PHOSPHORUS. 201 



acid require exactly 4 grains of soda, which 

 will be shown afterwards to be equivalent to an 

 atom. 



7%5 grains of anhydrous phosphate of soda, 

 then, contain just 4 grains of soda. The re- 

 maining 3-5 grains must be the phosphoric acid 

 of the salt. Thus, it appears, that anhydrou 

 phosphate of soda is composed of 



Phosphoric acid 3-5 

 Soda . . 4-0 



7-5 



3*5 of phosphoric acid just saturate 4 of soda, shown to 

 and 14 of protoxide of lead, each of which num- 

 bers, represents the atomic weight of the respec- 

 tive bases ; consequently, 3*5 is the atomic 

 weight of phosphoric acid. 



2. Many attempts have been made by che- Attempts 

 mists to determine the composition of phosphoric 

 acid by the rapid combustion of phosphorus. For ac 

 when phosphorus burns with flame, it unites 

 with the oxygen of the atmosphere, and is con- 

 verted into phosphoric acid. Now, as both 

 phosphorus and phosphoric acid are solids, it ap- 

 pears at first sight easy to determine the quan- 

 tity of oxygen with which the phosphorus has 

 combined, merely by weighing the phosphoric 

 acid produced. But in reality, this method, though 

 apparently easy, is not susceptible of perfect ac- 

 curacy. Phosphorus can scarcely be obtained 



16 



