1320.] Specific Gravity of Gases. 263 



Therefore an atom of phosphorus is 12 times heavier than an 

 atom of hydrogen. It is of course a multiple of 0-125; and 

 0-125 X 12 = 1-5 = the weight of an atom of phosphorus. 



The specific gravity of vapour of phosphorus = 0-8328. 

 This if we reckon the specific gravity of oxygen unity, will be 

 reduced to 0-75, and 0-75 x 2 = 1-5 = the weight of an atom 

 of phosphorus. It is obvious from all this that the true specific 

 gravity of phosphuretted hydrogen gas is 0-90277, and that it is 

 composed of 



Hydrogen 0-06944 



Phosphorus 0-83333 



0-90277 



For these numbers answer exactly to the requisite conditions : 

 •06944 X 12 = -83333 ; and 1-1111:1:: 0-90277 : -75. 



Now 0-9022, my old determination, differs from this only 

 __L— d part. The new determination 0-90325 differs only -tW*^ 

 parV. Both of these discordancies are considerably within the 

 limits of unavoidable errors from experiment. If we were to 

 take the average of the two sets of experiments, we should 

 obtain for the specific gravity of phosphuretted hydrogen gas 

 0-902735. Now this number accords with the calculated 

 specific gravity in its four first decimal places, and indeed 

 differs from it only by ^-r^rrth part, a quantity qmte inap- 



^"^I think then that it is demonstrated that the specific gravity 

 of phosphuretted hydrogen gas is 0-90277 ; and that sm atom of 

 phosphorus weighs 1-5. 



2 Bihydrognret of Phosphorus.— This gas was first particu- 

 larly examined by Sir H. Davy in 1812, though it had often been 

 noticed before by chemists. He obtained it by heating crystal- 

 lized phosphorous acid. It may be obtained hkewise by expos- 

 ing pure phosphuretted hydrogen to the direct rays of the sun. 

 It was in this last way that the quantity of it necessary tor 

 trying its specific gravity was obtained in my laboratory. Only 

 one experiment was made, on account of the difficulty of obtain- 

 ing this gas in quantity. The specific gravity was found 

 0-9653. 



From the experiments of Davy, and indeed from my own, 

 there cannot be the least doubt that this gas is composed of 



2 volumes hydrogen gas \ condensed into one volume. 



1 volume vapour of phosphorus j 



Hence its specific gravity must be equal to twice the specific 

 gravity of hydrogen gas + the specific gravity of vapour ot 

 phosphorus. 



