352 PHOSPHORUS. 



decomposed by sulphuretted hydrogen, afford respectively the 

 terhydrate, deutohydrate and protohydrate of phosphoric acid. 

 The statement of the decomposition of the metaphosphate of 

 lead by sulphuretted hydrogen, will be sufficient to ex- 

 plain how a hydrate of phosphoric acid comes to be formed in 

 all these cases : 



Before decomposition. After decomposition. 



Metaphosph. / P h s P>>oric acid- _, Metaphosph. of water 



of lead 1 ^ x y en -7 (" r tohydr. of phos. ac.) 



(.Lead ^/ 



Sulphuretted f Hydrogen . . -^S^ 



hydrogen [Sulphur . . . . _^ Sulphuret of lead. 



It will be observed that the sulphuretted hydrogen forms an 

 equivalent of water, at the same time that it throws down the 

 sulphuret of lead ; in this phosphate of lead, there is only one 

 equivalent of oxide of lead, and consequently only one equiva- 

 lent of water formed, but if there were two or three equivalents 

 of oxide, there would be two or three equivalents of water 

 formed ; or the phosphoric acid is always left in combination 

 with as many proportions of water as it previously possessed 

 of oxide of lead. Thus the different hydrates of phosphoric 

 acid are obtained, from the decomposition of the corresponding 

 phosphates of lead. 



In no decomposition of this kind, is there any transition from 

 one class of phosphates into another, because the decompo- 

 sitions are always mutual, and the products of a neutral charac- 

 ter. Hence an argument for retaining the trivial names, common 

 phosphates, pyrophosphates and metaphosphates, for there is 

 no changing, in decompositions by the humid way, from one to 

 the other, and the salts comport themselves so far quite as if 

 they had different acids. The circumstances may now be 

 noticed, in which a transition from the one class to the other 

 does occur : 



1 st. Changes without the intervention of a high tempera- 

 ture. When solutions of the metaphosphate and pyrophosphate 

 of water are warmed, they pass gradually into the state of com- 

 mon phosphate, combining with an additional quantity of 

 water ; and the metaphosphate of water appears then to become 

 at once common phosphate, without passing through the 

 intermediate state of hydration of the pyrophosphate. The 



