THE PHOTOGRAPHIC IMAGE. 387 



man's laud — peopled by so-called " molecular compounds," into which 

 the pure chemist ventures but timidly. Bat these compounds are 

 more and more urgiug their claims for consideration, and sooner or 

 later they will have to be reckoned with, even if they lack thatdefinite- 

 ness which the modern chemist regards as the essential criterion of 

 chemical individuality. The investigation may lead to the recognition 

 of a new order of chemical attraction, or of the old chemical attraction 

 in a difl'erent degree. The chemist who discourses here upon this sub- 

 ject at the end of the half century of photography into which we have 

 now entered will no doubt know more about this aspect of chemical 

 affinity ; and if I may invoke the spirit of prophecy in concluding, I 

 should say that a study of the photographic film with its invisible image 

 will have contributed materially to its advancement. 



