to that of the former in the proportion of 296 to 114 (A). 2. The globe being filled again with water, in two other days when the sunshine was less powerful, the quality was 197, and the quantity \( \frac{1}{8} \)th; but afterwards, when the weather became fine, the quantity was again 3.8 inches, and quality 342. 3. The globe being again filled with water, and exposed to the sun for two days, yielded 2.2 inches of air, of a quality equal to 233. 4. A similar globe, with poplar-cotton which had been used in former experiments, gave 2.53 inches, of a quality 280. 5. A small globe of 20 inches, with 17 grains of raw silk, gave one cubic inch of air, of the quality 253. 6. A large globe of 296 inches, filled with fresh water, and a small quantity of conserva rivularis, gave 1\( \frac{1}{2} \) cubic inch, of the quality only of 124. The water was changed to a brown colour. 7. On repeating the experiment with a small handful of the conserva, 13.14 cubic inches of air were produced, of the quality 246. The water was very faintly tinged towards the end of the experiment, of a greenish cast. 8. The globe of 46 inches, with 30 grains of raw silk used in many former experiments, produced in two days 1.6 cubic inches of air, of the quality 204. 9. A globe of equal capacity, with 15 grains of poplar-cotton, produced in the same time 1.28 inches, of the quality 260. In both these experiments, the water had acquired a faint greenish cast; but the colour of that with the cotton was deeper. On examining this water with a microscope, it was found to contain a great number of animalcules exceedingly small, and nearly of an oval figure; that with the silk contained them likewise, but not in such numbers: however, our author assures us, that in all cases in which the water acquired a greenish hue, he never failed to find them; and thinks, that from their presence alone, the colour of the water in the first instance universally arose.

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

(A) In all these experiments, the quality of atmospheric air is supposed to be 100. Dephlogisticated Air, not contribute any thing, considered as chemical substances, in the process of the production of pure air yielded by water; but if, on the contrary, they acted merely as a mechanical aid in its separation from the water, by affording a convenient surface for the air to attach itself to; in this case, any other body having a large surface, and attracting air in water, might be made use of instead of the silk in the experiment, and pure air would be furnished, though the body should be totally incapable of communicating anything whatever to the water.

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

The following experiments were made with a view to determine the quantity and quality of air produced by means of the heat and light of the sun from water alone. A large jar of clear glass, containing 455 cubic inches, being washed very clean, was filled with fresh spring water, inverted in a glass basin of the same, and exposed to the weather for 28 days. At the same time, another similar jar was filled with water taken from a pond in a garden in which many aquatic plants were growing, and exposed in the same place, and during the same period. The latter began to yield air in pretty large quantities on the third day, and continued to do so till the 14th; the former yielded little or none till the 14th, when it began to emit air, and continued to do so till the 24th. On removing the air produced, that from the spring-water was 14 inches in quantity, and 138 in quality; but from the pond water, 31½ in quantity, and 252 in quality. The colour of the waters was not changed; but both of them had deposited a considerable quantity of earth, which was found adhering to the surfaces of the glass basins in which the jars were inverted. As these basins, however, were very thick, and consequently had but little transparency, the sediment of the water was in a great measure deprived of the benefit of the sun's light; the experiment was therefore repeated with the following variation: In a large cylindrical jar of very fine transparent glass, 10 inches in diameter and 12 inches high, filled with spring-water, a conical jar, 9½ inches in diameter at the bottom, and containing 344 inches, was inverted, and the whole exposed to the sun for 21 days. Little air was furnished till the 7th day, when the liquor assumed a greenish cast, and a fine flinty sediment of the same colour, the green matter of Dr Priestley, beginning to be formed on the bottom, air was generated in abundance, and was furnished in pretty large quantities till the 18th, when it entirely ceased. The whole amounted to 40 cubic inches, and the quality 213.

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

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

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

In confirmation of this doctrine, the globe of 46 inches was filled with fresh spring-water, and two peach-leaves were exposed for 10 days to the sun. In four days the water seemed to be entirely exhausted; but, next day, the water acquired a greenish colour, and again produced air pretty plentifully, which appeared in bubbles on the leaves; and on the 6th day, 0.34 of a cubic inch of air was produced, of the quality 232. Next day it yielded 2½ths of a cubic inch, of the quality 297. The three succeeding days it yielded 1½ inches, the quality 307; after which an end was put to the experiment. Dephlogisticated Air.—On making other trials with leaves immersed in water already green and prepared to yield dephlogisticated air, it was found that they produced air in great quantity; but our author is of opinion, that all the appearances may be folved, by supposing that the air was produced in the mass of water by the green matter; and that the leaves, silk, &c. did no more than assist in making its escape, by affording a convenient surface to which it could attach itself, in order to collect together and assume its elastic form.

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

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

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

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

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

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

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

A phial containing 20 ounce measures of dephlogisticated air, and inverted into a solution of hepar sulphuris, was entirely filled with the latter in the space of two days.

The purity of dephlogisticated air is ascertained by its degree of diminution with nitrous air; which, like that of the diminution by liver of sulphur, or otherwise, is to be considered as a phlogistic process, or kind of burning, especially as a considerable degree of heat is thereby generated. Very great differences are perceived in this respect; and according to the quantity of diminution, the air is said to be two, three, or four times better than common air. It is not yet accurately determined how far this proportionable purity extends. Dr Priestley mentions some extracted from red lead five times as pure as common air. Another quantity, produced from a solution of mercury in nitrous acid, was so pure, that one measure of it mixed with two of nitrous air, which had been obtained in the first part of the same process, occupied only 0.03 of a measure. "Repeating the experiment (says he), I found, that two measures of nitrous air were rather more than sufficient to saturate one measure of the dephlogisticated air; so that possibly, had the former experiment been made with more circumspection, the diminution, extraordinary as it was, would have been somewhat greater. Indeed it cannot be supposed, that exactly two measures of nitrous air should be the precise quantity that would afford the greatest diminution. It should also be considered, that a small portion of air might be yielded by the water in which the experiments were made. Upon the whole, therefore, I am inclined to think, that, were it possible to make both the dephlogisticated and nitrous air in the greatest purity, and then to mix them in some exact proportion, the aerial form of both would be destroyed, the whole quantity seeming to disappear, as in the mixture of alkaline and acid air."

Notwithstanding this great degree of purity, the best dephlogisticated air is capable of being contaminated by some of the processes which affect the common air of our atmosphere. Dr Priestley having introduced a quantity of very dry, clean nails, into a receiver filled with dephlogisticated air, and inverted it in quicksilver, found, that about nine months after, one-tenth of the whole quantity had disappeared, though he could not perceive any rust upon the nails. The effects of combustion have already been related, viz., as producing a great quantity of pure fixed air; but putrefaction and animal respiration probably contaminate it in a manner similar to that of atmospheric air, though few or no experiments seem to have been made on this subject. Mr Cavallo, however, informs us, that "when an animal is confined in a quantity of dephlogisticated air, and is kept therein till it dies, that air is not rendered bad but that it will still be capable of considerable diminution by nitrous air. This seems to show, that dephlogisticated air is somewhat different from pure common air; or that common air is originally different from dephlogisticated air, lowered by the addition of phlogiston. The phenomenon is certainly very remarkable; and sometimes a quantity of dephlogisticated air, after having been breathed by an animal till it died, will appear by the nitrous test to be even better than common air. When the experiment is performed over lime-water (to absorb the fixed air produced in respiration), the diminution by a mixture of nitrous air is less than it would otherwise be; but it is still diminished much more than common air after an animal has died in it; which seems to intimate, that the death of the animal in dephlogisticated air is principally owing to the fixed air formed by the act of respiration. It may be said, that the inflammable principle discharged through the lungs of an animal, being perhaps combined with some other principle, requires a longer time to combine with the dephlogisticated air than the phlogiston of nitrous air; but this is only an hypothetical explanation of the abovementioned remarkable phenomenon, which requires many direct proofs."

Dephlogisticated air is much inferior to that of the vegetation common atmosphere in supporting vegetable life. This ill support has been ascertained by the experiments of Dr Priestley, Mr Fontana, Mr Scheele, Dr Ingenhouz, &c. Dr Priestley took three sprigs of mint, and having put all the roots into vials containing the same pump-water which had been for some time exposed to the atmosphere, introduced one of them into a jar of dephlogisticated air, another into a jar of common air, and a third into that which had been phlogisticated with nitrous air several months before, and in such a state, that one measure of it, and one of nitrous air, occupied the space of 1½ measures. This was done in April; and on examining them on the 12th of May following, it was found, that the plant in phlogisticated air had grown remarkably, much better than that in common air; while the plant in dephlogisticated air had a very sickly appearance. Examining them on the 26th of the same month, the appearance continued nearly as before; but it was now found, that though the plant in phlogisticated air had grown so well, the air was not sensibly improved by it, though the dephlogisticated air was injured by the plant which grew in it.

§ 3. Of the Composition of Dephlogisticated Air.

When Dr Priestley first discovered the existence of this Dr Priestley's fluid, having found that it was always procured by means of earthly substances; and that as it came over, the bubbles appeared full of fine white powder; he concluded, that it is composed of the nitrous acid and earth, with as much phlogiston as is necessary to its elasticity; and that the common atmosphere has as much more as is necessary to bring it into the mean condition in which we find it. It was not long, however, before this theory met with opposition. Dr Priestley himself, though induced, from the waste of the solid matter used in his experiments, to conclude that the air contained some quantity of earth, was nevertheless unable, by any method he could think of, to ascertain that quantity. His experiments were opposed by others made by Lavoisier; who insisted, that when solution of mercury was carefully distilled, the betwixt Dr metal was obtained in full quantity, or with scarce Mr Lavoisier any loss, notwithstanding the dephlogisticated air produced. This gentleman having put two ounces and one drachm of mercury into red precipitate, and afterwards revived it, lost a very few grains of the metal; which, he says, might be the weight of a little red matter that was found adhering to the neck of the vessel. The same thing was observed by Mr Fontana, who repeated the experiment often with less than a grain. The vessel he used had a neck of about two feet long; and he particularly remarks, that, in order to succeed in this experiment, the fire should be managed with very great dexterity; for if that be too strong, part of the precipitate will be volatilized, and then the result of the experiment is precarious.

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

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

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

On calcining metals in dephlogisticated air, very singular phenomena are observed, which seem to throw great light upon the composition of this fluid. One of the most simple of all phlogistic processes (says Dr metals, Priestley), is that in which metals are melted in dephlogisticated air. I therefore began with this, with a view to ascertain whether any water be produced when the air is made to disappear in it. Accordingly, into a glass vessel containing seven ounce-measures of pretty pure dephlogisticated air, I introduced a quantity of iron turnings, which is iron in thin small pieces, exceedingly convenient for these and many other experiments, having previously made them, together with the vessel, the air, and the mercury by which it was confined, as dry as I possibly could. Also to prevent the air from imbibing any moisture, I received it immediately in the vessel in which the experiment was made, from the process of procuring it from red precipitate, so that it had never been in contact with any water. I then fired the iron by means of a burning lens, and presently reduced the seven ounce-measures to 0.65 of a measure; but I found no more water after this process than I imagined it had not been possible for me to exclude, as it bore no proportion to the air which had disappeared. Examining the residuum of the air, I found one-fifth of it to be fixed air; and when I tried the purity of that which remained by the test of nitrous air, it did not appear that any phlogisticated air had been produced in the process: for though it was more impure than I suppose the air with which I began the experiment must have been, it was not more so than the phlogisticated air of the seven ounce-measures, which had not been affected by the process, and which must have been contained in the residuum, would necessarily make it. In this case, one measure of this residuum, and two of nitrous air, occupied the space of 0.32 of a measure. In another experiment of this kind, ten ounce-measures of dephlogisticated air were reduced to 0.8 of a measure, and by washing in lime-water to 0.38 of a measure. In another experiment, 7½ ounce-measures of dephlogisticated air were reduced to half an ounce-measure, of which one-fifth was fixed air, and the residuum was quite as pure as the air with which I began the experiment; the test with nitrous air, in the proportions above mentioned, giving 0.4 in both cases.

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

AER O L O G Y.

Dephlogisticated Air

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

Concluding from the preceding experiments, that iron, sufficiently heated, was incapable of saturating itself with pure air from the atmosphere, I then proceeded to melt it with the heat of a burning lens in the open air; and I presently found, that perfect iron was easily capable of being fused in this way, and continued in this fusion a certain time, exhibiting the appearance of boiling or throwing out air; whereas it was, on the contrary, imbibing air; and, when it was saturated, the fusion ceased, and the heat of the lens could make no farther impression upon it. When this was the case, I always found that it had gained weight in the proportion of \( \frac{7}{8} \) to \( \frac{24}{3} \), which is very nearly one-third of the original weight. The same was the effect when I melted steel in the same circumstances, and also every kind of iron on which the experiment could be tried. But I have reason to think, that with a greater degree of heat than I could apply, the iron might have been kept in a state of fusion somewhat longer, and by that means have imbibed more than even one-third of its original weight.

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

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

On attempting to revive this calx of iron in inflammable air, a very new and unexpected appearance took place. Having put a piece of iron saturated with pure air into a vessel filled with inflammable air confined by water, the inflammable air disappeared and the metal was revived; but on weighing it, he found that \( \frac{2}{1} \) grains out of \( \frac{11}{4} \) had been lost, besides the \( \frac{7}{4} \) ounce-measures of inflammable air which had vanished. Considering all these circumstances, the Doctor had now no doubt that the two kinds of air had united and formed either fixed air or water; and with a view to determine this point, he repeated the experiment in a vessel where the inflammable was confined by mercury, both the vessel and mercury having been previously made as dry as possible. In these circumstances he had no sooner begun to heat the iron, than the air was perceived to diminish, and at the same time the inside of the vessel to become cloudy, with particles of dew that covered almost the whole of it. These particles by degrees gathered into drops, and ran down in all places, excepting those which were heated by the sun-beams. On collecting the water produced in this experiment, by means of a piece of filtering paper carefully introduced to absorb it, he found it to be as nearly as possible of the same weight with that which had been lost by the iron; and also in every experiment of this kind, in which he attended to the circumstance, he found that the quantity of inflammable air which had disappeared was about double that of the dephlogisticated air set loose in the operation, supposing that weight to have been reduced into air. Thus, at one time, a piece of this flag absorbed \( \frac{5}{2} \) ounce-measures of inflammable air, while it lost the weight of about three ounce-measures of dephlogisticated air, and the water collected weighed two grains. Another time a piece of flag lost 1.5 grains, and the water produced was 1.7 grains. In a third case, where \( \frac{6}{2} \) ounce-measures of inflammable air were reduced to 0.92 of a measure, the iron had lost the weight of 3.3 ounce-measures of dephlogisticated air, or nearly two grains.

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

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

As these experiments of Dr Priestley tend very much to throw some light on the composition of dephlogisticated air, we shall here give an account of some others made by Mr Cavendish, as well as those of Dr Priestley and the French chemists, upon water:

From all which it is concluded by the most celebrated philosophers and chemists, That dephlogisticated air is one of the constituent and elementary parts of water, inflammable air being the other; though the opinion is still contested by some foreign chemists.

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

To try whether any vitriolic acid was produced by the phlogistication of air, 50 ounces of distilled water were impregnated with the fumes produced on mixing 52 ounce-measures of nitrous air with a quantity of common air sufficient to decompose it. This was done by filling a bottle with some of this water, and inverting it into a basin of the same; and then by a syphon, letting in as much nitrous air as filled it half full; after which, common air was added slowly by the same syphon, till the nitrous air was decomposed. When this was done, the distilled water was further impregnated in the same manner till the whole quantity of nitrous air was employed. The impregnated water was sensibly acid to the taste; and on distillation yielded first phlogisticated nitrous acid, then water, and lastly a very acid liquor consisting of dephlogisticated nitrous acid. By saturation with salt of tartar, 87½ grains of nitre, without any mixture of vitriolated tartar, or other vitriolic salt, were obtained.

These experiments having proved unsuccessful, Mr Cavendish next proceeded to try the effects of exploding dephlogisticated and inflammable air together in close vessels. He begins with relating an experiment of Dr Priestley; in which, it was said, that on firing a mixture of common and inflammable air by electricity, in a close copper vessel holding about three pints, a loss of weight was always perceived, on an average about two grains, though the vessel was stopped in such a manner that no air could escape by the explosion. It is also related, that on repeating the experiment, in glass vessels, the inside of the glass, though clean and dry before, immediately became dewy; which confirmed an opinion he had long entertained, that common air deposits its moisture by phlogistication. The experiment, however, did not succeed with Mr Cavendish, at least with regard to the loss of weight; which never exceeded the fifth part of a grain, and commonly was nothing at all. In these experiments Quantity of inflammable air necessary to produce common air, the greatest care was taken to observe with accuracy the diminution of air by the explosion, and quality of the remainder; from which it appeared, that 423 measures of inflammable air were nearly sufficient to phlogisticate 1000 of common air, and that the bulk of air remaining after the explosion is very little more than four-fifths of the common air employed; whence he concludes, that "when they are mixed in this proportion, almost all the inflammable, and about one-fifth of the common air, lose their elasticity, and are condensed into the dew which lines the glass."

To examine more exactly the nature of this dew, 50000 grain-measures of inflammable air were burnt with about 2½ times the quantity of common air, and the burnt air was made to pass through a glass cylinder eight feet long and three-fourths of an inch in diameter, in order to depolish the dew. The two airs were conveyed slowly into this cylinder by separate copper pipes, passing through a brass plate which stopped up one end of the cylinder; and as neither inflammable nor common air can burn by themselves, there was no danger of the flame spreading to the magazines from which they were conveyed. Each of these magazines consisted of a large tin vessel inverted into another just big enough to receive it. The inner vessel communicated with the copper pipe, and the air was forced out of it by pouring water into the outer vessel; and in order that the quantity of common air expelled should be 2½ times that of the inflammable air, the water was let into the outer vessels by two holes in the bottom of the same tin pan; the hole which conveyed the water into that vessel in which the common air was confined being 2½ times as big as the other. In trying the experiments, the magazines being first filled with their respective airs, the glass cylinder was taken off, and water let by the two holes into the outer vessels, till the airs began to issue from the ends of the copper pipes; they were then set on fire by a candle, and the cylinder put on again in its place. By this means upwards of 135 grains of water were left in the cylinder, which had no taste nor smell, and which left no perceptible sediment on being evaporated to dryness; neither did it yield any pungent smell during the evaporation; in short, it seemed pure water. In one of his experiments a little foamy matter was perceived, but it was found to proceed from the luting. On repeating the experiment with dephlogisticated, instead of common air, the produce was nitrous acid.

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

Dephlogisticated Air.

Conclusion from these experiments.

Dr Priestley's experiments.

His opinion concerning the composition of water.

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

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

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

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

The experiments of the French philosophers were of the same nature with those of Mr Cavendish, but conducted on a larger scale. The inference drawn from them was the same with that already mentioned, viz. that dephlogisticated and inflammable air in all cases are the two constituent parts of water. This opinion is adopted by Mr Kirwan in his Treatise on Phlogiston.

"The experiments of Mr Cavendish, and of Mr Kirwan," says he, "appear to me to leave no room to man's conclusion, that when very pure dephlogisticated and inflammable air are inflamed, the product is mere water (A); for when these airs are employed in the proper proportion, only 0.02 of the mixture of both airs retains its aerial form. Now it is impossible to suppose that all the water obtained pre-existed in these airs; that is, that 49 parts in 50 were mere water.

Notwithstanding these positive conclusions, however, by some of the most respectable names in this country, the evidences adduced have been unsatisfactory to some French chemists; who maintain, that Messrs Cavendish, Priestley, and Kirwan, are totally mistaken with regard to the production of water from dephlogisticated and inflammable air; contending, that the water obtained had previously existed in the air, and was not originally produced in the operation. The fact, indeed, becomes somewhat dubious from some experiments related by Dr Priestley himself, and of which we shall now proceed to give an account." One consequence of the hypothesis in question is evident, that if water really be produced by the dephlagration of either dephlogisticated or common air with inflammable air, the quantity of liquid obtained ought to increase in proportion to the quantity of the two airs consumed, and that without any limitation. This, however, is not the case, as Dr Priestley has observed. He had succeeded indeed with scales of iron and copper, as has already been related; and in the experiment with the latter, the production of water was so copious, that when only 3½ ounce-measures of air were absorbed, the water stood in drops on the inside of the vessel, and some of these ran down it. Water was also procured by firing dephlogisticated and inflammable air from iron by the electric spark in a close vessel, an experiment similar to those made by Mr Lavoisier at Paris. In his first experiment he put 3.75 ounce measures of a mixture of air, of which one-third was dephlogisticated, and two-thirds inflammable air from iron, in a close vessel, and, after the explosion, found in it one grain of moisture; but on repeating the experiment with half as much dephlogisticated as inflammable air, he could perceive no sign of moisture. The greatest difficulty, however, which he says he ever met with respecting the preceding theory, arose from his never having been able to procure any water when he revived red precipitate in inflammable air, or at least no more than might have been supposed to be contained in the inflammable air as an extraneous substance.

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

"The difficulty, with respect to what becomes of the two kinds of air, was not lessened by the attempts which I made to collect all that I could from repeated decompositions of inflammable and dephlogisticated air in a close vessel. As I had produced water in this process when no more than a single explosion was made at a time, I thought that by continuing to make explosions in the same vessel, the water would not fail to accumulate till any quantity might be collected; and I intended to have collected a considerable part of an ounce. And as I should know exactly what quantity of air I decomposed, I had no doubt of being able to ascertain the proportion that the water and air bore to each other. With this view a mixture was made of a large quantity of air, one-third dephlogisticated and two-thirds inflammable, from iron and oil of vitriol.—But though I had a sensible quantity of water at the first explosion (in each of which between four and five ounce-measures of the mixture of air were used), I was surprised to perceive no very sensible increase of the quantity of water on repeating the explosions. Having therefore expended 48 ounce-measures of the mixture, the process was discontinued; and, collecting the water with all the care that I could, I found no more than three grains, when there ought to have been eleven.

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

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

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

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

Dephlogisticated Air.

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

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

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

Sect. IV. Of Phlogisticated Air.

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

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

In order to disprove this assertion, he introduced four ounces of mercury to 50 cubic inches of common air, propelling to calcine the metal by keeping it for several days in a heat almost equal to that which is necessary to make it boil. After the expiration of the appointed time, 45 grains of precipitate per se were formed, and the air in the vessel was diminished by about 1/4th of its volume. In this state it did not precipitate lime water; but instantly extinguished candles, and killed animals immersed in it; no longer affording any red vapours, or being diminished by mixture with nitrous air. On distilling the precipitate produced, about as much dephlogisticated air was obtained as had been left by the common air in the calcination; and by recombining this with the noxious air left in the vessel, he recomposed a fluid nearly of the same goodness with common air. Hence he draws the following conclusions:

* Doctrine of phlogiston opposed by the foreign chemists. Phlogisticated Air.

93 Composition of atmospheric air.

94 Effects of respiration on air.

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

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

From these experiments, and many others similar, it appears that the doctrine of phlogiston had been carried too far by Dr Priestley and other British philosophers, and that the air consists of two kinds of fluids; one perfectly salutary, and friendly in the highest degree to animal life; the other altogether unfit for it. These two appear incapable of being converted directly into one another by any process, natural or artificial: for though both are destructible, yet they are always converted into other substances; from which, indeed, either the one or the other may be extracted at pleasure by employing the proper methods. The strongest arguments in favour of the transmutation of phlogisticated air into that of a purer kind, were drawn from the purification of noxious air by vegetation, and agitation in water. In the former case, however, it has been observed in the last section, that this seeming purification is no other than an exchange of the one air for the other; the vegetables absorbing the phlogisticated, and emitting the deplogisticated air in its stead. With respect to the agitation in water, the matter remained more dubious; and it is only in the last volume of Dr Priestley’s treatise that we have any purified by account of this being accomplished by an emission of agitation in purer air from the water.—“In the infancy of my experiments,” says he, “I concluded, that all kinds of air were brought by agitation to the same state; the purest air being partially phlogisticated, and air completely phlogisticated being thereby made purer; inflammable air also losing its inflammability, and all of them brought into such a state as that a candle would just go out in them. This inference I made from all the kinds of air with which I was then acquainted, and which did not require to be confined by mercury, being brought to that state by agitation in a trough of water, the surface of which was exposed to the open air; never imagining that when the air in my jar was separated from the common air by a body of water, generally about twelve inches in depth (adding that within it that without the jar), they could have any influence on each other. I have, however, been long convinced, that, improbable as it then appeared to me, this is actually the case.”

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

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

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

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

The apparatus used in these experiments, was that represented Plate VIII. fig. 4, and consists only of a crooked glass tube, whose ends are plunged into quicksilver contained in two glasses, in the middle part of which the air is confined betwixt the two portions of quicksilver. The air was introduced by means of a smaller tube, fig. 5, the tube M of the former figure being filled with quicksilver, the bent end of which was introduced into a jar DEF, filled with the proper kind of air, and inverted in water. The end C being stopped by the finger, the quicksilver was thus prevented from falling out, let the tube be placed in what position it would, until this pressure was removed. Upon introducing the crooked tube into the jar in the position represented in the figure, and removing the finger from the orifice at C, the quicksilver would descend; and by stopping this orifice again, any quantity of the fluid may be allowed to run out, and the empty space of the tube will be filled with the air desired. Having thus got the proper quantity of air into the tube ABC, it was held with the end C uppermost, and stopped with the finger; and the end A, made smaller for that purpose, being introduced into the end of the bent tube M, the air, on removing the finger from C, was forced into that tube by the pressure of the quicksilver in the leg BC. Thus he was enabled to introduce any quantity he pleased of any kind of air into the tube M; and by the same means it was in his power to let up any quantity of soap-ley, or other liquor which he wanted to be in contact with it. In one case, however, in which he wished to introduce air into the tube many times in the same experiment, he made use of the apparatus represented fig. 6, consisting of a tube AB, of a smaller bore, a ball C and a tube DE of a larger bore. This apparatus was first filled with quicksilver; and then the ball C and the tube AB were filled with air, by introducing the end A under a glass inverted into water, which contained the proper kind of air, and drawing out the quicksilver from the leg ED by a syphon. After being thus furnished with air, the apparatus was weighed, and the end A introduced into one end of the tube M, and kept there during the experiment; the way of forcing air out of this apparatus into the tube being by thrusting down the tube ED, a wooden cylinder of such a size as almost to fill up the whole bore, and by occasionally pouring quicksilver into the same tube, to supply the place of that pushed into the ball C. After the experiment was finished, the apparatus was weighed again, which showed exactly how much air had been forced into the tube M during the whole experiment; it being equal in bulk to a quantity of quicksilver, whose weight was equal to the increase of weight of the apparatus. The bore of the tube M, used in these experiments, was about the tenth of an inch in diameter; and the length of the column of air occupying the upper part of the tube was in general from 4ths to 1½ inches.—In order to force an electrical spark through the tube M, it was necessary to place an insulated ball at such a distance from the conductor as to receive a spark from it, and to make a communication between that ball and the quicksilver in one of the glasses, while the quicksilver in the other glass communicated with the ground.

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

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

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

"This being premised, we may safely conclude, that in the present experiments, the phlogisticated air was enabled, by means of the electrical spark, to unite to, or form a chemical combination with, the dephlogisticated air, and was thereby reduced to nitrous acid, which united to the soap-ley, and formed a solution of nitre; for in these experiments the two airs actually disappeared, and nitrous acid was formed in their room; and as it has been shown, from other circumstances, that phlogisticated air must form nitrous acid when combined with dephlogisticated air, the abovementioned opinion seems to be sufficiently established. And a further confirmation is, that no diminution of air is perceived when the electric spark is passed either through pure dephlogisticated or through perfectly phlogisticated air; which indicates a necessity for the combination of the two in order to produce nitrous acid. It was also found by the last experiment, that the quantity of nitre produced was the same that would have been obtained from the soap-ley, had they been saturated with nitrous acid; which shows, that the production of the nitre was not owing to any decomposition of the soap-ley.

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

"As far as the experiments hitherto published extend, we scarcely know more of the nature of the phlogisticated part of the atmosphere, than that it is not diminished by lime-water, caustic alkalis, or nitrous air; that it is unfit to support fire or maintain life in animals; and that its specific gravity is not much less than that of common air: so that though the nitrous acid, by being united to phlogiston, is converted into air possessed of these properties; and, consequently, though it was reasonable to suppose, that part at least of the phlogisticated air of the atmosphere consists of this acid united to phlogiston; yet it might be fairly doubted whether the whole is of this kind, or whether there are not, in reality, many different substances confounded by us under the name of phlogisticated air. I therefore made an experiment to determine whether the whole of a given portion of the atmosphere could be reduced to nitrous acid, or whether there remains not a part of a different nature from the rest, which nature would refuse to undergo that change. For this purpose, I diminished a similar mixture of dephlogisticated and common air in the same manner as before, until it was reduced to a small part of its original bulk; after which some dephlogisticated air was added, and the spark continued until no further diminution took place. Having by these means condensed as much as I could of the phlogisticated air, I let up some solution of liver of sulphur to absorb the dephlogisticated air; after which only a small bubble of air remained unabsorbed, which certainly was not more than $\frac{1}{15}$th of the bulk of the phlogisticated air let up into the tube; so that if there is any part of the phlogisticated air of our atmosphere which differs from the rest, and cannot be reduced to nitrous acid, we may fairly conclude, that it is not more than $\frac{1}{15}$th part of the whole."

Though these experiments had shown, that the chief cause of this diminution of airs is the conversion of the phlogisticated kind into nitrous acid, it seemed not unlikely, that when any liquor containing inflammable matter was in contact with the air in the tube, some of this matter might be burnt by the spark, and thereby diminish the air. In order to determine this, the electric spark was passed through dephlogisticated air included between different liquors; and the result of the experiments was, that when dephlogisticated air, containing only \( \frac{1}{2} \)th part of its bulk of phlogisticated air, was confined between short columns of soap-lys, and the spark passed through it till no farther diminution could be perceived, the air lost \( \frac{4}{5} \)ths of its bulk; which is not a greater diminution than might very likely proceed from the decomposition of the small quantity of phlogisticated air contained in it, as the dephlogisticated air might easily be mixed with a small quantity of common air while putting into the tube. When the same dephlogisticated air was confined between columns of distilled water, the diminution was rather greater than before, and a white powder was formed on the surface of the quicksilver beneath: the reason of which, in all probability, was, that the acid produced in the operation corroded the quicksilver, and formed the powder; and that the nitrous air produced by that corrosion united to the dephlogisticated air, and caused a greater diminution than would otherwise have taken place. When a solution of litmus was used instead of distilled water, the solution soon acquired a red colour; which grew paler and paler as the spark was continued, till it became quite colourless and transparent. The air was diminished by almost one-half, and might perhaps have been further diminished had the spark been continued.

When lime-water was let up into the tube, a cloud was formed, and the air was further diminished by about one-fifth; the remainder was good dephlogisticated air. In this experiment, therefore, the litmus was, if not burnt, at least decomposed, so as to lose entirely its purple colour, and to yield fixed air; so that, though soap-lys cannot be decomposed by this process, yet the solution of litmus can, and so very likely might the solutions of many other substances be. But there is nothing in any of these experiments which favours the opinion of the air being at all diminished by means of phlogiston communicated to it by the electric spark.

**Sect. V. Of Fixed Air.**

The discovery of this kind of air is as old as Van Helmont; who gave it the name of *gas silvestre*, from its being emitted in great quantity by burning charcoal. Subsequent discoveries showed, that a fluid of the same kind was plentifully produced by fermenting liquor, in almost every kind of combustion, and naturally generated in vast quantity in mines and coal-pits, where it is known by the name of the *choak-damp*; that it exists in a concrete state in alkaline farts, chalk, limestone, the shells of marine animals, magnesia alba, &c., in a very large proportion, constituting one-half, and sometimes more of their weight; and that it might always be extracted from the atmosphere, in unlimited quantity, by exposing certain substances to it.

On examining the nature of this fluid, it was found to manifestly acid, that it has now obtained a place among these substances under the name of *aërial acid*; or, more improperly, *cretaceous acid*, from its being fixed air, contained in great quantities in chalk, as has been already mentioned.

Fixed air is the heaviest of all permanently elastic fluids, excepting those derived from the mineral acids. Mr. Kirwan determines it to be to common air as 1500 of fixed air to 1000, the barometer being at 29.85, the thermometer at 64, and the fixed air being extracted from calcareous spar by marine acid, whose specific gravity was 1.0145. He observes, however, that though this air was obtained in the driest manner possible, and that the globe which contained it appeared perfectly free from moisture; yet, when carried into a room 27 degrees colder, the inside of the globe was covered with dew, which soon formed visible drops.—In its concrete state, fixed air is one of the heaviest bodies in nature. Mr. Kirwan, in the 71st volume of the Philosophical Transactions, gives an account of his ingenious method of finding the specific gravity of fixed air in its fixed state, when combined with calcareous earth; from which it appears, that fixed air, in that state, is prodigiously concentrated, and, were it possible to exist by itself in that concentrated state, it would be the heaviest body known, gold and platinum excepted.

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

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

| Weight of the box in air | 607.65 | |-------------------------|--------| | Its loss of weight in water | 73.75 | | Weight of the box and lime in air | 1043.5 | | Weight of the lime singly in air | 435.85 | | Loss of weight of the box and lime in water | 256.5 | | Loss of weight of the lime singly | 182.3 |

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

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

Losses of 100 grs. of marble = \(\frac{100}{2717} = 36.8\) grs.

Losses of 55.92 grs. of calcareous earth = \(\frac{55.92}{2.39} = 23.39\) grs.

Losses of 11.66 grs. of water = \(\frac{11.66}{35.05} = 1.75\)

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

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

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

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

In considering this subject farther, it occurred to Dr Priestley, that his experiments, in which charcoal was used, lay open to an objection, that since dry wood, and imperfectly made charcoal, yield fixed air, it might be said, that all the elements of fixed air are contained in charcoal; and though this substance alone, even with the assistance of water, will not yield fixed air, this might be effected by treating it with other substances without their importing anything to it; especially as the inflammable air procured from charcoal by means of water, appears to contain fixed air when decomposed with the dephlogisticated kind. In order to expel all the fixed air from charcoal, he made a quantity of it from dry oak, and pounding it while hot, instantly mixed four measures of it with one of red precipitate, and, putting them into an earthen retort, got, with a heat no greater than what was sufficient to revive the mercury, a large quantity of air, half of which was fixed. Afterwards the proportion of fixed air was less, and at last no fixed air at all was obtained; but as the residuum was worse than the common atmosphere, he hence inclined to believe, notwithstanding Mr Cavendish's experiments, that phlogisticated air may be composed of phlogiston and dephlogisticated air. In another experiment he found a better proportion of charcoal and red precipitate. This was by mixing one solution of ounce of precipitate with the same quantity of perfect char- charcoal hot from the retort in which it was made. Putting these into a coated retort, he expelled from them, by a strong heat, about 30 ounce-measures of air, the whole of which was the purest fixed air, leaving only about one-fortieth part unabsorbed by water, and this almost perfectly phlogisticated.

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

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

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

"I had always concluded, that no fixed air could be procured by the decomposition of inflammable air which had been produced by mineral acids, because I had not been able to do it with that which I had got by means of vitriolic acid; but I learned from Mr Metherie, that this is peculiar to the vitriolic acid, the remains of which, diffused through the inflammable air, procured by it, he conjectures, may actually decompose the fixed air produced in the process. For, as I have hinted before, when the inflammable air is produced from iron by means of spirit of salt, there is a very perceptible quantity of fixed air when it is united with dephlogisticated air. When I decomposed these two kinds of air in equal quantities, they were reduced to about 0.5 of a measure, and of this not more than about one forty-ninth part was fixed air. This experiment ought, however, to be added to the other proofs of fixed air being produced by the union of dephlogisticated air and phlogiston.

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

"That dephlogisticated air actually enters into the composition of the fixed air, in this experiment, is evident from the weight of the latter, which far exceeds that of the charcoal dispersed in the process. For, in this last experiment, the weight of the fixed air produced was 4.95 grains. Consequently, supposing the charcoal to be wholly phlogiston, as it is very nearly so, fixed air may be said to consist of 3.45 parts of dephlogisticated..." gisticated air, and 1.5 of phlogiston; so that the dephlogisticated air is more than three times the proportion of phlogiston in it.—I must not conclude, however, without observing, that, in one experiment, I never failed to produce fixed air; though it is not easy to see how one of its supposed elements, viz. dephlogisticated air, could enter into it. This is by heating iron in vitriolic acid air. In one of these experiments, four ounce-measures of the vitriolic acid air were reduced to 0.65 of an ounce-measure; and of the quantity lost three and an half measures were fixed air absorbed by lime-water, and the remainder weakly inflammable.”

Fixed air, even when pure and unmixed, is remarkably altered by the electric spark, part of it being thus rendered immiscible in water. Dr Priestley, having taken the electric spark for about two hours in a small quantity of fixed air confined by mercury, found, that after the operation one-fourth of it remained immiscible with water; though, before it, only one-thirtieth part had remained unabsoled. The inside of the tube had become very black; which, in other experiments of a similar kind with vitriolic acid air, he had observed to arise from the adhesion of a small quantity of mercury supersaturated with phlogiston. In another experiment, in which the spark was taken an hour and ten minutes in about half an ounce-measure of fixed air, one-fifth remained unabsoled, and the standard of the residuum was 0.9; though, before the operation, only one-thirtieth part had been absoled, and the standard of the residuum was 1.0. In this experiment, also, he observed, that the air was increased about a twentieth part. On taking the electric spark an hour in half an ounce of fixed air, as much residuum was left as had remained in five times the quantity of the same fixed air in which no spark had been taken. This residuum was also much purer than that of the original fixed air, the standard being 0.8; whereas that of the original fixed air had been, as before, 1.0. On repeating the experiment, he found the residuum still greater, but equally pure; and, in this case, a good quantity of black matter was observed adhering to the tube. Having taken the spark in a small tube containing 1/3rd of an ounce-measure of fixed air, the inside of the tube was clouded with black matter, and in the bottom was a small quantity of yellowish matter resembling sulphur; the residuum was between one-fourth and one-fifth of the whole, and less pure than formerly. This circumstance he also supposes to be a proof that fixed air may be composed of phlogiston and dephlogisticated air. Pursuing this experiment, by taking the electric spark three hours in a small quantity of fixed air, he observed that it was first increased, and then diminished about one-eighth of the whole; the inside of the tube being very black on the upper part, and below the mercury very yellow, for the space of a quarter of an inch all round the tube; but this space had been above the mercury in the beginning of the operation. One-third of the air remained unabsoled by water; but so impure, that the standard of it was 1.8, or almost completely phlogisticated.—Varying the process by using water impregnated with fixed air instead of mercury, the quantity of air was much augmented by that which came from the water; but thus the far greater part of it was incapable of being absorbed by lime-water; and on this occasion he observed, that water impregnated with fixed air is a much worse conductor of electricity than the same fluid impregnated with mineral acids. On still varying the circumstances of the experiment, by using common water instead of that which had absorbed fixed air, he found that the quality of the residuum was evidently better than that of the original fixed air.

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

With regard to the quantity of fixed air which may be expelled from different substances, Dr Priestley observes, that from seven ounces of whiting, the purest calcareous substance we are acquainted with, he expelled by heat 630 ounce-measures of air; by which means the whiting was reduced to four ounces. One third of this was somewhat phlogisticated; the standard being 1.36 and 1.38. Repeating the experiment, he obtained 440 ounce-measures of air from six ounces of whiting; about one-half of which was fixed air, and the remainder of the standard of 1.4. On moistening some calcined whiting with water impregnated with vitriolic acid air, he obtained 90 ounce-measures; of which the first portions were three-fourths fixed air, and the standard of the residuum 1.5; the latter had less fixed air, and the standard of the residuum was 1.44. The whiting was rendered black and hard, but became soft and white with spirit of salt. Three ounces and a quarter of lime fallen in the air, yielded 375 ounce-measures; of which about one-fifth was fixed air, and the standard of the residuum 1.4. Four ounces of white lead had yielded 240 measures of air when the retort melted. The residuum of the first process was one-third, the standard 1.36; and of the last the standard was 1.28, that with the common atmosphere being 1.23. Two ounces and three quarters of wood-ashes yielded, in a very strong heat, 430 ounce-measures of air; of the first portion of which one-tenth, of the second one-third, and of the third one-half, was fixed air. The standard of the residuum of the first portion was 1.6, and of the second 1.7. It extinguished a candle; so that the air came properly from the ashes, and not from any remaining particles of the charcoal mixed with them. After the process, the ashes weighed 839 grains; but by exposure to the air for one day, the weight was increased to 842 grains; and, perhaps with more heat than before, yielded 50 ounce-measures of air; of which about one-eighth was fixed air, and the standard of the residuum 1.38 and 1.41. A candle burned in this residuum, and the ashes were reduced to 789½ grains. Two ounce-measures of Homberg's pyrophorus burned in the open air, and then distilled in a retort, yielded 144 ounce-measures of air; of which one-half at first was fixed air, but at the last very little. The residuum of the first portion extinguished a candle, but that of the last burned with a blue lambent flame. The standards of both with nitrous air were about 1.8. The pyrophorus was then kept two days in the retort, with the mouth immersed in mercury; after which, on being taken out, it burned as strong as ever. Immediately before the burning, it weighed 428 grains; immediately after it, 449; but being spread thin and exposed to the atmosphere for a night, the weight was increased to 828 grains; though, on being well dried, it was again reduced to 486. Subjecting it to a greater heat than before, the matter yielded 110 ounce-measures of air; the first portions of which were half fixed air, but the last contained very little, and burned with a blue lambent flame. It was then reduced to 396 grains. The experiment was then repeated with a quantity of pyrophorus, which would not take fire in the open air; and on heating this substance in an earthen retort, five-sevenths of the first part of the produce was fixed air; but this proportion gradually diminished; till at last nine-tenths of the whole was inflammable air, burning with a lambent blue flame. This inflammable air being decomposed with an equal quantity of dephlogisticated air, yielded 0.86 of a measure of fixed air. Another quantity of pyrophorus, which burned very well, and which by exposure to the atmosphere had gained 132 grains, being again exposed to heat in an earthen retort, gave 180 ounce-measures of air; three-sevenths of the first portion of which was fixed, and the rest phlogisticated air; but afterwards only one-half was fixed and the rest inflammable, burning with a lambent blue flame; and at last it was wholly inflammable. This pyrophorus took fire again after being poured out of the retort, but not without the assistance of external heat. It had been red-hot through the whole mass at the first burning, and the surface was covered with white ashes; but all the inside was as black as ever it had been. Four ounces of dry ox-blood yielded 1200 ounce-measures of air, and it was conjectured that not less than 200 measures had escaped. It contained no fixed air. The first portion burned with a large lambent white flame, the middle portion fainter, and the last was hardly inflammable at all. The remaining coal weighed 255 grains, and was a good conductor of electricity.

Sect. VI. Inflammable Air.

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

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

The great quantity of inflammable air produced in Meteor warm climates has given occasion to some philosophers to suppose, that it may possibly have some share in producing certain atmospherical meteors. The weak lightnings without any explosion, which are sometimes perceived near the horizon in serene weather, are by them conjectured to proceed from inflammable air fired by electric explosions in the atmosphere. Mr Volta supposes that the ignis fatui are occasioned by the inflammable air which proceeds from marshy grounds, grounds, and is set on fire by electric sparks: but these phenomena can be accounted for in a more probable manner from the action of the electric fluid itself.

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

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

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

In extracting air from solid substances, the steam of water was always necessary; and thus inflammable air was produced from a great number of different ones. From sulphur treated in this manner in an earthen tube, inflammable air was obtained of a nature similar to that from oil of vitriol and iron. From arsenic, the produce was one-seventh of fixed air; but all the rest strongly inflammable, with a smell scarcely distinguishable from that of phosphorus. Twenty ounce-measures of this air weighed 4½ grains less than an equal quantity of common air. Both these experiments, however, were very troublesome, on account of the volatility of the matters, which sublimed and choked up the tubes. From two ounces of the feaces of iron, or firing cinder, which he has found to be the same thing, Dr Priestley obtained 580 ounce-measures of air; one-tenth of the first part of which was fixed air, but afterwards it was all inflammable. Forty ounce measures of this air weighed two grains more than an equal quantity of common air. From charcoal exposed to the red-hot flame of water, inflammable air was procured in great quantities. From ninety-four grains of perfect charcoal, that is, prepared with a strong heat so as to expel all fixed air from it, and 240 ounces of water, 840 ounce-measures of air were obtained, one-fifth part of which was fixed air; and the inflammable part appeared likewise, by decomposition, to have a quantity of fixed air intimately combined with it.—Three ounces of bones burnt black, and treated in this manner in a copper tube, yielded 840 ounce-measures of air; the water expended being 288 grains, and the bones losing 110 grains of their weight. This air, he observes, differs considerably from that of any other kind of inflammable air; being in several respects a medium between the air procured from charcoal and that from iron. It contains about one-fourth of its bulk of uncombined fixed air, but not quite one-tenth intimately combined with the remainder. The water that came over was blue, and pretty strongly alkaline; owing to the volatile alkali not having been totally expelled by the heat which had reduced the bones to blackness.

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

The following are the results of some of the Doctor's experiments.—Two hundred and sixty-seven grains, added to the weight of a quantity of iron, produced a loss of 336 grains of water, and an emission of 840 ounce-measures of air; and in another experiment, 140 grains added to the weight of the iron produced a loss of 240 grains of water, and the emission of 420 ounce-measures of air. "The inflammable air produced in this manner (says he) is of the lightest kind, and free from that very offensive smell which is generally occasioned by the rapid solution of metals in oil of vitriol; and it is extricated in as little time in this way as it is possible to do it by any mode of solution. The following experiment was made with a view to ascertain the quantity of inflammable air that may be procured in this manner from any given quantity of iron. Nine hundred and sixty grams of iron, when dissolved in acids, will yield about 800 ounce-measures of air; but, treated in this manner, it yielded 1054 measures, and then the iron had gained 329 grains in weight" (a).

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

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

(a) In these experiments, the Doctor seems not to have supposed that any particular kind of water was necessary for this production of inflammable air: but in the Memoirs of the Philosophical Society at Haarlem, it is asserted by Dr Deiman and M. Paets Van Troostwyk, that the experiment will not succeed when boiled or distilled water, or any other than that containing fixed air, is made use of; and to this air they attribute the calcination of the iron and production of inflammable air. This assertion, however, is contrary to what we find related by Mr Kirwan. See no 138. any acid: yet an acid vapour may be easily diffused through it, and may perhaps in many cases be obsti- nately retained by it, as no kind of air seems to be ca- pable of so great a variety of impregnations as inflam- mable air is."

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

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

Although from these experiments it did not appear that water was in any way essentially necessary to the production of this kind of inflammable air, it appeared manifestly to be so in the following: "At the time (fays he) when I dispersed any quantity of charcoal with a burning lens in vacuo, and thereby filled my receiver with nothing but inflammable air, I had no suspicion that the wet leather on which my receiver stood could have any influence in the case, while the piece of char- coal was subject to the intense heat of the lens, and placed several inches above the leather. I had also procured inflammable air from charcoal in a glazed earthen retort for two whole days successively, during which it continued to yield it without intermission. Also iron-filings in a gun-barrel, and a gun-barrel it- self, had always given inflammable air whenever I tried the experiment. These circumstances, however, de- ceived me, and perhaps would have deceived any other person; for I did not know, and could not have be- lieved, the powerful attraction between water and char- coal or iron, when the latter are intensely hot. They will find, and attract it, in the midst of the hottest fire, and through any pores that may be left open in a retort; and iron-filings are seldom so dry as not to have as much moisture adhering to them as is capable of enabling them to give a considerable quantity of in- flammable air. But my attention being now fully awakened to the subject, I presently found that the circumstances above mentioned had actually misled me; I mean with respect to the conclusion which I drew from the experiments, and not with respect to the ex- periments themselves, every one of which will, I doubt not, be found to answer, when properly tried.

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

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

This difference was for some time attributed to a quantity

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

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

Equal parts of dephlogisticated air and the inflammable kind produced from spirit of wine, were reduced to one measure, and by washing in lime-water to 0.6 of a measure. The standard of the residuum was 1.7. In another experiment, in which the vapour of the spirit of wine had passed through a tube filled with bits of crucibles, the first diminution was to 1.6, the second to 1.4, and the standard of the residuum was to 1.84; but in a third, the first diminution was to 1.2, the second to 0.9. Air procured by steam from red-hot platinum was reduced to 0.72 of a measure, and the standard of the residuum was 0.9. It contained no fixed air. Air from brimstone, with an equal part of dephlogisticated air, was diminished to 0.6, and no fixed air was found in the residuum. Its standard was 0.95. With inflammable air from arsenic, the first reduction was to 1.15, the second to 0.95. The standard was 0.82. With the inflammable air procured by a decomposition of alkaline air, the diminution by explosion was to 0.65; and no fixed air was contained in the residuum; the standard of which was 0.8. Inflammable air from ether resembles that from spirit of wine. The first diminution was to 1.36, the second to 1.2; and the standard was 1.9.

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

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

Inflammable air procured by distilling some rich mould in a gun-barrel had a very offensive smell, like that procured from putrid vegetables; it contained one-twentieth part of uncombined fixed air. When this was separated from it, and the remainder decomposed with dephlogisticated air, the first diminution was to 1.45, the second to 0.67, and the standard of the residuum was 0.6. The air procured from cast-iron has likewise a peculiarly offensive smell; and, on this account, the Doctor imagined, that it might contain more phlogiston than common inflammable air, so as to absorb more dephlogisticated air than the other. But this conjecture did not appear to be well founded; for on exploding it with dephlogisticated air in the proportions

giltton, informs us, that in his experiments he used "inflammable air extracted from clean newly-made filings of soft iron, in the temperature of 59°, by vitriolic acid whose specific gravity was 1.0973, and obtained over mercury, having very little smell, and what it had being very unlike the usual smell of inflammable air."—The weight of this air, when the barometer stood at 29.9, and the thermometer at 60°, was found to be to that of common air as 8.43 to 1000; and, consequently, near 12 times lighter. proportions already mentioned, the diminution was the same as with inflammable air produced from the malleable kind, viz. 1.56.

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

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

Inflammable air, when produced in the driest way possible, is exceedingly light, as has been already observed: but Dr Priestley has found, that by standing No. 5. Inflammable Air), it would be more natural to suppose, that water, like fixed air, consists of phlogiston and dephlogisticated air, in some different mode of combination.

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

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

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

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

Plants in general grow tolerably well in inflammable air, and the willow plant has been observed to absorb great quantities of it. Its inflammability is not diminished by the putrefaction of animal substances, nor does their putrefaction seem to be retarded by it. Animals confined in it are killed almost as soon as in fixed air; but insects, which can live a considerable time in phlogisticated air, live also a considerable time in this kind of air; but at last they become torpid, and appear to be dead, though they will still recover if removed into the open air. Mr Cavallo relates, that the Abbé Fontana, having filled a large bladder with inflammable air, began to breathe it in his presence; after having made a very violent expiration, in which case the effects are most powerful. The first inspiration produced a great oppression in his lungs, the second made him look very pale, and the third was scarcely accomplished when he fell on his knees through weakness. Birds and small quadrupeds, inclosed in small vessels of this air, died after a very few inspirations. Lastly, inflammable air appears to have a smaller share of refractive power than common air; for Mr Warfle informs us, that having placed an hollow triangular prism, of which the angle was 72 degrees, so as to half cover a large object-glass in one of Mr Dollond's perspectives, and to turn round as to make the frame of a window, at the distance of 1280 feet, seen partly through the prism and partly through common air, appear undivided. The inflammable air was then blown out of the prism, but no part of the apparatus was moved; when the frame of the window seen through the object-glass and the prism as before, seemed to separate about four inches.

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

Sect. VII. Sulphurated Inflammable Air.

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

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

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

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

Sect. VIII. Of Alkaline Air.

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

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

In prosecuting his experiments on alkaline air, Dr Proofs of Priestley concluded that it contains phlogiston, both from its being convertible into inflammable air by electric explosions, and likewise from its reviving the calcines of metals. In attempting to ascertain the quantity of lead revived in alkaline air, he met with two difficulties; the first, on account of some part of the calx being blackened and imperfectly revived; the second, that the lead completely revived was dissolved by the mercury employed to confine the air. To prevent this last inconvenience, he put the powdered masticot (the substance he chose to employ on this occasion) into small earthen cups, contriving to place them with their mouths upwards, in such a manner, that when the lead was revived by means of a burning lens, it would remain in the cup, and not mix with the mercury which supported it. The proportions of metal then revived, were six grains of lead in three ounce-measures, 16½ in three measures and an half, 13 in two and an half, and 12 in three and three-fourths; but the experiment on which he laid the greatest stress, was that in which 26½ grains of lead were revived in 7½ ounce-measures of alkaline air. In this proportion, 100 ounce-measures of alkaline air would revive 352 grains of lead; but an equal quantity of inflammable air from iron would have revived 480 grains of metal. This deficiency appeared somewhat surprising to the Doctor, considering that alkaline air is resolved into more than twice its bulk of the inflammable kind; though it is possible, that inflammable air from iron may contain more phlogiston than that into which alkaline air is resolvable.

On heating red precipitate in alkaline air, the mercury was revived as in other cases, and a considerable quantity of water was produced, though none appears. on reviving it with common inflammable air. "It has even (says he) run down in drops in the inside of a vessel which contained five ounce-measures of the air; and a considerable quantity of dephlogisticated air was found in the residuum." On throwing the focus of the lens on red precipitate, inclosed in this kind of air, till three measures of it were reduced to two, water was produced as usual, and the standard of the residuum was 1.7. In another experiment, a violent explosion took place before he could observe whether any water was produced or not.

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

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

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

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

out there being any proper decomposition of the air."

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

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

**Sect. VIII. Of Nitrous Air.**

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

In the production of this kind of air, great differences are perceived by a diversity in the strength of the acid. Thus, if we dissolve copper in strong nitrous acid, no nitrous air is produced, though the same materials will yield air in great quantity by the mere diffusion of water to dilute the acid. This is very properly explained by Doctor Priestley, from the property nitrous acid has of attracting phlogiston, which is evident from what happens in the solution of mercury. When strong spirit of nitre is poured upon this metal, the solution soon begins, and is very rapid, yet not a single bubble of elastic fluid is produced; but in a short time the acid next to the mercury is changed of an orange colour, which is an indication of its having acquired phlogiston, probably from the nitrous air. Nitrous air which is decomposed the moment it is formed, and before its particles are united into visible bubbles. The bubbles of air indeed break through the coloured acid, but they disappear the moment they come in contact with the pale-coloured acid. As soon as the whole quantity of acid has assumed the orange colour, nitrous air escapes from it in considerable quantity; but the mixture of water deprives the acid of its power of decomposing nitrous air. The strong and pale-coloured nitrous acid ought to be diluted with at least two or three parts of water to one of the acid, for the easy production of nitrous air from copper and mercury.

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

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

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

Nitrous air seems to be the most fatal to animal life of any. Even insects, which can bear phlogisticated and inflammable air, generally die the moment they are put into it. Frogs, snails, and other animals which do not respire very frequently, die in a few minutes, and generally do not recover even when taken out of this noxious fluid before they are dead. Plants perish very soon in nitrous air, and even in common nitrous air saturated with nitrous air; but Dr Priestley informs us, that "though in general plants die almost immediately in water impregnated with nitrous air, yet in one case of this kind, when the superfluous nitrous air was let out under water, so that no part of it was decomposed in contact with the water, the plant grew in it remarkably well."

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

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

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

One of the most remarkable properties of nitrous Diminished air, is its diminution with deplogisticated air; by deplogistic which means it becomes a test of the quantity of that cated air, kind of air contained in the atmosphere. With pure deplogisticated air, the diminution is almost nothing, at the same time that some quantity of nitrous acid is reproduced by the decomposition of the nitrous air; but as our atmosphere is always mixed with a considerable quantity of phlogisticated air, on which nitrous Nitrous air has no effect, the diminution in this case is never so considerable. Upon this principle the Eudiometer is constructed.

Another very remarkable property of nitrous air is its strong antiseptic power; in so much that animal matters may, by its means, be preserved for many months without corruption. This property, it was thought, might have been extremely useful on many occasions; but Dr Priestley, after a number of experiments on the subject, concludes in the following manner. "Nitrous air will indeed preserve meat from putrefaction; but after long keeping, it becomes very offensive both to the nostrils and palate, though the smell is not altogether that of putrefaction; and indeed the substance continuing quite firm, it could not be properly putrid."

Having formerly experienced the remarkable antiseptic power of nitrous air, I proposed an attempt to preserve anatomical preparations, &c., by means of it; but Mr Kay, who made the trial, found, that, after some months, various animal substances were shrivelled, and did not preserve their forms in this kind of air."

The specific gravity of nitrous air, as well as of other kinds, has been ascertained by Mr Kirwan. As it corrodes metals, he endeavoured to find its weight by comparing the loss sustained by the materials which produce it. Thus he found, that 14 grains of the materials produced 38.74 inches of nitrous air; and, consequently, by proper calculation, that the specific gravity of nitrous air is to that of atmospheric air as 195 to 1000.—"If this air (says he) had been obtained over water, or in strong heat, its weight would probably have been very different; as it is liable to be mixed with phlogisticated air, nitrous vapour, and a variable quantity of water. Nitrous vapour would render it heavier, and phlogisticated air or water probably lighter."

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

"In reviewing the experiments I had formerly made on this kind of air, I could not recollect any of them in which the pure nitrous acid was produced, excepting that with deplogisticated air, besides the experiment in which it was decomposed by the electric spark; which furnishes a strong objection to this hypothesis." To ascertain the matter more fully, the following experiments were made.

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

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

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

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

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

"I have formerly observed, how readily nitrous air is diminished by taking the electric spark in it. This experiment I have frequently repeated, in order more particularly..." Nitrous Air, particularly to ascertain the quantity and quality of the residuum. In one experiment half an ounce of nitrous air was reduced, an less than half an hour, to one quarter of its bulk. One-fourth of the residuum was still nitrous, and the rest phlogisticated. Taking the electric spark in a quantity of nitrous air till it was diminished to one-third, the whole was completely phlogisticated, not affecting common air at all, and extinguishing a candle. A white matter was formed with the mercury over which the spark was taken, which made the water admitted to it extremely turbid. In another process, the electric spark was taken in a quantity of nitrous air till it could be no more diminished, when it was reduced in bulk in the proportion of 10½ to 24. Letting it stand all night upon the mercury, it was increased in the proportion of 11½ to 24; seemingly by the acid uniting to the mercury and generating more nitrous air, since it had that smell. No water appeared after the process; and the water admitted to it acquired no acid taste, but an astringent one like that of water impregnated with nitrous air. There was a white powder formed, as in the former experiments.—To try if it were possible to make water imbibe the acid from the nitrous air, the electric spark was taken in it, with a small quantity of water over the mercury. But even this water did not acquire any acid taste, but only an astringent one.”

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

Sect. IX. Of Dephlogisticated Nitrous Air.

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

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

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

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

As the process for obtaining dephlogisticated nitrous air by means of iron is very tedious, the Doctor endeavoured to find another which might be attended with less inconvenience. This he accomplished by dissolving turnings of iron in a dilute solution of copper in nitrous acid (the same that remains after the production of nitrous air), mixing it again with an equal quantity of water. Without this precaution, he tells us, that though the iron will at first be acted upon very slowly, yet the mixture will at length grow so hot as actually to boil, and the process will be exceedingly troublesome; however it will be necessary, previous to any attempt to dissolve the iron, to heat the solution of copper, in order to expel all the nitrous air and superfluous Dephlogisticated nitrous air is absorbed by water almost as readily as fixed air, and in considerable quantity; the liquid taking up about one-half its bulk of air. After being thus saturated, the whole quantity of dephlogisticated nitrous air may be expelled pure by heat, and is easily received in vessels containing mercury. It was likewise observed, that as this kind of air much resembles fixed air in its properties of being imbued by water, and expelled again by heat, it resembles it also in this farther property, that all the air which has been actually incorporated with the water will not be imbued by water again. But the proportion of this part is three or four times greater than the corresponding part of fixed air; it is also considerably more phlogisticated. Water impregnated with it very soon parts with it again on being exposed to the atmosphere.—It discovers not the smallest trace of containing either acid or alkali. Its specific gravity is less than that of common air. On heating red precipitate in this kind of air, pure dephlogisticated air was produced without affecting, or being affected by, the nitrous air. Repeating the experiment with malleable iron, the quantity of it was enlarged, and the whole phlogisticated, without any mixture of fixed air. By heating bits of clean crucibles or retorts in this kind of air, it seemed to approach in quality to common atmospheric air; and the effects were always found to be the more considerable the longer the process was continued. On attempting, however, to determine whether this change in the constitution of dephlogisticated nitrous air was occasioned by means of heat or light, he heated it in earthen tubes; but found, that though these were glazed both on the outside and inside, and seemed perfectly air-tight both before and after the experiment, the air had escaped. By the electric spark it was rendered wholly immiscible with water, and brought to the standard of 1.45; so that the Doctor had no doubt of its being respirable. Yet this kind of air, though it admits a candle to burn so well in it, will not kindle pyrophorus, though the nitrous air from which it is produced would instantly set it on fire.

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

§ 1. Vitriolic acid Air.—This is always a combination of vitriolic acid with phlogiston, and consequently may be procured from any mixture of that acid in its highly concentrated state with phlogistic matters. Hence it is obtained from all the metals, gold and platinum excepted, on boiling them with strong oil of vitriol. It is also procurable from the same acid rendered black by any phlogistic matter. No greater heat is required to expel this kind of air than that produced by the flame of a candle. It is the heaviest of all aerial fluids, next to fluor acid air, being to common air as 2265 to 1000. Dr Priestley informs us, that a quantity of vitriolic acid thus impregnated with phlogiston, will yield many times more air than an equal quantity of the strongest spirit of salt.—When the vitriolic acid air is produced in great plenty, the top of the phial in which it is generated is commonly filled with white vapours. The air has also the same appearance as it is transmitted through the glass tube; and it is sometimes discoverable in the recipient. When such substances are put to the oil of vitriol as cause a great effervescence with that acid, care should be taken to add them by very small quantities at a time, and likewise to apply the heat by very slow degrees, lest the rapid production of air, and the heat attending it, should break the vessels. It is most equably produced by using strong oil of vitriol and charcoal; but in most cases the production of vitriolic acid air is attended with that of inflammable, and sometimes fixed or phlogisticated air. With ether about one-half of the first produce is inflammable; but the quantity lessens as the process goes on. The Doctor observed, that, when quicksilver was used, the acid was not turned black, as in other experiments of the like nature. He also observed, that iron yielded a little inflammable air together with the acid gas; but that the elastic fluid produced when zinc was used, contained about two parts of inflammable and one of acid air. Copper, silver, and lead, when heated in vitriolic acid, yield the purest vitriolic acid air, without any mixture of inflammable air; but the lead yields only a very small quantity, and requires a great degree of heat. It is procured in the greatest abundance from the fumes of burning sulphur, and is then called the volatile vitriolic, or sulphurous acid; for an account of the properties of which, see Chemistry, (Index).

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

The most remarkable property of this vapour is, that its colour may be made more or less intense by the mere circumstance of heat. It may be confined by being placed in vessels with ground-stoppers, or in tubes hermetically sealed, and thus exposed to the action of heat; in which case it will be found, that the colour of the vapour becomes considerably more intense in proportion as the glass vessel containing it is more or less heated; and that, on the contrary, the intensity of the colour diminishes as it is cooled. "It seems probable (says Dr Priestley), that if this vapour was not confined, but had room to expand itself, it would become colourless with heat. This at least is the case when it is combined with water. The phenomena I refer to are very common in the process for making dephlogisticated air, in which I first observed them. But the same things are observable in the process for producing any other kind of air in which much spirit of nitre is made use of; and likewise constantly in the common process for making spirit of nitre itself. It is, that when the heat is moderate, the vapour within the..." the glass tube or retort is red; but that, as the heat increases, it becomes transparent." The Doctor having observed that red lead, impregnated with nitrous vapour, may be preserved a long time without deliquescing or losing its acid, made use of a composition of this kind for procuring the nitrous vapour with which he filled his tubes. By imbibing this vapour the medium lost its red colour and became white. "I put (says he) a small quantity of this white minium into a glass tube closed at one end; then holding it to the fire, make it emit the red vapour till the whole tube is filled with it; and having the other end of the tube drawn out ready for closing, as soon as the vapour begins to issue out of that end, I apply my blowpipe and seal it. By this means I conclude that the tube is filled with a pure red vapour, without any mixture of nitrous air, and perhaps common air also." For a further account of the properties of nitrous acid air, see Chemistry, (Index.)

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

After the marine acid has yielded all the air that can be expelled from it, it is extremely weak, so that it can but barely corrode iron. The gas itself is considerably heavier than common air, the specific gravity of the two being in the proportion of five to three; a cubic inch weighing 0.654 grains. It is very fatal to animal life, but less so than pure nitrous air; for flies and spiders live longer in marine acid than in nitrous air. In dipping a candle into a jar of this air the flame is extinguished; but the moment before it goes out, and also when it is afterwards first lighted again, it burns with a green or light-blue flame, like that of common salt thrown into a fire. Its diminution by the electric spark is barely perceptible. Ice is dissolved by it as fast as if it touched a red-hot iron. It is partly absorbed by almost every substance containing phosphorus, and the remaining part becomes inflammable. Oil of olives absorbs it very slowly, and oil of turpentine very fast; by which they both become almost black, and the remainder of the air is inflammable. Essential oil of mint absorbs marine air pretty fast, becoming brown, consistent, and so heavy as to sink in water; and its smell is in great measure fluor acid altered. Ether absorbs it very fast, and has its colour air, &c., altered by the impregnation, becoming first turbid, then yellow, and at last brown. The air over the ether is strongly inflammable. A small bit of phosphorus changed smoked and gave light in this acid air; and the elastic inflammable fluid was but little diminished in twelve hours. On malleable air, the admixture of water, about four-fifths of the gas were absorbed, and the rest was inflammable. This change was also effected by a great number of other substances: some of which, however, required a considerable time to produce their effect; such as crusts of bread not burned, dry wood, dry flesh, roasted pieces of beef, ivory, and even flints. See Chemistry, (Index.)

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

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

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

Sect. XI. Of Hepatic Air.

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

**Sect. XII. Of Atmospheric Air.**

The two component parts of our atmosphere, viz., deplogificated and phlogificated air, have been so fully treated of under their respective sections, that little remains to be said in this place, excepting to determine the proportion in which they are usually met with in the common air. The only regular set of experiments which have been made on this subject are those of Mr Scheele. He constructed an eudiometer, consisting of a glass receiver, which could contain 34 ounces of water, and a glass cup containing a mixture of one pound of iron-filings, and an equal weight of flowers of sulphur moistened; which cup standing upon a glass supporter, was inserted in the former receiver, which, when this was in it, could contain 33 ounces of water. To the outside of the glass tube or receiver, was affixed a slip of paper, to the height of a third of the tube, containing 11 divisions, each corresponding to one ounce of water. This paper was varnished over with oil varnish, to prevent its being spoiled by water. The whole then was placed in water, which gradually rose as the air was diminished. This mixture would serve four times before the power of diminishing air was lost. He carefully compared the height of the air therein with the barometer and thermometer, both before and after the experiment; in eight hours the experiment was completed. With this instrument he examined the goodness of the common air in Stockholm every day for a whole year, and found the diminution never to exceed $\frac{1}{10}$, nor to fall short of $\frac{9}{10}$; so that upon a medium it may be estimated at $\frac{9}{10}$. During the months of January and February it was $\frac{9}{10}$. The 23rd of March it was $\frac{9}{10}$, though the atmospheric cold increased, and the barometer stood higher than before. The 19th of April it was $\frac{9}{10}$, though the barometer and thermometer did not vary, and so stood till the 21st. In May and June it stood between $\frac{8}{10}$ and $\frac{9}{10}$. The 30th of July it stood at $\frac{9}{10}$. From the 3rd to the 15th of September at $\frac{9}{10}$. The 6th of October at $\frac{9}{10}$, during a high storm; but after it stood between $\frac{8}{10}$ and $\frac{9}{10}$, till the 4th of November, when it fell to $\frac{8}{10}$, and continued between $\frac{8}{10}$ and $\frac{9}{10}$ to the 20th, when it rose to $\frac{9}{10}$. The 21st it fell to 8, and stood between $\frac{7}{10}$ and $\frac{8}{10}$ till the 8th of December, when it rose to $\frac{9}{10}$; and from thence to the 31st it stood between $\frac{8}{10}$ and $\frac{9}{10}$.

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

**Sect. XIII. Of the artificial Production of Airs of different Kinds.**

§ 1. Fixed Air, or Aerial Acid. The artificial methods of producing this are principally three, viz. by fermentation, by heat, and by acids.

(1.) By fermentation. When vegetable or animal substances, especially the former, are fermented, they yield a great quantity of fixed air. In breweries, on the surface of the fermenting liquor, there is always a stratum of fixed air reaching as high as the edge of the vats; so that if these vessels are deep, and the fermenting liquor much below their edges, the above-mentioned stratum may be some feet in thickness. The same phenomenon is observable in the fermentation of wines in general; and it is owing to the production and elasticity of fixed air, that fermenting liquors, when put into close vessels, often burst them with great violence. The case is the same whatever substance it is that undergoes the vinous fermentation, though the quantity of fixed air produced is not the same in all substances, nor even in the same substance at different times. From 42 cubic inches of beer Dr Hales obtained 639 cubic inches of air in 13 days. From a quantity of sugar undergoing Of Artificial Air undergoing the vinous fermentation, Mr Cavendish obtained so much fixed air, that out of 100 parts of the former 57 appeared to have been volatilized and converted into fixed air.

But though a vast quantity of fixed air escapes during this process of fermentation, a very considerable portion still remains united with the fermented liquor, and to this it owes all its briskness and agreeable pungent acidulous taste; for when the fixed air is totally evaporated, the liquor becomes entirely 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 phial (fitted with a ground stopple and tube), capable of containing 1½ 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:

| Madeira Port of six years old | 1½ oz. | |-----------------------------|--------| | Hock of five years measure | | | Barrelled claret | | | Tokay of 16 years | | | Champagne of two years | | | Bottled cider of 12 years | |

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

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

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

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

During this production or extrication of fixed from atmospherical air, the latter is commonly supposed to be considerably diminished, though Mr Lavoisier and Mr Scheele have now rendered that opinion doubtful. If a piece of charcoal be burned by throwing the focus of a lens upon it when contained in a glass receiver inverted in water, after the apparatus is cooled, the water will have mounted a small way into the receiver. The diminution, however, is limited, and depends on several circumstances. Dr Hales has observed, that, in equal receivers, the air suffers a greater diminution by burning large candles than small ones; and likewise that, when equal candles are made use of, the diminution is greater in small than in large receivers. The cause of this phenomenon probably is, that the air contained in the receiver cannot all come into contact with the flame of the candle; whence, as soon as the air which is nearest the flame becomes contaminated, the candle is extinguished. Thus the author of a Concise Treatise on the Various Kinds of Permanently Elastic Fluids, has diminished the air of an inverted receiver one fifth 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 thirty-sixth only; Dr Hales found it to be diminished of one twenty-fifth 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 Of Artificial one grain of phosphorus, when the experiment was tried with a receiver inverted in water, upon the surface of which a small quantity of oil had been introduced; but when the receiver was inverted in quicksilver, the absorption was constantly between two one-fourth and two three-fourth inches for each grain. Mr Cavallo mentions his having often repeated the experiment of burning phosphorus in a glass tube inverted in water, by applying the closed part of the tube, wherein the phosphorus was contained, to a pretty strong fire, when he always observed that the utmost diminution of the inclosed air effected by this means was full one-fifth.

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

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

Even the electric spark separates fixed air from common atmospherical 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 electrification produced no sensible effect. "To determine," says the Doctor, "whether the cause of the change of colour was in the air or in the electric matter, I expanded the air which had been diminished in the tube by means of an air-pump, till it expelled all the liquor, and admitted fresh blue liquor in its place; but after that, electricity produced no sensible effect, either on the air or on the liquor; so that it was evident that the electric matter had decomposed the air, and had made it deposit something that was of an acid nature."

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

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

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

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

In Mr Boyle's, Dr Boerhaave's, and Dr Hales's works, and in other books, the quantities of elastic fluid generated in various processes, and by divers substances, are mentioned with distinction; but as those writers were not acquainted with the characteristic properties of fixed air, we do not know whether the elastic fluid mentioned by them was pure fixed air or not. From animal substances, mixed with spirit of nitre, and sometimes heated a little, in order to facilitate the production of elastic fluid, Dr Priestley obtained, in general, fixed air; but whereas the fixed air produced by a similar process with vegetable substances is mostly mixed with nitrous air, this is mixed with an elastic fluid, which is seldom nitrous in a very slight degree, but is often phlogisticated air, viz. in such a state as extinguishes a candle, does not diminish common air, nor is itself diminished by nitrous air. Towards the end of the process, the Doctor remarks, "that when, by means of a strong heat, the produce of air is very rapid, and the air full of clouds, it is, like air, produced from vegetable substances in the same circumstances, slightly inflammable, burning with a lambent greenish, or bluish flame."

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

The principal facts are the following:

1. When calcaceous earths, alkalis, and magnesia, in their usual state, are mixed with acids, they cause an effervescence; and consequently the production of a permanently elastic fluid, namely, fixed air.

2. These substances retain the fixed air very obstinately; so that a strong fire is necessary to expel it from magnesia, and the strongest is not sufficient to expel it entirely from fixed alkalis, and especially from calcaceous earths (a). When these substances are treated with acids, they yield the fixed air, because they have a stronger attraction to those acids than to the fixed air.

3. The calcaceous earths which are insoluble in water, when deprived of the fixed air become soluble in it. Thus lime-stone is not soluble in water, but lime (viz. lime-stone deprived of its fixed air) is soluble in water. And if those substances, deprived of their fixed air, are put in a situation proper to recover their lost fixed air, they lose the property of being soluble in water. Thus, when lime-water is exposed to fixed air, the lime absorbs the fixed air; and, losing at the same time its property of being soluble in water, is precipitated from it in the state it was before calcination, viz. of a calcaceous earth insoluble in water, and capable of effervescing with acids.

4. Alkalis, both fixed and volatile, when deprived of their fixed air, become more caustic, and more powerful solvents, incapable of crystallization, and of effervescing with acids. But if to those alkalis, and also earths rendered more caustic, their fixed air be restored, they acquire at once all the properties they had before they were deprived of the fixed air, viz. they become more mild, effervescing with acids, recovering their weight, &c.

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

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

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

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

(2.) By Acids. Let a glass tube, open at both ends, be bent, by means of a blow-pipe and the flame of a candle, nearly into the shape of an S, as it is represented by A.B., and fix a cork D to one of its extremities, so as to fit the neck of a common phial, that may hold about four or five ounce measures. The hole through the cork may be made with an iron wire red-hot, and the tube may be fastened in it with a bit of soft wax, so as not to let any air go through. Fill a similar phial, or any glass receiver K, with water, and Cavallo invert it after the manner shown above, in a basin HI, air, 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 A.B., to the bottle, and putting it in the situation F.G., 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 F.G., will immediately begin to effervesce, showing a frothing, and an intestine motion accompanied with heat, that may be felt by applying the hand to the outside of the fluid. The elastic fluid called fixed air is copiously emitted from this mixture, and passing through the bent tube, will go into the bottle K, as appears by the bubbles which come out of the tube, and, puffing... Sect. XIII.

AER O L O G Y.

O. artificial air 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 into it, the flame will be instantly extinguished. The same thing will happen if a lighted piece of wood is let down in it.

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

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 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 refits 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 putrefactive 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 considerable. Sir William Lee, baronet, in two of his letters to Dr Priestley, informs him of his having found, that flesh-meat, even in the hot season, could be preserved wholesome for several days, by only washing it two or three times a day in water impregnated with fixed Of Artificial fixed air. "We have been enabled," says he, "to preserve meat as perfectly sweet and good to the extent of ten days, as at the first killing; and there seems no doubt it might be preserved much longer." He has even recovered some meat that had begun to change. This useful discovery, Sir William justly observes, may be very beneficial to the public, especially to butchers. "Particularly a butcher," says he, "who deals pretty largely, affirms me he found the greatest success from it, and only objects that the veal was a little discoloured, though kept perfectly sweet."

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

§ 2. To produce Inflammable Air.—The process for making this sort of gas is the same as that for making fixed air: one of the materials only must be different, viz. iron-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 mixture has a displeasing smell, even when mixed with a very large quantity of common air; so that if any considerable quantity of it comes out of the bottle, before the cork with the bent tube be applied to it, &c., its smell may be perceived all over the room in which the experiment is made, but this smell is not particularly offensive.

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

In this experiment we see, that inflammable air follows the general rule of all other combustible substances, namely, that of burning only when in contact with common air; thus the flame of this gas, whilst burning, is observable only on that surface of it which is contiguous to the common air; so that if the bottle be closed, the flame is put out immediately, because the air is intercepted from it. But if the inflammable air were put in such a situation as to expose a very great surface to the common air, it is plain, that by this means its combustion would be accelerated, so as 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 manner: When the bottle is to be inverted into the basin, in order to let it be filled with the inflammable gas, instead of filling it entirely with water, let half of it remain filled with common air; then invert it, and let the other half, which is now filled with water, be filled with inflammable air after the usual manner; and when the bottle is full, remove it in the manner shown above, and approach it to the flame of the candle, by which means the inflammable air takes fire; but now it explodes all at once with a large flame and a considerable report, sometimes breaking the bottle in which it is contained. In this case, the bottle being filled with equal parts of inflammable and common air, these two elastic fluids were mixed together, so that almost every particle of the one touched every particle of the other, and hence the sudden combustion was occasioned. The force of this explosion is so very considerable, that some pistols have been contrived, which are charged with a mixture of air and inflammable gas, and being fired by means of an electric spark, are capable to drive a leaden bullet with great violence. Sometimes those pistols are made of glass (but in this case they are not charged with a bullet), and it is very diverting to show that pistols are charged and explode by the combustion of an invisible substance.

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

By taking electric sparks in any kind of oil, spirit inflammable of wine, ether, or spirit of sal ammoniac, Dr Priestley obtained inflammable air. The oil, or other liquid obtained from quinone, was confined in a glass tube by quicksilver, and various substances were cemented in the upper part of the tube, through which the sparks being sent, went to the quicksilver through the oil; but after that a few sparks had been taken, a quantity of inflammable air was generated, &c. Left the production of inflammable air should be attributed to the cement which fastened the 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-fillings in a very diluted state, gives, by the affluence of a moderate degree of heat, a mixture of different airs, partly fixed, partly common air, and partly phlogisticated air. See further the article Aerostation.

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

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

Among the semi-metals, zinc gives the weakest nitrous air, when dissolved in nitrous acid. The elastic fluid 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 the best nature in the nitrous acid; therefore it is plain, that the metals whose nature is more uniform must be preferred for this purpose. Accordingly, brass yields nitrous air of a more uniform nature than iron; copper is superior to brass; but pure mercury is still superior to copper; and indeed this is the metal which, considering its nature, uniformity of substance, and easy solution, is upon the whole the most useful for this purpose.

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

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

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

In order to observe the principal property of this elastic fluid, which is that of diminishing the bulk of common air, let a glass tube, closed at one end, and about nine inches long, and half or three quarters of an inch in diameter, be filled with water, and inverted in water; then take a small phial, of about half an ounce-measure, filled with common air, and plunging it under the water contained in the same bath where the inverted tube is kept, let that quantity of air enter into the tube, which will go to the top of it, the water subsiding accordingly. Let a mark be made, either with a file or by sticking soft wax on the tube, just opposite to the surface of the water in it, which will mark how much of the tube is filled by that given measure of air. After the same manner, fill the same small phial (which we shall call the measure) again with air; throw that air into the tube, and put a mark on the tube coinciding with the level of the water in it. In this manner let four or five measures be marked on the tube. Now, if three measures of common air are put § 4. To procure Dephlogisticated Air.—This is no other than exceedingly pure atmospherical air, entirely free from those heterogeneous vapours which contaminate the air we commonly breathe. The easiest method of procuring this air is to put some red-lead into the bottle, together with some good strong oil of vitriol, but without any water. Let the red-lead fill about a quarter of the bottle, and the vitriolic acid be about the same quantity or very little less; then apply the bent tube to the bottle, and proceed in the same manner as above. But it must be remarked, that without heat this mixture of red-lead and vitriolic acid will not give any dephlogisticated air, or it yields an inconsiderable quantity of it; for which reason the flame of a candle (that of a wax taper is sufficient) must be applied under the bottom of the bottle; which for this purpose must be rather thin, otherwise it will be easily cracked.

In this manner the red-lead will yield a good quantity of elastic fluid, the greatest part of which is dephlogisticated air; but not the whole quantity of it, for a good portion of fixed air comes out with it. In order to separate the fixed from the dephlogisticated air, the inverted bottle, when filled with the compound of both, as it is emitted from the red-lead, must be shook in the basin for impregnating water with fixed air; by which means the water will absorb the whole quantity of fixed air, and leave the dephlogisticated air by itself.

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

There are not the only advantages which might be expected from dephlogisticated air. It has been found by experience, that animals will live much longer in this kind of air than in an equal quantity of common air; whence it is supposed, that the breathing of it must be much more healthy, and contribute to longevity much more than the common atmosphere. Nay, there are not wanting some who attribute the longevity of 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 expense and trouble of the most ready methods of procuring it, have hitherto prevented any fair trial from being made. Yet philosophers, considering the probability there is of this kind of air being salutary in many diseases, have bestowed some pains in attempting to find out methods of procuring it easily and in large quantity; concerning which we have the following observations in Cavallo’s Treatise on Air.

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

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

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

(a) In this operation the flame of the candle, when once applied, must be kept continually near it; and when the mixture does not produce any more elastic fluid, or the operation is required to be intermittent, care should be taken to remove the extremity of the bent tube from the water first, and then to take off the flame of the candle from under the bottle; otherwise, if the flame of the candle be first removed, the materials within the bottle condensing by cold, the water immediately enters, which in an instant fills the bottle, and generally breaks it. that animals will not live longer in a quantity of dephlogisticated air when it stands in contact with lime-water, than they will when no lime-water is used. In what manner a difference so enormous can take place, between philosophers in other respects so accurate, we can by no means determine. It is plain, however, that if 27,000 inches of common air are necessary for a person in one hour, the same quantity of dephlogisticated air cannot be breathed longer than four hours, nor even so long, with any real advantage. Mr Cavallo indeed allows only 12,000 inches for four hours; but though this might no doubt sustain life for that time, the person must at best expect nothing from it superior to the common atmosphere, if he was not materially injured by it.

A very ready method of procuring dephlogisticated air in large quantity, is by means of nitre; and on the supposition that 12,000 inches are sufficient for four hours, (or for 40 hours, as he limits the Abbé Fontana's supposition), Mr Cavallo proceeds in the following manner:

"The instruments necessary for the production of dephlogisticated air from nitre are the following; viz. earthen retorts, or earthen vessels with a straight neck, somewhat in the shape of Florence flasks, but with a longer neck, these being cheaper than the retorts, and answering as well;—a small furnace, in which the earthen retort must be kept red-hot; a common chimney-fire is not sufficient. These furnaces may be very easily made out of large black lead crucibles. The nitre must be put into the retort or other vessel, so as to fill half or nearly three quarters of its belly; then a bent glass tube is fitted to the neck of the earthen vessel, in such a manner as not to let any elastic fluid escape into the open air. The bellute or cement for this or similar purposes is made by mixing together whiting and drying oil. The retort being put into the furnace, must be surrounded with lighted charcoal, which is to be supplied according as it wastes; in short, the belly of the retort must be kept quite red-hot, or rather white-hot, for about three hours at least. If, instead of the retort, the other described earthen vessel be used, care should be had to place it with the neck as little inclined to the horizon as possible, lest the nitre should stop the neck and break it."

The air is then to be received into large glass jars, as is usual in other experiments on air.

"The retort or other earthen vessel that is used for this purpose cannot serve for more than once, because it generally breaks in cooling; and besides, the decomposed nitre cannot easily be taken out of it. The retort capable of holding a pound of nitre (the quantity necessary for producing 12,000 cubic inches of dephlogisticated air) for this operation, costs at least half-a-crown; the other earthen vessels in the shape of Florence flasks, but with longer necks, cost about 18d. a-piece, or 2s.; so that the price of these vessels forms a considerable part of the expense. 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, dephlogisticate 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 expenses at present of artificial necessity 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 through 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 congeal into lumps by the cold blast.

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

§ 6. To procure Marine Acid Air, which is nothing other than the marine acid itself, and which without any addition becomes a permanently elastic fluid; put some sea-salt, or common kitchen salt, into the usual bottle in which the materials for producing elastic fluids are generally put, so as to fill about a fourth part of it, and upon this salt pour a small quantity of good concentrated vitriolic acid; then apply the bent tube to the bottle, and introduce it through the quicksilver into the receiver, filled with and inverted in quicksilver after the usual method, and the elastic fluid is copiously produced.

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

When water has absorbed a good quantity of this elastic fluid, it acquires the properties of nitrous acid; and when heated, it yields a large quantity of nitrous air. When the nitrous acid air is combined with essential oils, a considerable effervescence and heat are produced, nearly in the same manner as when the nitrous acid itself is poured upon those oils.

§ 8. Fluor Acid Air.—Put some of those minerals called fluors, or fusible spars, pulverized, into the usual bottle, and upon it pour some concentrated oil of vitriol; then adapt the bent tube, &c. The fluor acid air is at first produced without the help of heat; but in a short time it will be necessary to apply the flame of a candle to the bottle, by which means a considerable quantity of this elastic fluid is obtained.

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

Hepatic Air. See Sect. XI. supra.

INDEX

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

Alkaline air: Its properties, 146. Contains phlogiston, 147. Converted into inflammable air, 148.

Animals: Cause of their death in dephlogisticated air, 61. Effects of inflammable air on them, 141.

Air: Inflammable air produced from it by the red-hot steam of water, 124. After gaining most of their weight by absorption from the atmosphere, 122.

Atmosphere consists of two very different kinds of fluids, 24, 93. The proportions of these, 178. The upper parts of it more salubrious than the lower, 179.

B. Black’s (Dr) discoveries, 21. His theory concerning fixed air attacked at first, but now universally received, 23. Boyle’s discoveries, 17.

Calcination of metals: Mr Lavoisier’s experiments on it, 92. His conclusions therefrom with regard to the composition of atmospheric air, 93.

C. Calcified iron: Remarkable phenomenon attending its calcination with a burning-glass, 70. Cavalle’s conclusions from Dr Ingenhouz’s experiments, 38. His method of collecting inflammable air from ponds, 119. Cavendish’s experiments on water, 75. On the production of nitrous acid, 101, 102. Charcoal yields a great quantity of fixed air, 16.—totally convertible into inflammable air, 129. Its excessive attraction for water, 132.

Combustion, whether common air is diminished by it, 58, 183.

Contagion of the plague, of a heavy sluggish nature, 5.

Copper: Dr Priestley’s experiments to produce water by its means, 73. Is not affected by alkaline air, 146.

Cotton-wool: Quantity of dephlogisticated air produced by its means from water, 45.

Cretaceous acid: An improper name for fixed air, 107.

D. Darkness: Its effects on the production of air, 42. Dephlogisticated air discovered by Dr Priestley, 24. First obtained by means of a burning-glass from precipitate per se, 25. Why called dephlogisticated, 26. Produced from a great variety of substances, ibid. Discovered by Mr Scheele, 28. May be obtained without the use of nitrous acid, 29. Produced in greatest quantities by a sudden and violent heat, 30. Method of procuring it from different substances, 31. How it is produced by nature, 32. Method of obtaining it from water, 36. From the leaves of plants, 37. By means of raw silk, 41. From various other substances, 45. Quantity of it produced from water, 46. Of the cause of its production, 47. At what times it is produced of the best quality, 48. Found in sea-water, 53. How to preserve it in large quantity, 54. It produces intense heat, 55.

Explodes violently with inflammable air, 56. Burns violently with pyrophorus, 57. Is diminished by combustion, 59,—and by nitrous air, 60, 154. In what manner it may be contaminated, 61. Does not support vegetation, 62. Of its component parts, 63. Does not contain earth, 65. Whether it contains any nitrous acid, 66. Imbibed by calces of metals, 67. By iron, 68. Mr Cavendish’s experiments on its composition, 75. Nitrous acid produced from a mixture of it and inflammable air, 77. Supposed to be one of the component parts of water, 81, 82, 83. Effects of the electric spark on it when inclosed between different liquors, 105. Dr Priestley’s experiments on the production of fixed air from it, 110.

Dephlogisticated nitrous air, how procured, 160. Its component parts, 161. Best method of procuring it, 163. Made to approach to the nature of atmospheric air, 164.

Diminution of air, supposed to be owing to phlogiston emitted into it, 89.

E. Earth is not a component part of dephlogisticated air, 65.

Effervescence between acids and alkalis occasioned by fixed air in the latter, 21.

Eider-down: Dephlogisticated air produced by its means from water, 45.

Electric spark: Its effects on dephlogisticated air inclosed between AEREOLOGY.

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

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

Finery-cinder, the same with scales of iron, confits of the metal united with deplogificated air, 124.

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

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

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

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

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

G.

Gold: A peculiar kind of air produced from its solution, 175. A beautiful experiment with it, ibid. Green matter observed by Dr Priestley in glass jars producing deplogificated air, proved to be of an animal nature, 40.

H.

Hales, Dr, his discoveries, 18, 19.

Heat: Its effects on fixed air, 115.

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

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

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

Human hair produces deplogificated air with water, 45.

I.

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

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

Inflammable air: Method of burning it in the deplogificated kind, 59. Water produced from a mixture of inflammable and deplogificated air, 77. Quantity of it necessary to phlogisticate common air, 78. This kind of air produced in mines, from putrid waters, &c., 117. Great quantities generated in hot climates, 118. Mr Cavallio's method of collecting it from ponds, 119. Meteors thought to be produced by it, 120. Different kinds of inflammable air, 121. Extracted from various substances by heat, 122. More air procured by a sudden and violent than by a gradual heat, 123. How procured from water and other fluid and solid substances, 124. Portions of inflammable air procured from iron by means of steam, 125. Of the constituent parts of inflammable air, 126. No acid contained in it, 127. Water necessary to its production according to Dr Priestley, 128. Denied by Mr Kirwan, 138. Charcoal totally convertible into it, 129. Experiment showing the necessity of water for the production of inflammable air, 131. Is not pure phlogiston, 133. Priestley's analysis of different kinds of it, 134. Fixed air generated in its decomposition, 135. Fixed air convertible into it, 136. Has a great propensity to unite with water, 137. Dr Priestley's conclusion with regard to its component parts, 139. Its absorption by water, 140. Its effects on vegetation and animal life, 141. Has little refractive power, 142. Schemes to employ it for various purposes, 143.

J.

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

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

K.

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

L.

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

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

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

Light: Effects of it in the production of deplogificated air, 36. Effects of light without heat, 43. Of artificial light, 44.

Lint produces deplogificated air, 45.

Litmus: Its solution decomposed by taking the electric spark in deplogificated air confined over it, 105.

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

M.

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

Marble, why it sometimes bursts with froth, 5.

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

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

Metallic vapours, their poisonous qualities, 5.

Metallic calces imbibe deplogificated air, 67.

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

Mineral waters contain air, 19, 20.

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

Mofettes, their nature, 5.

Mustard, its effects on air, 35.

N.

Nitre yields a great quantity of deplogificated air, 28.

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

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

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