fure those differences, or the refpective degrees of purity, as indicated by the quantity of the diminution, with great exactness.

To give ac inftance of the accuracy, or fenfibility, of this teft:-the Author obferves that, if he did not deceive himself, he has by its taeans perceived a real difference between the air without doors, and that of his ftudy after a few perfons had been with him in it. And further, a vial of air having been fent him from the neighbourhood of York, it appeared not to be fo good as the air near Leeds; that is, the diminution, on the addition of an equal quantity of nitrous air, was not fo great in the former cafe as in the latter. He even thinks it poffible, by means of this teft, to diftinguifh fome of the different winds, ar the quality of the air in different seasons of the year.

As the nature of our work will not permit us to enter into the more complicated experiments relating to nitrous air, we hall confine ourselves to an enquiry into the chemical nature, or compofition, of this fingular fluid, as collected from this fection, and from the observations contained in the fecond part of this work, where the Author refumes the confideration of this fubject, and recites the many additional discoveries relating to it, made by him fince the publication of the first part.

Nitrous air on being agitated with water, after the fame manner in which the Author had formerly impregnated water with fixed air, appeared to him to communicate a very acid taste to the water; and thence he was led to fufpect that the nitrous acid was contained in it. It appears, however, from a letter of Mr. Bewly's, a correfpondent of the Author, printed in an Appendix to this work, that by agitating water with nitrous air alone, the latter will not be decompounded, or communicate to the water any fenfibly acid impregnation; but that the prefence of common air is abfolutely neceffary to produce thefe effects: and he accounts for the deception which may naturally be occafioned, on the tasting of the water after fuch agitation, by attributing it to the admixture or commenftruation of the common with the nitrous air, in the neck of the vial, and at the very inftant of applying the latter to the mouth. The juftice of the Author's fufpicion, that the nitrous acid is contained in water impregnated with nitrous air, is confirmed by the fame correfpondent; who obferves that-nitrous air, thus decompounded by atmospherical air, and afterwards neutralifed by the addition of a fixed vegetable alcali, furnished him with real crystals of nitre.

In the profecution of his numerous and curious experiments on this fubject, related in the fecond part of this work, the Author difcovered that nitrous air was decompounded, or re

folved

folved into its conftituent principles, by an admixture of common air, which lets loose the acid contained in it, and separates it from the phlogifon, which he fuppofes to be its other conftituent principle. At the fame time, the phlogifton, entering into and combining itself with the common air, produces a diminution of it, in the fame manner as that principle was found to do, in a variety of other proceffes. There is reason however to fufpect that the greatest part of the very confiderable diminution, obfervable on the mixture of nitrous with common air, proceeds from the great change produced in the nitrous vapour; which from a ftate of confiderable expanfion, in the form of an elastic fluid, is thus reduced into its smallest poffible dimenfions, and condenfed into the fize of a small drop or two of nitrous acid.

The union of the nitrous acid with the phlogifton, or other principle with which it is combined in nitrous air, is indeed fo ftrict, or their affinity to each other is fo ftrong, that this acid, as we have found, will not leave the phlogifton, although a fixed alcali, or even a cauftic calcareous earth, diffolved in water, be prefented to and agitated with it, unless common air be admitted. The nitrous acid, contained in nitrous air, will, for inftance, pafs through a folution of falt of tartar, or through lime water, and will bear being long agitated with thefe fluids, without being neutralifed, or fentibly condenfed. But on inverting the vial, and fuffering common air to enter through the liquor, it immediately and vifibly diffolves the union between the acid and the other principle; and leaves the former at liberty to combine itfelf with the alcali, or cauftic earth of the lime.

It appears however, from fome of the Author's experiments, that nitrous air alone is capable of being abforbed by, or dif· folved in water, by long agitation. In fome of these cafes we fhould apprehend either that it is decompounded by the air which, as M. de Luc has lately fhewn, is obftinately retained by all water; or probably that water is capable of receiving a fmall portion of it in an undecompounded ftate; in the fame manner as vitriolic ether, which is ufually confidered as infoluble in water, may be totally diffolved in it by adding freth parcels of that Auid to it.

Out of the many experiments relating to this fluid we fhall felect one, which prefents a very amufing phenomenon, that first cafually occurred to the Author, and for fome time exercised his fagacity; not only in endeavouring to account for the caufet of it, but likewife in difcovering the effential circumftances on which the appearance depended, fo as to be enabled to repeat the experiment at his pleafure. He at length fucceeded in both

thefe

thefe particulars, and thereby procured a more intimate knowledge of the conftituence of nitrous air than he had been able to acquire before.

On mixing nitrous with common air in an inverted jar placed in a trough of water, when the diminution of the air was nearly completed the jar began to be filled with the most beautiful white fumes, refembling the falling of a very fine now. On endeavouring however to repeat the experiment, he was frequently unfuccefsful, and fuftained the mortification of baulking the expectations of his friends, to whom he meant to exhibit it. After many trials and reflections on the subject, he at length discovered the effential circumftances on which the appearance depended; and particularly that it was produced by the volatile alcali emitted from the water, which was in a flight degree putrid. The experiment made in the following manner exhibits this curious appearance to the beft advantage.

The smallest drop of any volatile alcaline liquor, fuch as fpirit of hartshorn, or fal ammoniac, or a fmall piece of the folid volatile falt, is put into a tall glass jar containing common air, the mouth of which is stopped with a cork. This jar is introduced within a larger jar inverted, and containing nitrous air. The moment the cork is removed, by means of a particular contrivance for that purpose, the white clouds abovementioned begin to be formed at the mouth of the jar, and prefently descend to the bottom, fo as to fill the whole, were it ever fo large, as with fine fnow.'-Or a piece of volatile falt, inclosed in a bit of gauze, muflin, or a small net of wire, is fufpended in a jar of common air. Soon after the admiffion of a quantity of nitrous air to it, and when the redness produced by the mixture begins to go off, the white cloud, like fnow, begins to defcend from the falt, as if a white powder was fhaken out of the bag that contains it;' and this appearance will last about five minutes.

This white powder, and the white clouds attending this mixture, are nothing more, as the Author juttly concludes, than a nitrous fal ammoniac, extemporaneously formed; in consequence of the decompofition of the nitrous air effected by the common air, which receives the phlogifton of the former, and at the fame time lets loofe its acid, which is now at liberty to unite with the fumes of the volatile alcali, and produce the neutral or ammoniacal falt under the form of a white cloud, or of a pow der refembling fnow.

In the seventh and eighth fections, the Author investigates the nature and caufe of the injury done to the air by the fumes of burning charcoal; and of the fimilarly and equally noxious impregnation which it receives from the calcination of metals. In REV. Aug. 1774. L

both these cafes, as well as in all others in which the air is rendered incapable of fupporting life and flame, it fuffers a diminution; the cause of which we fhall have occafion to speak of hereafter. In the eighth fection we meet with a fingular obfervation of the very confiderable and noxious diminution of common air, effected folely by the effluvia of paint, made with white lead and oil. Several pieces of paper daubed with this paint, being put into an inverted jar ftanding in water, greatly diminifhed the air contained in it; as was evident from the rifing of the water between one-fifth and one fourth of the whole space. The remaining air was hereby rendered incapable of effervescing with nitrous air, and was in the highest degree noxious. This air, like other diminifhed airs, was made wholesome by agitating it in water deprived of air :—an obfervation which, as Dr. Percival has lately remarked, fhews the utility of the practice of placing veffels of water in rooms lately painted.

The air which is the fubject of the next fection, poffeffes, like the reft, many remarkable properties, and is diftinguished from them by the Author, by the appellation of Acid Air. The honourable Mr. Cavendish firft obferved it; having procured it from a diffolution of copper in the marine acid t. Dr. Prieftley however, in the profecution of his experiments on this fubject, difcovered that the metal was not requifite to the production of this remarkable fluid; that it is furnished by the fpirit of falt alone, and is nothing more than the fume or vapour of that acid; which being once raised from it by means of heat, is not liable to be condenfed by cold, like the vapour of water and other fluids, but becomes a permanently elaftic fubftance, which extinguishes flame, and is heavier than common air. When all this air has been expelled from a given quantity of the marine acid, what arifes afterwards is a real vapour condenfable, like that of water, by cold. The remaining liquor is found to be a weak acid, barely capable of diffolving iron.

This term the Author has chofen to apply to all these elastic Auids indifcriminately, as a lefs exceptionable, appellation than that of vapour. Subftances that poffefs a permanent degree of elasticity, and that have in common fo many of the other properties of the air we breathe, ought rather, he thinks, to be diftinguished by the title of air, than by that of vapour; as this laft term has hitherto conftantly conveyed with it the idea of an elaftic matter readily condenfable in the common temperature of the atmosphere.

See his ingenious paper on Factitious Air, in the 56th volume of the Phil Tranf. and our Review, vol xxxvii. Dec. 1767, P• 44°, where fome mention is made of this fingular fluid.

The moft obfervable property of this air or vapour is, that while it is contained in an inverted glafs veffel ftanding in quickfilver, it preferves its aerial or elaftic form; but on prefenting water to it, the greateft part of it vanishes, and is found to be condensed into a strong acid fpirit. Two grains and a half of rain water will absorb no less than three ounce measures of this air. The water thus faturated now weighs twice as much as it did before, and is converted into a concentrated spirit of falt, ftronger, the Author obferves, than any he ever met with.

In the course of his experiments on this new fubject of inquiry, with a view to discover its properties, and its chemical relations to other bodies, the Author foon difcovered that, it had a ftrong affinity to phlogifton, fo as to extract it from numerous bodies abounding in that principle; particularly from inflammable spirits, expreffed oils, oil of turpentine, charcoal, phofphorus, bees wax, and, which is fingular, even from fulphur: forming, in this laft inftance, a ftrong exception to the common table of affinities, as here this vapour of the weakest of the three mineral acids breaks the union of the vitriolic, or the moft potent, with phlogifton, diflodging the former, and uniting with the latter. It is not one of the leaft obfervable refults of these various combinations, that this acid air, which before extinguished flame, is now converted into an inflammable compound, in all its fenfible qualities refembling inflammable air.

Some mifcellaneous experiments form the subject of the tenth and laft fection of the first part.-But we now find it impoffible even to skim over this truly ingenious performance in the compass of one article. We fhall therefore refume the confideration of it in our next or a future number.

The attention which we have deservedly bestowed on this work has enabled us to detect a few Errata that have escaped the Author's correction, fome of which materially affect the sense. We suffer fo much ourselves by unavoidable inaccuracies of the prefs, that we readily lay hold of the present opportunity of gratifying the Author, his readers, and ourselves, by marking the following:-Page 123, line 3 of the note, for acid, read air. Page 260, line 7, dele with. Page 306, line 2, for necet, read recent. Page 311, line 25, for healed, read treated; and page 324, line 15, for it, read iron.