1st. The equable diffusion of oxygene and nitrogene through every part of the atmosphere, which can hardly be supposed to depend on any other cause than an affinity between these principles.[181] [181] That attraction must be called chemical, which enables bodies of different specific gravities to unite in such a manner as to produce a compound, in every part of which the constituents are found in the same proportions to each other. Atmospheric air, examined after having been at perfect rest in closed vessels, for a great length of time, contains in every part the same proportions of oxygene and nitrogene; whereas if no affinity existed between these principles, following the laws of specific gravity, they ought to separate; the oxygene forming the inferior, the nitrogene the superior stratum. The supposition of the _chemical_ composition of atmospheric air, has been advanced by many philosophers. The two first evidences have been often noticed. 2dly. The difference between the specific gravity of atmospheric air, and a mixture of 27 parts oxygene and 73 nitrogene, as found by calculation; a difference apparently owing to expansion in consequence of combination. 3dly. The conversion of nitrous oxide into nitrous acid, and a gas analogous to common air, by ignition. 4thly. The solubility of atmospheric air undecompounded in water. ATMOSPHERIC AIR, then, may be considered as the least intimate of the combinations of nitrogene and oxygene. It is an elastic fluid, permanent at all known temperatures, consisting of,73 nitrogene, and,27 oxygene. It is decomposable at certain temperatures, by most of the bodies possessing affinity for oxygene. It is soluble in about thirty times its bulk of water, and as far as we are acquainted with its affinities, incapable of combining with most of the simple and compound substances. 100 cubic inches of it weigh about 31 grains at 55° temperature, and 30 atmospheric pressure. NITROUS OXIDE is a gas unalterable in its constitution, at temperatures below ignition. It is composed of oxygene and nitrogene, existing _perhaps_ in the most intimate union which those substances are capable of assuming.[182] Its properties approach to those of acids. It is decomposable by the combustible bodies at very high temperatures, is soluble in double its volume of water, and in half its bulk of most of the inflammable fluids. It is combinable with the alkalies, and capable of forming with them peculiar salts. 100 grains of it are composed of about 63 nitrogene, and 37 oxygene. 100 cubic inches of it weigh 50 grains, at 55° temperature, and 30 atmospheric pressure. [182] For it is unalterable by those bodies which are capable of attracting oxygene from nitrous gas and nitrous acid, at common temperatures.