Prof. Draper on the Chemical Action of Light. 391 



Thus, under the iufluence of the sunshine, plants can decompose 

 many botUes, such as carbonic, sulphuric 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 are best 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 of 

 a deep mahogany-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 atmosphere of dry carbonic 

 acid ; the sides of the vessel are incrusted with crystals which 

 have slowly sublimed from it, and which in colour resemble the 

 ferridcyanide of potassium. 



The chemical properties of these two vai'ieties of phosphorus 

 are wholly different. Indeed there is scarcely a point in which 

 they may not be said to be unlike. The common kind shines in 

 the dark, the red does not ; the common is soluble in a variety 

 of menstrua, which do not act on the other : thus one of the 

 best methods of preparing red phosphorus is to expose a solution 

 of the common in sulphuric aether to the light ; a red powder, 

 the substance in question, precipitates. Contrasted together, 

 the one displays a range of affinity which the other does not ; 

 nor do these properties seem to leave them when they are united 

 with other bodies. Thus, the active or white phosphoi-us, when 

 united with hydrogen, yields a gas which is spontaneously com- 

 bustible in the aii' ; the red, or passive variety, yields a hydrogen 

 compound of the same constitution, but devoid of the property 

 of spontaneous combustibility. 



It shordd be understood, that though other agents, as a high 

 temperature, can impress this remarkable change upon phos- 

 phorus, none do it with more energy or more completely than 

 the solar rays. I found by exposing a stick of white, or active 

 phosphorus, to the prismatic spectrum, that it is the more refran- 

 gible rays which are the most effective, those rays which were 

 formerly termed deoxidating. Thus the rays which are most 

 efficient in setting oxygen free from the bodies vrith which it is 

 united, have also the quality of impressing such a change on 

 those bodies that they oxidize subsequently with great difficulty. 

 It follows, that the true cause of such decompositions is the im- 

 pression which the light makes on the elementaiy substance : 

 thus if phosj)lioric acid is decomposed by the solar rays, the de- 

 composition is owing to the phosphorus being thrown into the 

 red or j)assive state, a state in which its affinity for oxygen has 

 almost entirely disappeared. 



