MKM..III XIX.] MODIFIED CHI.oKlNi: 279 



can be shown by removing the tubes, after they have 

 been in the spectrum for an hour or two, into the gleams 

 of daylight. One by one they exhibit after a time a 

 rise, the order being the green first, then the yellow and 

 the orange, and at last the red. And if at the same time 

 a tube which has been kept in the dark be exposed 

 along with them, they will all rise before it, showing 

 that modification had set in and been going on in them 

 all ; that it had been more active in the green than in 

 the yellow, in the yellow than in the orange, in the or- 

 ange than in the red; and, had the exposure to the spec- 

 trum been long enough, the liquid in every one of the 

 tubes would have risen. 



VII. The Indigo Ray forms the Hydrochloric Acid as 

 well as produces the Preliminary Modification. 



It only remains now to inquire whether the rays caus- 

 ing the production of the hydrochloric acid are those 

 which effect the modification of the chlorine ; in other 

 words, whether the first stage of the process is brought 

 about by the same agent which carries on the second. 

 The experiment I have just described shows that modifi- 

 cation is most actively produced by the indigo ray, and 

 it is easy to show that it is the same ray which carries 

 on the second part of the process ; for, if before placing 

 the tubes in the prismatic spectrum we expose them to 

 the daylight, so that the liquid has just commenced to 

 rise in each, and then to the spectrum, it will be found 

 that the liquid of the tube in the indigo rises most rap- 

 idly, and the others in the order stated before. There- 

 fore we perceive that the same ray commences, carries 

 on, and completes the process. 



Few substances can exceed in sensitiveness to licrht a 



^j 



mixture of chlorine and hydrogen previously modified. 

 Brought into the obscure daylight of a gloomy chamber, 



