LIGHT AND THE QUANTUM 115 



flow of liquids have yet shown any discernible 

 deviation from its laws. Yet we know that water 

 is only an aggregation of molecules, that such 

 an idea as that of constant density must be in- 

 valid, and that if our vision could be amplified 

 and our apparatus could be made more delicate, 

 we should make observations not at all in ac- 

 cord with the theorems of hydrodynamics. Still 

 it ought to be possible to enlarge the old science 

 of hvdrodynamics so as to include molecular 

 motions, and among the theorems of this larger 

 science the former theorems would still appear 

 as important corollaries for the limiting case 

 of an infinite number of molecules. Before this 

 could be done it would be necessary to decide a 

 number of questions; for example, whether, 

 when two molecules collide, the impact is instan- 

 taneous or lasts a certain definite time, or 

 whether it begins at infinite distance, becoming 

 appreciable only at close approach. 



We have made a little closer approach to the 

 solution of such problems in the science of elec- 

 tricity, where that which was once called the 

 electric fluid is now regarded as an assemblage 

 of electrons. The electric field invented by Max- 

 well has been adopted into electron theory, so 

 that we regard each electron as producing a 



