THE APPENDIX. NUMBER I. EXPERIMENTS and OBSERVATIONS relating to fome of the Chemical Properties of the Fluid, commonly called FIXED AIR; and tending to prove that it is merely the VAPOUR of a particular ACID. In two Letters to the Reverend Dr. Priestley: By William Bewly. NY fuccessful investigation of that part of philosophy, in which you have lately made so extenfive and rapid a progrefs, cannot be communicated to the publick any where with so much propriety, and advantage, as in the company of those fingular and important discoveries which will be given in the new Volume of your Observations, now in the press. With great pleasure, therefore, I comply with your late request, to tranfmit to you the particulars of my Obfervations on Mepihtic or Fixed Air; the general refults of which I formerly communicated to you. They tend, if I do not deceive myself, to throw a new and just light on the real nature and chemical properties of that fluid; the extenfive diffufion of which throughout the universe, where it forms a conftituent principle of almost all known bodies, renders it a fubject deferving of an accurate and minute investigation. In the present letter I fhall principally confine myfelf to thofe Obfervations only which first convinced me of the existence of an acid in fixed air : ------ a point which has been contefted, or, at leaft, left dubious, by other inquirers. The experiments which I fhall relate in a subsequent letter will, I expect, fatisfactorily evince, that this acid is not a fubftance extrinfecal to fixed air, or cafually floating in it, and separable from it; but, on the contrary, that it is a neceffarily conftituent principle of this fluid; and even that fixed air itself is no other than this very acid; or, in other words, that it is a peculiar and diftinct acid spirit, fui generis, which, on its being expelled, by the power of a fuperior acid, or the force of fire, from the various earths, falts, &c. with which it is combined, inftantly affumes the form of an elastic vapour, greatly resembling common air; which form it permanently retains, till it meets with any of thofe numerous bodies which have an affinity to it, and which have been deprived of, or are not already faturated with it. By thefe bodies this acid vapour is condenfed, or reduced into a liquid or fixed ftate; in which ftate it combines with them, in a manner in no respect different from that in which the vitriolic or any other acid is united with the various falts, earths, or other fubstances, with which they form neutral compoundsSuch, at least, is the fyftem which I have been naturally led to deduce from the following experiments. Even the bare prefence of an acid, in fixed air, has, as I have already observed, been doubted of. So lately even as the laft year Dr. Brownrigg*, to whom this new branch of Chemical philofophy is fo highly indebted, obferved, that though Mephitic air, imparts to the waters impregnated with it a brisk and pungent tafte, which has ufually been ftiled fubacid; yet it differs from all acid fpirits in not ftriking a red colour • Phil. Tranf. vol. 64. part 2. for the year 1774. p. 369. with the blue tinctures of vegetables; adding, that not only no change of this kind could be obferved to have been effected by it, in the numerous experiments made by himself and several other gentlemen; but likewife that he had" for feveral days fufpended "pieces of linen, that had been dyed blue with fresh ચંદ juice of violets, in the mephitic air of Spa water, "and alfo in that of chalk; and when the linen was "taken out of the faid air, did not perceive its blue "colour in any wife changed, although the famé "pieces of dyed linen were inftantly turned of a green "colour, when exposed to the fumes of the spirit of hartfhorn." Whether therefore," he adds, "and under what relations, this aerio-faline fpirit "may merit the title of an acid, I leave to the determination of others." In the Appendix to your former Volume, your ingenious correfpondent, Mr. Hey t, has likewife shewn that water impregnated with fixed air, produced no change of colour in the fyrup of violets; and that it did not effervefce with either the fixed or the volatile alcali.- -The fact is, that fixed air is so rare a vapour, and the Mephitic Acid, as I fhall already venture to call it, is so greatly diluted in water, which is even faturated with it, that many of the blue juices refift its action upon them; while others, more senfible tefts of acidity (fuch as infufions of Litmus, Cyanus, or Corn-flower, and a few others) readily anounce its acid quality.As to its not producing an effervescence with alcalis, it will appear from the following experiments, that fuch effervefcence is, from the very nature of the thing, impoffible in the present Experiments and Obfervations, &c. p. 288. 1ft edit. cafe, in which the very contrary of an effervefcence must take place. In all other cafes, when an acid is added to a mild alkali, the mephitic acid, as being the leaft powerful of all the acids, is expelled, in its ftate of vapour, or in elastic bubbles, which conftitute the appearance called an effervefcence; whereas when the mephitic acid itself is added to an alcali, it is condenfed, and filently abforbed in it. It may be neceffary to premise that, in feveral of the following experiments, I found it most convenient, as well as productive of greater accuracy and expedition, to take the inverted phial out of the bason, after every fresh introduction of fixed air, for the purpose of agitating more freely the liquor contained in it; and that I took care to use a bason or cup of a very small diameter, and which contained a very fmall quantity of fluid; in order to guard, as much as poffible, againft diffipation of the fixed air, during the procefs. It may be proper likewife to obferve, that I may not incur a fufpicion of plagiarism, that fome scattered hints, relative to a few of the following Obfervations, have been formerly inserted by me in a certain anonymous publication. The experiment, by which I firft detected the prefence of an acid in fixed air, fome years ago, is as follows. I have repeated and diversified it on the prefent occafion, and with the fame event. EXPERIMENT I. Having accurately adapted, to the mouth of a phial containing spirit of vitriol, a cork, in which a glafs tube was inferted, which was drawn out at its farther extremity, fo as to terminate in a bore nearly capil lary; |