THE PERIODIC LAW. 169 



yet the cause of gravitation is even to-day a mere speculation, and no 

 link has yet been discovered to connect the three phases of attraction, 

 gravitation between masses, cohesion and adhesion between molecules, 

 and chemical affinity between atoms. The fact of evolution is univer- 

 sally recognized, much is known and more is a matter of speculation as 

 to how and why evolution has taken place, but why an organism tends 

 to resemble its progenitor and why it tends to vary are as unknown as 

 before the days of Darwin and \Yallace. So the Periodic Law is, after 

 all, a mere statement of fact. But it is a statement which has already 

 exerted vipon chemistry an influence as great as that of gravitation 

 and evolution in their respective fields, and is destined more and more 

 to become the very foundation of chemistry. We may hope and con- 

 fidently expect that it will eventually lead us to the real constitution 

 of matter. 



