IV. 



NEW CHEMICAL REACTIONS PRODUCED BY 

 LIGHT. 



1868-69. 



MEASUEED 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 timeli- 

 ness of application, are able to produce great effects. A 

 series of these, of a novel and beautiful character, dis- 

 covered in 1868, and further 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 

 96 



