Mr Clark on the Pyrophosphate of Soda. 307 



which it contained. Lastly, then, does the common phosphate 

 of soda contain one proportion of oxygen, and one proportion 

 of hydrogen, more than what is commonly supposed, in com- 

 bination with the salt, with its alkali, or with its acid ? 



These are questions, which a more extended course of ex- 

 periments than we yet possess must answer. 



If we were to consider nothing else than the difference in 

 the action of heat on crystals of the phosphate and the pyro- 

 phosphate of soda, the probability would be very strong in- 

 deed, that the last proportion of water, which is expulsible 

 from the phosphate only by a red heat, is not water of crystal- 

 lization in combination with the salt, but rather water, or the 

 elements of water, in combination with the acid or base. But 

 I made some experiments on another salt, which weigh very 

 much against this probability. 



The researches of Dr Berzelius and of Professor Mitscher- 

 lich have well established that the arseniates are similar to the 

 phosphates, in their constitution, in their properties, and even 

 their form. I was induced therefore to try whether a change 

 could be produced by heat on the arseniate of soda, similar to 

 that remarkable one, which tjie phosphate of soda had under- 

 gone. The appearances I found the same; but the effects 

 were different. For, the arseniate of soda submitted to a sand- 

 bath heat lost all its water, except one proportion, which it lost 

 when urged by a red heat, just as the phosphate of soda had 

 done ; but then the watery solution of the arseniate which had 

 been heated red hot, and even to fusion, did not produce dif- 

 ferent precipitates with other salts ; and, being set aside for 

 crystallization, it gave the old crystals back again, which had 

 the property of retaining as obstinately as before the last pro- 

 portion of water. Now all this looks very like, as if the phos- 

 phate and the arseniate of soda possessed the common proper- 

 ty, of retaining one proportion of water, expulsible only by a 

 red heat ; and as if the heat, which expels this water from 

 them both, produces on the phosphate of soda an additional 

 change, which it does not produce on the arseniate of soda. 

 In this view, the expulsion of the last proportion of water by 

 heat would be, not the cause of the conversion of phosphate of 



