IT is not here intended to give a chemical account of the different kinds of air. It is well known to all, in any degree conversant with the subject, that besides the air which we breathe, there have been discovered other species of permanently elastic invisible fluids, possessing very different properties. Of these, one of the best known, and the only one as yet applied to medical purposes, is the Fixed, Fixable, or Mephitic Air, or Gas.

This kind of air is naturally contained in a great variety of substances; and is set free in many processes of art. Every kind of vinous fermentation extricates a large quantity of it, which floats on the surface of the fermenting liquor. It is expelled from alkaline salts, and from absorbent earths, by the action of fire, and of acids. The properties by which it is principally distinguished, are, its extinguishing flame, being destructive to animal life when inspired pure and in large quantity, and being readily absorbed by water, to which it gives a slightly acid taste, accompanied with a degree of brifk-nefs and spirit. It has the general properties of a weak acid, of a very volatile nature.

The idea of its medical virtues seems to have arisen chiefly from some experiments of its great antiseptic power when applied pure; and from the discovery of its presence in large proportion in some of the most celebrated mineral waters, and also in other substances very efficacious in the cure of particular diseases.

At present, fixed air is considerably employed in medicine, chiefly in the following ways.

The celebrated antiemetic mixture of Rive-rius, composed of a spoonful of lemon juice and a scruple of salt of wormwood taken in the act of effervescence, is suppofed to owe its efficacy chiefly to this principle, which is set free during the combination. On this foundation, its use is extended to many diseases in which a tendency to putridity is suspected. Any other acid may be substituted with equal advantage as far as the fixed air is concerned.

By various contrivances, the air expelled from an absorbent earth by the addition of an acid is received into water, in which it dissolves, communicating to the liquor the qualities above-mentioned. This becomes a very grateful beverage in fevers and putrefcent diseases, being cooling, antiseptic, and at the same time cau-sing a temporary glow in the stomach. It should frequently be made fresh, and kept well corked, as the aerial spirit very readily flies off again from the water. As fixed air is in reality an acid, water impregnated with it may be neutralized by an alkali, thus forming an agreeable neutral julep, which is found to be an useful refrigerant and diuretic. If a few clean iron filings are thrown into water to be impregnated with fixed air, so much of the iron is dissolved by its means, as to produce an artificial chalybeate, possibly not inferiour in virtue to the most celebrated natural ones.

As the antiseptic powers of fixed air seem to be most considerable when a stream of the pure air itself is thrown upon the matter to be sweet-ened, some practitioners have injected bladders full of it by way of clyster where the primae via were loaded with highly putrid feces. On this principle, too, patients with ulcerated lungs have been directed to respire fixed air as it rose from an effervescing mixture; and, notwithstanding the inftantly fatal effects of breathing this air absolutely pure, it was found to be perfectly innoxious when thus passing through a body of atmospherical air. The same vapour has also been received upon the naked surface of cancerous and other putrid ulcers, with a view to sweeten and correct the discharge.

In these methods has this substance been directly applied to medical purposes. But its application has by some been supposed to be indirectly much more extensive, as they have attributed to it the efficacy of various other remedies. Fermented and fermentable liquors, fruits and fresh vegetables, have been thought useful in putrescent habits and diseases chiefly as affording a large supply of this principle, which might be absorbed by the stomach and intes-tines. The use of Wort in the Sea Scurvy (fee the article Frumentum) was professedly sug-gested by Dr. Macbride on this supposition.

From what has been above said, the experienced reader will easily perceive in what claffes of diseases this medicine may be expected to be serviceable. There is one disorder, however, in which its proposed use may not appear so deducible from its obvious qualities: this is, the stone in the kidneys and bladder. It is well known that the medicines from which most of late years has been expected in these cases, are lime-water and caustic alkali, the direct chemical opposites to fixed air. But as the nature of urinary calculi is acknowledged to be very variouas, it is not unreasonable to propose op-posite solvents for them. That calcareous earth may be rendered soluble in water by the medium of fixed air, is not to be doubted; and there are calculi which are certainly calcareous. From indisputable experiments it appears, that pieces of calculi have undergone a solution out of the body in water impregnated with fixed air; but it can scarcely be hoped that the men-struum will reach the bladder in so concentrated a state. cases have, however, been published, in which manifest relief was obtained from the free use of a beverage of water satu-rated with fixed air; and it has this advantage above alkaline solvents, that substances abounding in fixed air are more friendly to the general health than alkalescent ones. It has been pro-posed to throw pure fixed air directly into the bladder by a suitable apparatus; but no experiment of this kind has been made public.

The reader who willies for further information concerning the medical uses of fixed air, may consult the ingenious Commentary on the subject by Dr. Dobfon.