A HISTORY OF SCIENCE 



manifests in some degree. If we assume that the pow- 

 er which holds one atom to another is the same which in 

 the 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 

 themselves 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 an- 

 other water-like solution 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 knowl- 

 edge solely upon the fact that the thing had been 

 done before over and over, always with the same result. 

 Why the silver forsook the nitrogen 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 transpar- 

 ent. More than that, no one can explain with certainty 

 just what is meant by the familiar word soluble itself. 

 That is to say, no one knows just what happens when 

 one drops a lump of salt or sugar into a bowl of water. 

 We may believe with Professor Ostwald and his fol- 



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