Mr Clark on the Pyrophosphate of Soda. 305 



I do not enter into any statement of the grounds of my own 

 opinion ; because I anticipate an occasion when I shall dis- 

 cuss the subject at full length. At present, therefore, I shall 

 content myself with simply expressing my conviction, that, 

 with respect to the constitution of the phosphates, the weight 

 of evidence lies decisively on the side of Dr Berzelius. 



According to his tables, a combining proportion of dry 

 phosphate of soda is 16.741, this quantity containing 7. parts 

 of oxygen ; namely 5. in the phosphoric acid and 2. in the 

 soda ; and this dry phosphate of soda corresponds, as the at- 

 tentive reader must have observed, with what I have called, 

 in this paper, dry pyrophosphate of soda. 



I shall first consider the water of crystallized pyrophosphate 

 of soda; because its proportion in this salt is least liable to 

 doubt ; and then I shall consider the water of the crystalhzed 

 phosphate of soda. 



Crystals of Pyrophosphate of Soda. 

 . In the preceding part of this paper, there has been given 

 two experiments on the water of hard well-formed crystals of 

 this salt, where the water lost was .4072. I made only one 

 other experiment on crystals, which were much softer, but 

 which were pounded, and dried in blotting paper. The water 

 was .4059. 



Dried pyrophosphate of soda - 16.741 16.741 



Water - - 11.521 11.438 



The water in the former of these contains 10.24 oxygen ; 

 and in the latter 1 0.1 7. It is probable, therefore, that this 

 salt contains 10 proportions of water. 



Crystals of Phosphate of Soda. 

 The water in crystals of this salt has likewise been already 

 stated. But phosphate of soda retains much water, mechani- 

 cally, among its crystals; and this mechanical water is a 

 source of difficulty ; because you cannot pound and dry them, 

 without risk of losing water by efflorescence. I took an ap- 

 proximate method. I put some crystals in the corner of a 

 silk handkerchief and pounded them with a wooden mallet, 

 drying , them as they were broken with the handkerchief. 



