ix SULPHUR AND PHOSPHORUS 181 



over water. This gas is phosphuretted hydrogen ; but some- 

 times mixed with hydrogen. Liquid caustic potash may be 

 used instead of hydrate of lime, in order to prevent the 

 generation of hydrogen. 



Phosphoretted hydrogen gas has the following pro- 

 perties : (i) When bubbles of it come into the atmosphere, 

 they instantly take fire ; an explosion is produced, and a ring 

 of white smoke ascends, which is phosphoric acid : (2) It 

 is unfit for respiration, and for supporting combustion : (3) 

 Its specific gravity is 0*85, common air being denoted by 

 unity : (4) Water absorbs -^th of its bulk of this gas : 

 (5) If the gas be electrified, the phosphorus is thrown 

 down, and there finally remains the bulk of the gas of pure 

 hydrogen " (New System, II. 456-457). 



The same gaseous compound is formed when a solution 

 of phosphorous acid is heated. 1 (Gengembre, 1783, he. tit.) 

 Water is evaporated, phosphoretted hydrogen escapes and 

 finally a residue of phosphoric acid remains. In this action 

 the phosphorous acid undergoes a process of simultaneous 

 oxidation and reduction, one portion being oxidised to 

 phosphoric acid by another portion, which loses all its 

 oxygen, and is converted into phosphoretted hydrogen. 

 A similar process occurs when the gas is made from phos- 

 phorus by the action of an alkali : in this case a part of the 

 phosphorus is oxidised with the help of oxygen derived 

 from the decomposition of water, whilst another portion 

 combines with the hydrogen of the water to form phosphor- 

 etted hydrogen. 



Phosphoretted hydrogen not an acid. Berthollet points 

 out in his memoir on " Sulphuretted hydrogen " that : 



"Water in which phosphoretted hydrogen is dissolved, 

 shows no sign of acidity ; and solutions of potash, lime and 

 ammonia, do not appear to absorb more of the gas than 

 pure water." 



1 Kopp attributes this discovery to Pelletier (1790), seven years after 

 Gengembre's paper was read at the Paris Academy, but I have not 

 been able to find any reference to it in Pelletier's papers. 



