MEMOIR XXIX.] CHEMICAL FORCE IN THE SPECTRUM. 



rapidly increasing concentration of the rays occasioned 

 by the peculiarity of prismatic dispersion towards the 

 red end will give a deceptive preponderance in that di- 

 rection. Without entering further into this discussion, 

 it is sufficient for my present purpose to understand that 

 the decomposition in question is accomplished by rays 

 between the fixed lines B and F. 



The two absorptive media, potassium bichromate and 

 cupro-ammonium sulphate, so often and so usefully em- 

 ployed in actino - chemical researches, corroborate this 

 conclusion. Plants cannot decompose carbonic acid, nor 

 can they turn green in rays that have passed through a 

 solution of the latter salt. They accomplish both those 

 results in rays that have passed through the former. 



The decomposition of carbonic acid, and the produc- 

 tion of chlorophyl by the less refrangible rays of the 

 spectrum, afford thus a striking illustration that chem- 

 ical changes may be brought about by other than the 

 so-called chemical rays. 



. Case of tlie Colors of Flowers. 



/ / 



The production and destruction of vegetable colors by 

 the agency of light have, of course, long been a matter of 

 common observation. Little has, however, been done in 

 the special examination of the facts, and that little for 

 the most part by Herschel. 



We have only to examine his Memoir in the Philo- 

 sophical Transactions (Part II., 1842) to be satisfied that 

 nearly every radiation can produce effects. Thus the 

 yellow stain imparted by the Corcliorus Japonica to pa- 

 per is whitened by the green, blue, indigo, and violet 

 rays. The rose-red of the Ten-weeks-stock is in like man- 

 ner changed by the yellow, orange, and red. The rich 

 blue tint of the Viola odorata, turned green by sodium 

 carbonate, is bleached by the same group of rays; that is, 



