38 



becomes gradually, yet very slowly, darkened by the deposition of 

 tar arid carbon from vapour of bisulphide of carbon. The reaction 

 with previously dried slaked lime commences at 108 Fahr., and 

 continues through the whole range of temperature up to redness. 

 At a red heat the sulphur of the bisulphuret of carbon and other 

 sulphur compounds unites with the lime and forms sulphide of cal- 

 cium. Practically very high temperatures are useless, as the hydro- 

 carbons of gas begin to be decomposed about the melting-point of 

 lead, and to deposit their carbon upon the hot lime. Fortunately 

 injurious temperatures are not required. I have frequently freed gas 

 from every trace of sulphur, so that upon combustion no sulphurous 

 acid was generated, by employing lime so heated as not to deposit 

 any carbon, and removing the sulphuretted hydrogen evolved in the 

 hot tube by ordinary hydrate of lime. 



The same gas passed through hot clay gradually darkens the clay 

 by forming sulphide of iron, and, when the blackness has reached 

 the end of the tube containing the clay, lead-paper is blackened by 

 the passing gas. The clay treated with an acid evolves sulphide of 

 hydrogen. Carbonic acid is evolved in both cases. It is thus proved 

 that bisulphide of carbon, in the presence of hydrogen passing over 

 hot hydrate of lime, is decomposed, and that its sulphur becomes 

 united to hydrogen. Coal-gas always contains a considerable quantity 

 of hydrogen, so that, if it contain vapour of bisulphide of carbon, the 

 process I have the honour to describe will effect its removal. The 

 same process will break up the impure naphthalin compound and 

 convert its sulphur into sulphide of hydrogen ; and the employment 

 of clay in the ordinary purifiers, before the gas passes through the 

 hot ones, will so arrange the elements of certain other sulphur com- 

 pounds as to enable the manufacturer to remove their sulphur as 

 sulphide of hydrogen. Sulphocyanide of ammonium is decomposed 

 by the heated lime, and its sulphur is liberated as sulphide of hydro- 

 gen. The only requisite for complete success was that no injury 

 should be done to the light-giving materials of gas while removing 

 the impurities. I have passed the principal illuminating constituents 

 of coal-gas through the hot lime and clay, and find that they are 

 not injured. The temperature which suifices for purification is not 

 high enough for injury. The photometer shows that coal-gas is not 

 injured. 



