PHOSPHATES. 349 



bodies, or soft solids, without crystallization. All these salts 

 contain only one atom of base to one of acid, like the proto- 

 hydrate of the acid itself. The trivial name metaphosphates 

 was applied to the class by myself, to mark the cause of the 

 retention of peculiar properties by their acid, when free and in 

 solution, namely that it was not then simply phosphoric acid, 

 but phosphoric acid together with water.* This is the least 

 stable of the hydrates of phosphoric acid being converted rapidly, 

 by the ebullition of its solution, into the terhydrate. If the 

 terms metaphosphoric acid and pyrophosphoric acid are employed 

 at all, it is to be remembered that they are applicable to the 

 proto and deutohydrates, and not to the acid itself, which is the 

 same in all the hydrates. But to prevent the chance of mis- 

 conception, metaphosphate of water and pyrophosphate of water 

 might be substituted for these terms. 



A solution of the terhydrate of phosphoric acid, evaporated 

 in vacuo over sulphuric acid, crystallizes in thin plates, which 

 are extremely deliquescent. When heated to 400, that hydrate 

 loses a portion of water, and becomes a mixture of the deuto 

 and protohydrates ; and by heating it to redness for some time, 

 the proportion of water may be reduced to one equivalent, or 

 perhaps even less than this. But at that high temperature much 

 of the hydrated phosphoric acid passes off in vapour. The so- 

 lution of phosphoric acid is not poisonous, nor when concen- 

 trated does it act as a cautery, but it injures the teeth from its 

 property of dissolving phosphate of lime. A solution of the 

 latter salt in phosphoric acid has been prescribed in rickets, a 

 disease which indicates a deficiency of earthy phosphates in the 

 system. The phosphate of soda also is administered as a mild 

 aperient ; its taste is saline, but not disagreeably bitter. 



Phosphates. The formation of three classes of phosphates 

 from the three basic hydrates of phosphoric acid, affords an 

 excellent illustration of the formation of compounds by sub- 

 stitution. The quantity of fixed base, such as soda, with which 

 phosphoric acid combines in the humid way, being entirely 

 regulated by the proportion of water previously in union with 

 the acid, which is simply replaced by the fixed base. Thus, the 



* Researches on the arseniates, phosphates and modifications of phosphoric 

 acid. Phil, Trans. 1833, p. 253 ; or Phil. Mag. 3rd series, vol. 4, p. 401. 



