458 ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX. 



The chemical properties of these two varieties of phos- 

 phorus are very 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 methods of preparing 

 red phosphorus is to expose a solution of the common in 

 sulphuric ether to light a red powder, the substance in 

 question, precipitates. Compared together, the one dis- 

 plays 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 phosphorus, 

 when united with hydrogen, yields a gas which is spon- 

 taneously combustible in the air; the red or passive va- 

 riety yields a hydrogen compound of the same constitu- 

 tion, but devoid of the property of spontaneous combus- 

 tibility. 



It should be understood that though other agents as 



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a high temperature can impress this remarkable change 

 upon phosphorus, none can do it with more energy or 

 more completely than the solar rays. I found by expos- 

 ing a surface of white or active phosphorus to the pris- 

 matic spectrum that it is the more refrangible rays that 

 are the most effective. Thus the rays most efficient in set- 

 ting oxygen free from the bodies with which it is united 

 have also the quality of impressing such a change on 

 those bodies that they oxidize subsequently with diffi- 

 culty. It follows that the true cause of such decompo- 

 sitions is the impression which the light makes on the 

 elementary substance. Thus if phosphoric acid be de- 

 composed by the solar rays, the decomposition is owing 

 to the phosphorus being thrown into the red or passive 

 state a state in which its affinity for oxygen has almost 

 entirely disappeared. 



These considerations enable us to explain what takes 



