In the shape of food the outer world supplies the organism with all the materials necessary for the building up of the constantly wasting organic structures; and, in the shape of heat, there comes from the outer world that other element necessary for structural changes, development and growth--the element of force. But the task of directing all the outward materials to the development and maintenance of the organism--in other words, the task of the director-general of the organic economy falls to the protoplasm. Now this wonderful substance, chemically and physically the same in the highest animal and in the lowest plant, has been all along the puzzle of the biologist. How is it that in man protoplasm works out human structure; in fowl, fowl structure, &c. &c., while the protoplasm itself appears to be everywhere the same? To Professor Yaeger belongs the great merit of having shown us that the protoplasms of the various species of plants and animals are not the same; that each of them contains, moreover, imbedded in its molecules, odorant substances peculiar to the one species and not to the other. That, on the other hand, those odorous substances are by no means inactive bodies, may be inferred from their great volatility, known as it is in physical science that volatility is owing to a state of atomic activity. Prevost has described two phenomena that are presented by odorous substances. One is that, when placed on water, they begin to move; and the other is, that a thin layer of water, extended on a perfectly clean glass plate, retracts when such an odorous substance as camphor is placed upon it. Monsieur Ligeois has further shown that the particles of an odorous body, placed on water, undergo a rapid division, and that the movements of camphor, or of benzoic acid, are inhibited, or altogether arrested, if an odorous substance be brought into contact with the water in which they are moving. Seeing, then, that odorous substances, when coming in contact with liquid bodies, assume a peculiar motion, and impart at the same time motion to the liquid body, we may fairly conclude that the specific formative capacity of the protoplasm is owing, not to the protoplasm itself, since it is everywhere alike, but to the inherent, specific, odoriferous substances. I shall only add that Professor Yaeger's theory may be carried farther yet. Each metal has also a certain taste and odour peculiar to itself; in other words, they are also endowed with odoriferous substances. And this may help us to explain the fact that each metal, when crystallizing out of a liquid solution, invariably assumes a distinct geometrical form, by which it may be distinguished from any other. Common salt, for instance, invariably crystallizes in cubes, alum in octohedra, and so on.