430 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX. 



of hydrochloric acid produced measures the quantity of 

 the acting radiation. This I proved by using a gas- 

 flame of standard height, and a measuring lens (page 

 265) consisting of a double convex, five inches in diame- 

 ter, sectors of which could be uncovered by the rotation 

 of pasteboard screens upon its centre, the quantity of hy- 

 drochloric acid produced in a given time being propor- 

 tional to the area of the sector uncovered. The same was 

 also proved by using a standard flame and exposing the 

 gases during different periods of time. The quantity 

 of hydrochloric acid produced is proportional to the time. 

 The following experiment illustrates the phenomena 

 arising durino; the actinization of a mixture of chlorine 



O O 



and hydrogen, and substantiates several of the foregoing 

 statements. 



The diverging rays of a lamp were made parallel by a 

 suitable combination of convex lenses. In the resulting 

 beam a chlor- hydrogen actinometer was placed, there 

 being in front of it a metallic screen, so arranged that it 

 could be easily removed or replaced, and thus permit the 

 rays of the lamp to fall on the actinometer or intercept 

 them. 



On removing the screen and allowing the rays to fall 

 on the sensitive mixture in the actinometer, an expansion 

 amounting to half a degree was observed. In 60 sec- 

 onds this expansion ceased. 



The volume of the mixture now remained stationary, 

 no apparent change going on in it. At length, after the 

 close of 270 seconds, it was beginning to contract and 

 hydrochloric acid to form. 



At the end of 45 seconds more a contraction of half a 

 degree had occurred : the volume of the mixture was 

 therefore now the same as when the experiment began, 

 this half degree of contraction compensating for the half 

 degree of expansion. 



