SOME UNSOLVED SCIENTIFIC PROBLEMS 



to its acceptance or its refutation. Our century has 

 given experimental proof of the existence of the atom, 

 but has not been able to fathom in the same way the 

 exact form or nature of this ultimate particle of matter. 

 Equally in the dark are we as to the explanation of 

 that strange affinity for its neighbors which every atom 

 manifests in some degree. If we assume that the power 

 which holds one atom to another is the same which in 

 case of larger bodies we term gravitation, that answer 

 carries us but a little way, since, as we have seen, gravi- 

 tation itself is the greatest of mysteries. But again, how 

 chances it that different atoms attract one another in such 

 varying degrees, so that, for example, fluorine unites 

 with everything it touches, argon with nothing? And 

 how is it that different kinds of atoms can hold to them- 

 selves such varying numbers of fellow-atoms oxygen 

 one, hydrogen two, and so on ? These are questions for 

 the future. The wisest chemist does not know why the 

 simplest chemical experiment results as it does. Take, 

 for example, a water-like solution of nitrate of silver, 

 and let fall into it a few drops of another water-like solu- 

 tion of hydrochloric acid; a white insoluble precipitate 

 of chloride of silver is formed. Any tyro in chemistry 

 could have predicted the result with absolute certainty. 

 But the prediction would have been based purely upon 

 previous empirical knowledge solely upon the fact that 

 the thing had been done before over and over, always 

 with the same result. Why the silver forsook the ni- 

 trogen atom, and grappled the atom of oxygen, no one 

 knows. Nor can any one as yet explain just why it is 

 that the new compound is an insoluble, colored, opaque 

 substance, whereas the antecedent ones were soluble, 

 colorless, and transparent. More than that, no one can 



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