LECTURE IX. J HISTORY OF CHEMISTRY. 153 



constitution, but that, on the contrary, it assumed the function 

 of the base. 6 Graham showed, in 1833, how ordinary phos- 

 phoric acid and all its salts may be regarded as compounds of 

 one atom of phosphoric acid, P 2 O 5 , and three atoms of base 

 which can be completely or partially replaced by water. Thus, 

 according to him, ordinary (neutral) sodium phosphate consists 

 of one atom of phosphoric acid combined with two atoms of 

 soda and one of water : on mixing its solution with silver 

 nitrate, the silver salt with three atoms of silver is precipitated, 

 whilst sodium nitrate and nitric acid remain in the solution 

 together. Exceptions to Richter's law (already observed in 

 similar cases by Berthollet), where the mixture becomes acid 

 when the . solutions of two neutral salts are mixed, 7 were thus 

 explained. In this case two atoms of soda and one of water 

 were exchanged for three atoms of silver oxide. 



Another very important result of Graham's investigation is 

 met with in the analysis of pyrophosphoric acid and its com- 

 pounds. Graham shows that on heating the sodium salt men- 

 tioned above to over 350, the water it contains is driven off, 

 and that sodium pyrophosphate, identical with the salt already 

 known, is in this way produced. This salt is not, however, 

 isomeric with the original one, as had been supposed, but 

 differs from it by containing one atom of water less ; and this 

 is of essential importance in regard to the nature of the acid. 

 The white precipitate, too, which is produced by silver salts, 

 only contains two atoms of silver oxide ; and it is thus a quite 

 general property of pyrophosphoric acid, to saturate only two 

 atoms of base (or of water), which distinguishes it very sharply 

 from ordinary phosphoric acid. In the latter the ratio of the 

 oxygen in the base to that in the acid is as 3 to 5 ; in the other 

 acid it is as 2 to 5. 



Graham finds further that on heating the acid sodium 

 phosphate, which consists, according to him, of one atom of 

 phosphoric acid, one atom of soda, and two atoms of water 



6 Phil. Trans. 1833, 253 ; A.C.R. No. 10 ; Annalen. 12, I. 7 Ber- 

 thollet, SUit. Chim. I, 117 ; E. I, 85. 



