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 ab- 

 solute 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 farther illustrated in subsequent years, 

 may be exhibited by subjecting the vapours 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 3 feet long and 3 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 



