PHOSPHORIC ACID. 353 



metaphosphate of barytes also, which is an insoluble salt, is 

 gradually dissolved, when boiled in water, and becomes com- 

 mon phosphate. The easy transition from the one class of 

 phosphates to the other, then witnessed, forbids the supposi- 

 tion that they contain different acids. It is remarkable that we 

 may have pyrophosphates of potash and of ammonia in solution, 

 and perfectly stable, but not in the dry state. These salts do 

 not crystallize. The pyrophosphate of ammonia, indeed, when 

 allowed to evaporate spontaneously, appears to crystallize, but 

 in the act of becoming solid, it passes into common phosphate 

 (the biphosphate of ammonia, 2HO, NH 4 O-f PO 5 ), which is the 

 salt that forms crystals. 



2nd. Changes with the intervention of a high temperature. 

 If a single equivalent of phosphoric acid, anhydrous, or in any 

 state of hydration, be calcined, at a temperature which may fall 

 a little short of a red heat, (1) with a single equivalent of soda 

 or its carbonate, the metaphosphate of soda will be formed ; 

 (2) with two equivalents of soda or its carbonate, the pyrophos- 

 phate of soda will be formed ; and (3) with three equivalents of 

 soda or its carbonate, a common phosphate of soda will be formed. 

 Hence, the formation of none of these classes is peculiarly the 

 effect of a high temperature. Again, a tribasic phosphate, con- 

 taining one or two equivalents of a volatile base, such as water 

 or ammonia, loses the volatile base, when ignited, and 

 the acid remains in combination with the fixed base. Hence, 

 common phosphate of soda (HO, 2NaO + PO 5 ) is converted by 

 heat into pyrophosphate (2NaO + PO 5 ,) the original observation 

 of Dr. Clark ; and the biphosphate of soda (2HO, NaO + PO 5 ) 

 into metaphosphate of soda (NaO + PO 5 ). The acid remains 

 in combination with the fixed base left with it, and the salt 

 produced may be dissolved in water without assuming basic 

 water. 



The metaphosphate of soda is susceptible of a remarkable 

 conversion, by the agency of a certain temperature, and exhibits 

 a change of nature, without a change of composition, such as 

 often occurs in organic compounds, but rarely admits of so 

 satisfactory an explanation. This particular salt, in common 

 with all the other phosphates, combines with water, which be- 

 comes attached to the salt, in the state of constitutional water, 

 or water of crystallization; The metaphosphate of soda, so 

 hydrated, when dried at 2 1 2, retains one equivalent of water, 



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