39 



The quantity of tar in gas as supplied to consumers, and the evil 

 of its presence as a source of sulphur, are not considered as, I think, 

 they deserve to be. It is exceedingly rare to find gas free from tar, 

 and I never yet met with tar which did not contain both nitrogen 

 and sulphur. Part of this tar is combined with ammonia in some 

 manner, and may be obtained by passing gas through a bottle con- 

 taining pebbles moistened with hydrochloric acid. Part is united to 

 naphthalin, as I have already mentioned : part is united to benzole 

 vapour, and part to other hydrocarbon vapours, such as paraffin, if 

 two instances within my own knowledge be sufficient to justify a 

 statement in reference to gas in general. In one instance I passed 

 gas through a metal vessel filled with a number of wire-gauze dia- 

 phragms and kept below 32 Fahr. Some cakes of solid paraffin 

 were found floating upon the water which had been placed in the 

 vessel before commencing the experiment, and a mixture of tarry oils 

 which had deposited the paraffin. In another instance, an old gas- 

 holder was about to be replaced by a new one, and on the water of 

 the tank in which the old gas-holder had worked, there was found up- 

 wards of a thousand gallons of a dark-coloured fluid. All but two 

 carboys was sold to a tar distiller. These two carboys were left ex- 

 posed to the air without corks for some time, and when the manager 

 of the gas-works went to get me some of the fluid for examination, 

 he found that the whole contents had evaporated. I had previously, 

 however, obtained about half an ounce of the mixture. It con- 

 tained paraffin, naphthalin, and the oils which accompany paraffin. 

 Nearly a fifth of its weight of solid pitch was obtained by distilling 

 off the hydrocarbons. A quantity of sulphide of hydrogen and 

 ammonia were evolved during the distillation, and some of the most 

 stinking compounds I ever met with produced from coal. From 

 these two instances it is clear that some, or perhaps all, of the volatile 

 hydrocarbons in gas possess the power of upholding tar with them 

 in their vapours ; and it is proved that this tar is no inconsiderable 

 source of the sulphurous acid produced by the combustion of gas as 

 at present purified. I have obtained tar containing sulphur from 

 every specimen of commercial benzole I have examined ; and as this 

 will evaporate at common temperatures without leaving a residue, 

 we are justified in the presumption that tar thus united to benzole 

 exists in gas. 



