That the molecules of the elements, i. e., the smallest individual parts which exhibit the properties of the elements, consist of yet smaller parts, or atoms, is undoubted. The generalization holds, with few exceptions, that the elementary molecules contain each two atoms. The exceptions are exhibited by the elements phosphorus, arsenic, cadmium, and mercury, the two former being possessed of molecular weights four times as great as their atomic weights, while the molecular weights of the two latter are equal to their atomic weights. No conclusive explanation has as yet been given of this fact; it remains a true residual phenomenon.
Again, the atoms of the elements are possessed each of a certain definite binding power. Each is capable of uniting with a fixed maximum number of other atoms, but this binding power is not always completely exercised. Why does this power vary? How is its action modified by the conditions under which it is exercised? Can the known facts concerning the action of this binding power, or valency as it is called, be brought within the scope of any definite and workable hypothesis? These questions are to be solved by the researches of the chemists of the future.
Once more, the properties of certain elements vary considerably with variations in the conditions of those elements. Oxygen, when exposed to the action of the electric discharge, is not split up into any form of matter other than itself, nor does it combine with any other form of matter, nevertheless its properties are largely modified. The molecular weight of ozone—the new form of oxygen produced by the action of the electric discharge—is known to be one and a half time greater than that of ordinary oxygen. But, nevertheless, no complete explanation of the facts, of which this special fact is a representative, has yet been given. Allotropy remains a residual phenomenon in chemical science.
Many animal instincts, e. g., the curious instinct which prompts the cuckoo to lay a single egg in a nest not her own, connected as this instinct undoubtedly is with the similar but less perfectly developed instinct of the American Molothrus bonariensis, have not as yet been completely brought within the sphere of any wide generalization.
Why should the use of its sting inflict injury, if not death, upon the bee?
Why do variations in structure or function arise suddenly in various animals?
These questions, and many questions similar to these, await their full explanation.
Science advances by slow but sure steps; she carefully propounds hypotheses, and carefully marks off those phenomena which these hypotheses leave unexplained. She is aware that the phenomena occurring in that immense sphere assigned to her are not always to be explained by one, but often by many hypotheses. Phenomenon is