OF THE MANUFACTURE OF GAS. 



73 



ant increase or diminution, from any variation of temperature 

 between the limits that are usually employed in gas-making, 

 but the relative amount and also the quality of the liquid and 

 gaseous products, depend very considerably upon the tempe- 

 rature to which the materials are exposed in the retorts. As 

 a general rule, the lower the heat the more do the liquid 

 products increase at the expense of the gaseous ones ; whilst 

 the higher the heat the greater is the yield in gas, the 

 quantity of the liquids being at the same time diminished ; 

 but not only does the relative quantity of the gas produced 

 thus vary, its quality also depends essentially upon the heat 

 employed, that produced at low temperatures being usually 

 superior to that evolved at higher ones. 



The gas thus generated contains several constituents which 

 require to be removed, before it is fit for use as a light-giving 

 material ; but it is not my intention at present to discuss the 

 methods used in the purification of gas, or indeed to describe 

 more minutely the usual processes of manufacture, since 

 these have been so fully and clearly delineated in an excellent 

 paper read before the society last year by Mr. Leigh ; I 

 therefore confine myself to some general observations upon 

 the relative value of the constituents of coal or other gas, 

 to considerations respecting the quantity and quality of the 

 purified gas obtainable froni the materials in general use, and 

 the methods by which both the one and the other may be 

 increased. 



The constituents of purified gas are hydrogen, light carbu- 

 retted hydrogen, carbonic oxide, defiant and other gases, 

 having the general formula Cn Hn, the vapours of hydro- 

 carbons having the formula Cn Hn and Cn H(n— e), and 

 other hydrocarbons whose formulae are unknown : in addition 

 to these, coal gas usually contains small quantities of nitrogen, 

 oxygen, and bisulphuret of carbon vapour; but, for our 

 present purpose, these may be entirely disregarded. 



It has always been maintained that hydrogen and carbonic 



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