PHOSPHOROUS ACID3. 427 



(b.) The sticks of phosphorus are also covered witli a white in- 

 crustation, which was formerly supposed to be an oxide of phos- 

 phorus. 



(c.) If the phosphorus be at the same time exposed to the light, 

 the incrustation becomes brown, water being decomposed, as is 

 evinced by the evolution of phosphuretted hydrogen, and the forma- 

 tion of the phosphorus and phosphoric acids.* 



(d.) When phosphorus is burned in less atmospheric air than is 

 necessary to its entire consumption, there remains a red substance, 

 supposed by some to be a hyd rated oxide, but the exact composition 

 of the oxides of phosphorus has not yet been ascertained. I have 

 observed that the same substance is formed when phosphorus is burn- 

 ed in oxygen gas. 



(e.) Phosphorus being burned in highly rarefied air, produces a 

 " red solid, comparatively fixed, and requiring a heat above 212 

 for its fusion a white, and easily volatile substance, which is com- 

 bustible, soluble in water, and has acid properties, and a substance, 

 which is strongly acid, and not volatile, even at a white heat.f The 

 first appears to be a mixture of unburned phosphorus, and phospho- 

 rous acid ; the second to be phosphorous acid ; and the third to be 

 phosphoric acid."J 



The white volatile solid unites with bases, and forms salts, that 

 are called phosphites. This acid absorbs oxygen from the air, and 

 becomes phosphoric acid, and on account of its avidity for oxygen, 

 it precipitates the salts of mercury, silver, platinum and gold. 



PHOSPHOROUS ACID OF DAVY. 



REMARK. According to Sir H. Davy, as already stated, what has 

 been usually called phosphorous acid, is a mixture of the phosphorous 

 and phosphoric. 



1. PREPARATION. 



(a.) Sublime phosphorus through corrosive sublimate, and a limpid 

 fluid is obtained, a compound, as is supposed of chlorine and phos- 

 phorus. 



(6.) Mix this product with water, and apply heat till the liquid is 

 of the consistence of syrup ; it is said to be a solution of pure phos- 

 phorous acid in water, and it becomes solid and crystalline on cool- 



* I have a bottle of sticks of phosphorus which were put up for me in London, 25 

 years ago. Having been distilled in hydrogen gas, they were then white, but they 

 are now covered by a lively red crust, and the water, which has never been chang- 

 ed, is decidedly acid. 



t According to the views now entertained of the pure glacial phosphoric acid, it 

 should be volatilized at a lower heat than that stated in the text, on the authority of 

 Sir H. Davy. 



t Henry, 10th Ed. Vol. I, p. 371. 



