OF SODA. 271 



I made use of this salt, which is perfectly neu- 

 tral, to determine the atomic weight of phospho- 

 ric acid. After analyzing it in the way described 

 in a preceding part of this work, I formed it by 

 dissolving in water 35 grains of pure phosphoric 

 acid, and adding to the solution 180 grains of 

 crystallized carbonate of soda. The solution, 

 after tins addition, was perfectly neutral, and 

 yielded phosphate of soda to the very last drop. 

 Now, 35 grains of phosphoric acid represent 10 

 atoms, and 180 grains of crystallized carbonate of 

 soda contain 40 grains of soda, equivalent to 10 

 atoms. This experiment proves decisively, that 

 the atom of phosphoric acid weighs 3-5, and that 

 phosphate of soda is composed of an atom of 

 acid united to an atom of base. The water of 

 crystallization was determined by exposing the 

 salt to a red heat. I have already stated, that 

 when the salt is quite free from efflorescence the 

 water amounts to 12 atoms, for 21 grains of the 

 salt lose 13'5 grains of water. But I have fre- 

 quently found the water less than this, obviously 

 from an incipient efflorescence. In some of my 

 earlier experiments it amounted only to 11 

 atoms ; but in my later experiments in which I 

 was at the requisite pains to prevent efflores- 

 cence, the water was always 12 atoms. The con- 

 stituents of the salt are 



