264 Dr. Thomson on the [Oct^ 



Twice the sp. gr. of hydrogen gas. = 01388 

 Sp. gr. of vapour of phosphorus . . = 0'8333 



0-9722 



Thus we see that its true specific gravity is 0"9722. The differ- 

 ence between this quantity and the specific gravity which we 

 found is about -yrirth part — a quantity which will not be consi- 

 dered as great for a first experiment. In all probability, the 

 whole of the phosphuretted hydrogen gas had not been decom- 

 posed by the action of the solar rays. A mixture of about 

 one-ninth of phosphuretted hydrogen gas would have given 

 hydroguretted phosphorus the specific gravity which we found. 



It is obvious that this gas is a compound of two atoms hydro- 

 gen and one atom phosphorus. Hence the weight of an integrant 

 particle of it is 1-75 ; for 



2 atoms hydrogen = 0*25 



1 atom phosphorus = 1'5 



r75 



Its specific gravity, supposing that of oxygen to be unity, will 

 be 0"875, and 0-875 x 2 = 1-75. Thus we see that its atomic 

 weight is just twice that of its specific gravity. In this respect, 

 it ao-rees with the other oaseous bodies which we have examined 

 m this essay. 



IV. Iodine, 



Iodine is easily converted into a vapour ; but it would be 

 difficult to determine the specific gravity of this vapour experi- 

 mentally. There is no great difficulty, however, in demonstrat- 

 ing from the specific gravity of hydriodic acid gas, and its known 

 composition what the specific gravity of vapour of iodine must 

 be. The same gas puts it in our power to determine the weight 

 of an atom of iodine with accuracy. 



Hydriodic acid gas is obtained when we bring iodine and 

 phosphorus in contact with each other. A great deal of heat is 

 evolved, and the hydriodic acid gas is expelled in considerable 

 quantity. It must be received over mercury, and its specific 

 gravity must be taken as soon as the gas is collected, because 

 mercury has the property of decomposing it when they are left 

 in contact of each other. The following were the specific gravi- 

 ties of this gas as ascertained in my laboratory : 



By first trial 4-3758 



By second trial 4-3756 



By third trial 4-3756 



Mean 4-3756{> 



