It is well enough to note, that, when we use the word volume or measure, in speaking of the atmosphere or any gaseous body, we adopt the theory of Gay-Lussac, who discovered that gases unite with each other in definite proportions whenever they enter into combination. This theory led to important results; for by knowing the elements of a compound gas, we easily determine its specific gravity. It has been attempted to apply the principle to organic bodies; but it has not yet been carried to a full and satisfactory conclusion. It may be noticed, too, that Dalton affirmed that simple substances unite with each other in definite weights to form compound substances, thus supporting the idea of Lussac. These discoveries were made about the same time, Dalton having the credit of originating them. Various modifications of the principle have been from time to time presented to public attention. Whether the constituents of the atmosphere are chemically or mechanically combined,--one of the things about which the learned are not fully agreed,--it is found to be chemically the same in its constituents, all over the world, whether collected on mountains or on plains, on the sea or on the land, whether obtained by aëronauts miles above the earth or by miners in their deepest excavations. On the theory of its mechanical combination, however, as by volume, and that each constituent acts freely for itself and according to its own laws, important speculations (conclusions, indeed) have arisen, both as regards temperature and climatic differences. It should be observed, that volume, as we have used the word, is the apparent space occupied, and differs from mass, which is the _effective_ space occupied, or the real bulk of matter, while density is the relation of mass to volume, or the quotient resulting from the division of the one by the other. Those empty spaces which render the volume larger than the mass are technically called its pores. Has the composition of the atmosphere changed in the lapse of years? On this point both French and German philosophers have largely speculated. It is computed that it contains about two millions of cubic geographical miles of oxygen, and that 12,500 cubic geographical miles of carbonic acid have been breathed out into the air or otherwise given out in the course of five thousand years. The inference, then, should be, that the latter exists in the air in the proportion of 1 to 160, whereas we find but 4 parts in 10,000. Dumas and Bossingault decided that no change had taken place, verifying their conclusion by experiments founded on observations for more than thirty-five years. No _chemical_ combination of oxygen and nitrogen has ever been detected in the atmosphere, and it is presumed none will be. * * * * * The atmosphere possesses, as may be readily imagined, many important characteristics. One of these is Weight.