MEMOIR XXX.] ON BURNING GLASSES AND MIRRORS. 45 1 



Any artificial coloration by the addition of extraneous 

 bodies does not increase the rate of decomposition, but 

 retards it. This is precisely what ought to be expected. 

 A compound atom has its grouping destroyed by the 

 action of light upon its own parts, and is in no manner 

 concerned in what is taking place in other atoms around. 

 They therefore cannot increase the effect on it; but, on 

 the contrary, they may greatly diminish the action on 

 the mass by exerting a special absorption themselves. 

 Thus the chloride of gold retards the decomposition of 

 chlorine water, when mixed therewith, in the same man- 

 ner as if it were placed in a trough in front of the water, 

 and intercepted the impinging beam. 



Experiments similar to the foregoing were made with 

 a solution of ferric oxalate mixed with alcohol, ammonia 

 citrate of iron, tincture of turmeric, sodic chloride, etc. 

 In every instance it was clear that the action of the light 

 is strictly molecular ; that it is impressed on the group of 

 atoms, and not on the mass; and that when various bod- 

 ies are conjointly exposed to the sun, each one undergoes 

 its own specific change, independently of and unaifected 

 by all the rest. 



These experiments, with others of a like kind, made 

 many years ago, have an important bearing on some 

 recently published by Professor Vogel, Captain Abney, 

 Captain Waterhouse, and others on imparting increased 

 sensitiveness to collodion by mixing it with variously 

 colored substances. I repeated their experiments as care- 

 fully as I could, and should have thought that my want 

 of success was due to unskilfulness had I not borne in 

 mind the foregoing considerations. 



2. We may next inquire, To what extent and m what 

 manner is the decomposition of a compound body affected 

 by the FREQUENCY of vibration of a ray ? 



