PHOSPHORUS. 203 



the acid) : 100 (the phosphorus in the acid) : : 1 

 (an atom of oxygen) : 0-74074. = atomic weight 

 of phosphorus. But we have just seen that its 

 true atomic weight is 3-5. 



Let us suppose the quantity of oxygen to be 

 two atoms. This would make the atomic weight 

 of phosphoric acid 3-481. For 135 : 100 : : 2 : 

 1-4815 = atomic weight of phosphorus. 



It appears from this, that phosphoric acid is a 

 compound of two atoms oxygen, and one atom 

 phosphorus; and that the atomic weight of 

 phosphorus, according to the result of Davy's 

 experiments, is 1-4815. But we have seen that 

 phosphoric acid weighs 3-5 ; therefore, the atom 

 of phosphorus must in fact weigh 1-5, or nearly 

 2 per cent, more than results from Davy's syn- 

 thetic experiment. Hence, 100 phosphorus 

 must combine with only 133i of oxygen, to form 

 phosphoric acid ; for 133^ : 100 : : 2 : 1-5. 

 That this is the true composition of the acid will 

 appear from the following experiments. 



3. Phosphorus and hydrogen combine in two 

 proportions, and form two gaseous compounds 

 called phosphuretted hydrogen, and bihydroguret 

 of phosphorus ; the former of which was first 

 distinguished as a peculiar substance by M. Gen- 

 gembre, the latter by Sir Humphrey Davy. 



(1.) Phosphuretted hydrogen gas may be ob- C 

 tained pure, by putting fresh phosphuret of lime phosph 

 into a small retort, filling the retort with water, g e en hydr< 



