260 Dr. Thomson on the [Oct. 



by pouring muriatic acid on pounded sulphuret of antimony in a 

 retort, and exposing the mixture to a moderate heat. It may 

 be procured hkevvise by mixing together equal weights of clean 

 iron filings and flowers of sulphur with a sufficient quantity of 

 water, and heating the mixture in a retort till it becomes black. 

 Considerable heat is evolved during the combination of the iron 

 and sulphur, which keeps the mixture boiling long after it is 

 removed from the fire. It was by this last process that the 

 sulphuretted hydrogen prepared in my laboratory for determining 

 its specific gravity was prepared. Its specific gravity was 

 found as follows : 



By first trial M780 



By second trial 1-1796 



By thn-d trial M796 



Mean M7906 



There have been various previous experiments to determine 

 the specific gravity of this gas ; though I have reason to believe 

 that no preceding chemist has come so near the truth as we 

 have done. 



Kirwan found it ri06* 



Thenard l-231t 



Gay-Lussac and Thenard 1-1912 J 



DaVy 1-1967 



Kirwan's gas was no doubt contaminated with common air, or 

 perhaps even hydrogen gas ; for I have found that when sulphu- 

 retted hydrogen gas is obtained by the process which he 

 employed, it usually contains a mixture of hydrogen gas. 



We have a simple method of determining the specific gravity 

 of this gas from the data established in the preceding part of 

 this paper. When sulphur is heated in hydrogen gas, that gas 

 is gradually converted into sulphuretted hydrogen gas without 

 any alteration in its volume. It follows from this that sulphuret- 

 ted hydrogen gas is a compound of 



1 volume hydrogen gas | ^^^^^^^^^^ j^^^ one volume. 

 1 volume vapour oi sulphur J 



Consequently the specific gravity of this gas is equal to the 

 specific gravity of hydrogen gas + the specific gravity of 

 vapour of sulphur. 



Sp gr. of hydrogen gas 0-0694 



vapour of sulphur 1-1111 



1-1805 



• On Phlogiston, p. 3. f Ann. de Chim. xxxii. 267. 



% Recherches Pliysico-Chim. i. 191. 



