IV 



NEW CHEMICAL EEACTIONS PRODUCED BY LIGHT 



1868-1869 



MEASUKED by their power, not to excite vision, but 

 to produce heat — in other words, measured bj their 

 absolute energy — the ultra-red waves of the sun 

 and of the electric light, as shown in the preceding arti- 

 cles, far transcend the visible. In the domain of chem- 

 istry, however, there are numerous cases in which the 

 more powerful waves are ineffectual, while the more mi- 

 nute 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 illustrated 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. 



A glass tube three feet long and three inches wide, 

 which had been frequently employed in my researches on 

 radiant heat, was supported horizontally on two stands. 

 At one end of the tube was placed an electric lamp, the 

 height and position of both being so arranged that the axis 

 of the tube, and 'that of the beam issuing from the lamp, 



ao3) 



