1820.] Specific Gravity of Gases. ' 2Q1 



Thus we learn that the true specific gravity of sulphuretted 

 hydrogen gas is 1-1865. Our determination differs from this 

 only by -,., ^'^^--j-g- — a difference certainly within the limits of una- 

 voidable error from experiment. It, therefore, serves to confirm 

 and establish the accuracy of the calculated specific gravity. 



A volume of sulphur vapom- is obviously equivalent to an 

 atom. As sulphuretted hydrogen is composed of one volume 

 sulphur vapour -f one volume hydrogen gas, it follows that it is 

 a compound of 



1 atom sulphur = 2"000 



1 atom hydrogen = 0*125 



2-125 



Hence the weight of an atom of it is 2-125. If we reckon 

 the specific gravity of oxygen gas unity, then the specific 

 gravity of sulphuretted hydrogen gas will be 1*0625. Now 

 r0625 X 2 = 2*125; so that the atomic weight of this gas is 

 just double its specific gravity. In this property, it agrees with 

 almost all the gases which we have had occasion to mention in 

 this essay. 



The volume of the two constituents of this gas is reduced to 

 one-half when they unite together. This shows the strong- 

 affinity which exists between them and the intimacy of the 

 combination which takes place. 



III. Phosphorus. 



None of the compounds of phosphorus and oxygen are 

 gaseous ; but it forms tvv'o gaseous combinations with hydrogen. 

 These will be sufficient to enable us to determine with precision 

 the weight of an atom of phosphorus, and to show that it agrees 

 with carbon and sulphur in having an atom whose weight is a 

 multiple of the atomic weight of hydrogen. 



1. Fhosphuretted Hydrogen Gas. — Though this gas had been 

 noticed by Gengembre and Kirwan as early as the year 1783, it 

 can scarcely be said to have been known as a chemical substance 

 till I published a set of experiments on it in 1816.*' I pointed 

 out the method of procuring it in a state of purity, determined 

 its properties, and ascertained its composition. This gas . is 

 procured in a state of purity by putting phosphuret of lime into 

 pure water. Phosphuretted hydrogen gas immediately begins 

 to be evolved, and continues to make its escape for more than 

 24 hours. It is best to receive it over mercury ; for water, 

 unless it be quite free from air, speedily alters its nature. The 

 specific gravity of a quantity of pure gas obtained in this way in 

 my laboratory was found to be as follows: 



• Annals of Philosophy, viii. 87. 



