Under the article Air, in the First Part, we have mentioned the general properties of the different kinds of this invisible fluid, as well as of the medical properties of each species. We must now speak of air chiefly in a chemical view; and though, perhaps, in this and some other parts of the present article, we may seem to deviate from our professed intentions of confining ourselves to medical chemistry, such is the rapid progress of the science, and so much closer is its connection with physiology advancing, that, even while we are writing what may at first appear irrelevant, will become of the greatest importance.

We speak first of oxygenous gas as the least compounded form of air; and though, as we have said, the aerial form is the purest in which the different bases can be represented, yet it is doubtful whether this gas has any basis, and whether it is not strictly and purely caloric. It combines, under different degrees of temperature with all the gases or their bases, with inflammables and with metals, but not with earths or alkalis. The temperature required for the union is different; and when it is in so high a degree as to dissipate the body in vapour, inflammation, strictly so called, is produced. By the French chemists, the union of oxygen with different bodies is styled combustion, but in a loose, and, we think, an unscientific sense. This gas constitutes 0.22 of atmospheric air, and its specific gravity is 0.00135.

Oxygen combines with bodies in different proportions. With nitrogen, it forms atmospheric air: in larger proportions, successively, nitrous, nitric oxide, and nitric acid. With hydrogen, water: and, as has lately been rendered probable, in a larger proportion, muriatic acid. With carbon, it forms plumbago, charcoal, gaseous oxide of carbon, carbonic acid, and carbonic acid gas. With sulphur, oxide of sulphur, sulphureous and sulphuric acid. With phosphorus, oxide of phosphorus, phosphorous, and phosphoric acid. With carbon and hydrogen, it forms what are styled the hy-drocarbonats and the vegetable acids; and, with the addition also of nitrogen, it forms animal substances. Oxygen unites with metals, forming what former chemists styled calces, now oxides. From these it is separated by phosphoric salts, and, indeed, by animal substances of every kind: a fact of importance in explaining the action of oxygenated remedies. It is separated also from almost every compound by light: a circumstance to which we shall return in that article.

Nitrogenous or azotic gas constitutes 0.78 of our atmosphere, and may afford the portion that we breathe with peculiar pleasure, and by which the process, styled animalization,may be chiefly assisted; it is probably absorbed in respiration. Its specific gravity is 0.00115, and some very delicate vegetable blues are rendered green by it. Vegetation, respiration, and combustion, are at once checked by this gas when unmixed; and azote is the distinction of animal substances. It constitutes, with oxygen, nitrous acid, and is the chief ingredient in ammonia. It refuses to unite with water, and dissolves phosphorus and carbon in small quantities.

With oxygen it forms atmospheric air, and in succession nitrous oxide, nitrous gas, and nitric acid; with hydrogen, ammonia; with sulphur and phosphorus, sulphurated and phosphorated nitrogen gas. With carbon, nitrogen, and oxygen, it forms animal substances; should the oxygen be in excess, the result is animal acids. With the proportion of oxygen, which forms nitrous oxide, viz. 37 of oxygen and 33 of nitrogen, it will support flame, and suffers no diminution when mixed with oxygen gas. It does not change even the most delicate vegetable colours, for in this state it recedes from its alkalinity, nor does it combine with alkalis. With atmospheric air, it is highly grateful when breathed, which supports the idea formerly hazarded, that azote, so far as it can be safely introduced, is pleasing and salutary. Water absorbs one half the bulk of nitrous oxide.

The nitrous gas contains 44 parts of oxygen, but does not change vegetable colours. Its specific gravity is 0.001343. It forms, with oxygen, nitric acid, and water absorbs 0.118 of its bulk. It supports neither vegetation nor respiration, and only in a few instances combustion.

Hydrogen gas, the inflammable air of Priestley, is very light, for its specific gravity is only 0.000094. It is highly inflammable, and burns when oxygenous gas or atmospheric air is contiguous; or detonates, when mixed with them, on the application of a burning body. It extinguishes flame, and will not support respiration. It dissolves sulphur, forming hepatic gas; and united with phosphorus and carbon, it forms gases highly fetid.

Hydrogen, with oxygen, forms water; with a less proportion it has been lately thought to produce muriatic acid; with nitrogen, ammonia; with sulphur and phosphorus, hepatic air and sulphurated hydrogen. With carbon and oxygen it produces the hydrocarbonates and vegetable substances; with carbon, nitrogen, and oxygen, animal substances and animal acids. Water is composed of 14.42 of hydrogen, to 85.58 of oxygen; ammonia of four parts of nitrogen, and one of hydrogen.

Carbonic acid gas was formerly known by the name of fixed air, and its discovery was the germ of the whole of pneumatic chemistry. Its basis, carbon, in its purest form, is the diamond; and 17.88 of carbon with 82.12 of oxygen, forms the gas. It is the chief basis of vegetable, though it occurs also in animal substances.

With oxygen and iron it forms plumbago, and with a larger proportion, successively, charcoal and gaseous oxide of carbone; with metals, what are styled, in the new language, carburets. With oxygen and hydrogen, it forms the hydrocarbonates, alcohol, ether, oils, wax, resins, camphor, starch, sugar, jelly, tanin, and all the variety of vegetable acids, with the mucous, laccic and se-bacic acids. With nitrogen, oxygen, and carbon, it forms the various gums and resins, cork, gelatin, albumen, fibrin, urea; the prussic, zoonic, uric, and amnic acids. Common charcoal of wood contains 36.14 of oxygen the gaseous oxide of carbone, 74.11 of oxygen.