3 1 2 CHEMICAL A CTION OF LIGHT. 



certain plausible theories for the most part of a very dubious 

 character respecting the central fire, Plutonism and Nep- 

 tunism, the stratification of the planets, the formation of moun- 

 tains, valleys, and basins. Our mineralogists, thinking far 

 less of the chemical molecular constitution of the different 

 formations than of their crystalline constitution, have minutely 

 studied the physical qualities and geometrical forms of the 

 integral parts of the rocks ; but neither have condescended to 

 direct their inquiries to the layer of soil trodden underneath 

 their feet. Yet this very layer of arable earth, to which all 

 bodies must return after death what they have taken from it 

 during life, this much despised humus, furnishes all our agri- 

 cultural products, the very foundation and support of our 

 material existence. 



To touch industrial occupations to meddle with trade, 

 commerce, or agriculture is unworthy of Science ! Such is 

 the silly cry of the many distinguished savants who pride 

 themselves on what they call their "freedom from selfish con- 

 siderations." 



Be it so ; but then you ought surely to be consistent, and 

 never regard science as a profession or a bread-winner. 



ON THE CHEMICAL ACTION OF LIGHT. 

 It is no easy study to investigate the modifications and 

 chemical effects which the terrestrial surface is capable of 

 receiving or undergoing, either from the direct rays of the sun, 

 or from diffused light. It requires new methods of inquiry, 

 methods frequently of extreme delicacy, as the labours of 



