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



of light acts in bringing about decomposition, and the 

 changes it undergoes ; 2d, the nature of the impression 

 made on the material group, the decomposition of which 

 ensues. The observations I proposed offering in relation 

 to the former of these points being completed, I may pass 

 to some remarks respecting the latter. 



An examination of many cases of the decomposition 

 of bodies by light has led me to the conclusion that its 

 cause is to be attributed to the inability of the group of 

 molecules affected to withstand the periodic impulses 

 communicated to them. Of those molecules some, per- 

 haps, take on a vibratory motion more readily than the 

 others, and the continuance of a given group becoming 

 impossible, a rearrangement ensues. 



But in other cases the mechanism of decomposition is 

 undoubtedly different : a change is impressed on one of 

 the elements acted upon, which weakens its affinity for 

 the others. Thus, under the influence of the sunshine, 

 plants can decompose many bodies, such as carbonic, sul- 

 phuric, and phosphoric acids. 



The nature of these changes may be best illustrated by 

 tracing the complete course through which any one of 

 these substances passes. The chief facts may be seen in 

 the case of phosphorus. This substance, when freshly 

 made, commonly exhibits a white waxy appearance, but 

 when exposed to the sunshine, it turns to a deep mahog- 

 any-red. If the exposure has been long continued, or the 

 effect hastened by the action of a burning-lens, the change 

 of aspect is very striking. It is analogous to that which 

 sulphur exhibits when heated to 400 or 500. I have a 

 specimen which has been kept for many years in an at- 

 mosphere of dry carbonic acid ; the sides of the vessel 

 are incrusted with crystals, which have slowly sublimed, 

 and which in color resemble the ferrid cyanide of po- 

 tassium. 



