These Observations on the MEDICINAL USES OF FIXED AIR have been before published in the Second Volume of my Essays; but are here reprinted with considerable additions. They form a part of an experimental inquiry into this interesting and curious branch of Physics; in which the friendship of Dr. Priestley first engaged me, in concert with himself. Manchester, March 16, 1774. In a course of Experiments, which is yet unfinished, I have had frequent opportunities of observing that FIXED AIR may in no inconsiderable quantity be breathed without danger or uneasiness. And it is a confirmation of this conclusion, that at Bath, where the waters copiously exhale this mineral spirit,[15] the bathers inspire it with impunity. At Buxton also, where the Bath is in a close vault, the effects of such _effluvia_, if noxious, must certainly be perceived. Encouraged by these considerations, and still more by the testimony of a very judicious Physician at Stafford, in favour of this powerful antiseptic remedy, I have administered fixed air in a considerable number of cases of the PHTHISIS PULMONALIS, by directing my patients to inspire the steams of an effervescing mixture of chalk and vinegar; or what I have lately preferred, of vinegar and potash. The hectic fever has in several instances been considerably abated, and the matter expectorated has become less offensive, and better digested. I have not yet been so fortunate in any one case, as to effect a cure; although the use of mephitic air has been accompanied with proper internal medicines. But Dr. Withering, the gentleman referred to above, informs me, that he has been more successful. One Phthisical patient under his care has by a similar course intirely recovered; another was rendered much better; and a third, whose case was truly deplorable, seemed to be kept alive by it more than two months. It may be proper to observe that fixed air can only be employed with any prospect of success, in the latter stages of the _phthisis pulmonalis_, when a purulent expectoration takes place. After the rupture and discharge of a VOMICA also, such a remedy promises to be a powerful palliative. Antiseptic fumigations and vapours have been long employed, and much extolled in cases of this kind. I made the following experiment, to determine whether their efficacy, in any degree, depends on the separation of fixed air from their substance.