116 PROGRESS OF SCIENCE IN THE CENTURY. 



definite evolution or absorption of heat. The object 

 of thermal chemistry is to measure the energy of 

 chemical changes by thermal methods, and thus to get 

 nearer the fundamental problem of the dynamics of 

 chemical affinity. 



Photochemistry. — There are few problems more 

 fascinating and more important than those which are 

 raised when we try to follow the transformations of 

 sunlight. Chemical processes in the sun give rise to 

 radiant energy, which is propagated with great ve- 

 locity (3 + 10*^ cm. per second) through space, 

 with the ether for its hypothetical vehicle. When it 

 reaches the earth, part of it passes into the form of 

 heat and thence into many other forms, while part 

 of it acting on green plants resumes the form of 

 chemical energy. The radiant energy of sunlight is 

 utilised by the green leaves to split up the carbonic 

 acid of the atmosphere and to build up the complex 

 substances which furnish food and fuel, not to speak 

 of the most valuable super-necessaries of life. 



I^or does the radiant energy affect plants only, it 

 has a subtle influence on many animals, modifying 

 for instance the process of coloration, and above all 

 producing those chemical changes in the retina which 

 are associated with vision. In the volume of this 

 series which deals with Inventions due notice will be 

 taken of photography (Daguerre, 1838), which de- 

 pends on the chemical reactions produced by light on 

 a sensitive surface. But the retina was the first sen- 

 sitive surface, and we may therefore say that it was 

 in the consideration of problems primarily physio- 

 logical and secondarily technical that photochemistry, 

 like thermochemistry, had its beginnings. 



We have just mentioned the effect of light upon 

 the human eye, and as an illustration from the other 



