452 ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX. 



From the beginning of optical chemistry, investigations 

 have been made for the purpose of determining the ac- 

 tion of rays of different refrangibHities. Almost a hun- 

 dred years ago it had been shown, in special cases, that 

 there is an antagonism between the opposite ends of the 

 spectrum. Thus the phosphorescence excited in Can- 

 ton's preparation by the violet end of the spectrum is ex- 

 tinguished by the red. As respects colored compounds, 

 Grotthus showed that the active ray is very commonly 

 of the tint complementary to that which it destroys. 



More recently this branch of the subject has been ex- 

 amined to a great extent, and the behavior of all kinds 

 of substances in the solar spectrum made known. The 

 general result is this, that on wave-length or, what is 

 the same thing, frequency of vibration, the number of 

 impulses it can communicate in a given period of time 

 depends the power of a ray to break down the union of 

 any group of atoms. A compound that may resist a 

 slowly recurring motion may be unable to maintain it- 

 self when the impulses increase in frequency. 



So numerous and well known are the photographic 

 and other changes brought on by light that I need not 

 occupy space with a description of them here. I shall 

 only refer to some curious instances of antagonism or in- 

 terference, the details of which will be found in the orig- 

 inal Memoir. Hitherto they have been very much over- 

 looked. 



Two rays may be so placed in relation to each other 

 that their motions may conspire or may antagonize ; and 

 as one or other of these conditions ensues, the chemical 

 result will correspond. 



When iodine vapor is permitted to have access to a 

 surface of polished silver, the silver tarnishes, the tar- 

 nished film increasing in thickness as the exposure to 

 the iodine is prolonged. It assumes in succession colors 



