It will now be evident that in all chemical phenomena we have the combined exercise of the great physical forces, and evidences of some powers which are, as yet, shrouded in the mystery of our ignorance. The formation of minerals within the clefts of the rocks, the decomposition of metallic lodes, the germination of seeds, the growth of the plant, the development of its fruit and its ultimate decay, the secret processes of animal life, assimilation, digestion, and respiration, and all the changes of external form, which take place around us, are the result of the exercise of that principle which we call chemical. By chemical action plants take from the atmosphere the elements of their growth; these they yield to animals, and from these they are again returned to the air. The viewless atmosphere is gradually formed into an organized being, the lordly tree upon whose branches the fowls of the air have their homes, and the human animal, exalted by being charged with a spiritual soul: yet the tree and the man alike are gradually resolved again into thin air. The changes of the mineral world are of an analogous character; but we cannot trace them so clearly in all their phenomena. The planet on which we live began its course charged with a fixed quantity of physical force, and this has remained constant to the present moment, and will do so to the end of time. By influences external to this earth the balance of these forces is continually disturbed; and in the effort to restore the equilibrium, we have the production of all the varied forms of matter, and the manifestation of each particular physical principle or power. As motion and attraction, balanced against each other, maintain the earth in her elliptical orbit, so the opposition of forces determines the existence of the amorphous rock, the light-refracting crystal, the fixed and flowering plant, and the locomotive animal. An eternal round of chemical action is displayed in nature. Life and death are but two phases of its influences. Growth and decay are equally the result of its power. FOOTNOTES: [212] Dr. Priestley appears to have been the first to observe the peculiar property of the diffusion of gases. Dr. Dalton, however, first drew attention to the important bearing of this fact on natural phenomena, and he published his views on _The Miscibility of Gases_ in the Manchester Memoirs, vol. v. The following extract from his memoir _On the Constitution of the Atmosphere_ will exhibit its bearings:-- It may be worth while to contrast this view of the constitution of the atmosphere with the only other one, as far as I know, that has been entertained.