108 THE POPULAR SCIENCE MONTHLY. 



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 ex- 

 ceptions, 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 expla- 

 nation 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 maxi- 

 mum number of other atoms, but this binding power is not always com- 

 pletely 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 ex- 

 posed 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 in- 

 stinct undoubtedly is with the similar but less perfectly developed in- 

 stinct of the American Molothrus bonariensis, have not as yet been 

 completely brought within the sphere of an}' 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 vari- 

 ous 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 hy- 

 potheses leave unexplained. She is aware that the phenomena occur- 

 ring in that immense sphere assigned to her are not always to be 

 explained by one, but often by many hypotheses. Phenomenon is 



