NEW CHEMICAL REACTIONS. 71 



other a dweller in England,* who never in their inquiries 

 consciously set a practical object before them whose only 

 stimulus was the fascination which draws the climber to a 

 never-trodden peak, and would have made Caesar quit his 

 victories for the sources of the Nile. That the knowl- 

 edge brought to us b"y those prophets, priests, and kings 

 of science is what the world calls " useful knowledge," the 

 triumphant application of their discoveries proves. But 

 science has another function to fulfill, in the storing and 

 the training of the human mind; and I would base my 

 appeal to you on the specimen which has this evening been 

 brought before you, whether any system of education at 

 the present day can be deemed even approximately com- 

 plete, in which the knowledge of Nature is neglected or 

 ignored. 



CHAPTER IV. 



NEW CHEMICAL REACTIONS PRODUCED BY LIGHT. 



1868-69. 



MEASURED by their power, not to excite vision, but to 

 produce heat in other words, measured by their absolute 

 energy the ultra-red waves of the sun and of the electric 

 light, as shown in the preceding articles, far transcend the 

 visible. In the domain of chemistry, however, there are 

 numerous cases in which the more powerful waves are 

 ineffectual; while the more minute waves, through what 

 may be called their timeliness of application, are able to 

 produce great effects. A series of these, of a novel and 

 beautiful character, discovered in 1868, and further illus- 

 trated in subsequent years, may be exhibited by subjecting 

 the vapors of volatile liquids to the action of concentrated 

 sunlight, or to the concentrated beam of the electric light. 

 Their investigation led up to the discourse on " Dust and 

 Disease" which follows in this volume ; and for this reason 

 some account of them is introduced here. 



* Faraday, 



