300 Mr Clark an the Pyrophosphate of Soda. 



very curious, and till now unobserved effect of heat may be 

 verified by any body within reach of an apothecary's shop. 



It was now evident that if the yellowness of the precipitate 

 was owing to an impurity, that impurity must be one which 

 heat expels. I was accordingly about to submit the phosphate 

 of soda to a red heat, and to pass the vapours which might 

 come from it through a solution of nitrate of silver; when I 

 observed a new circumstance, which proved that the change 

 was more fundamental than could be accounted for by the 

 presence of any slight impurity. 



When to a solution of the undried phosphate of soda, you 

 add nitrate of silver till no farther precipitate appears, you get 

 a mother liquor which is acidulous in its action on vegetable 

 colours ; but if you use in the same manner a solution of the 

 dried phosphate of soda which yields a white precipitate, you 

 get a mother liquor, which in its action on vegetable colours, 

 is neutral. The reason why the yellow precipitate leaves an 

 acid mother liquor, is that the phosphate of silver contains 

 more oxide (two-sevenths more according to Dr Thomson; 

 one-half more according to Dr Berzelius, but at all events 

 more oxide) than the phosphate of soda. This excessive pro- 

 portion of oxide, separating from solutions that are neutral, 

 must leave them acid. But the white precipitate, as it leaves 

 a neutral solution, must either contain a less proportion of ox- 

 ide of silver, or be precipitated from a salt which contains a 

 greater proportion of soda. In either case, the salt, which 

 produces the white precipitate with nitrate of silver, cannot be 

 the same, as the phosphate of soda, which produces the yel- 

 low precipitate, with nitrate of silver. I suspected, therefore, 

 that a red heat had produced some constitutional change on 

 the phosphate of soda. 



In search of more decisive evidence of the production of 

 such a change, I dried and heated red about a pound weight of 

 crystallized phosphate of soda, bought at an apothecary's. 

 The heat which I employed in this experiment did not fuse 

 the salt ; but, in after experiments, the heat was commonly 

 urged to the temperature of fusion. The salt which had been 

 dried and heated red was dissolved in water, concentrated, and 

 set aside for crystallization, I thus obtained a plentiful crop 



