548 



GAS, COAL 



flame, and consequently the light is diminished. Tho following table shows tho 

 maximum percentages of aqueous vapour which can be present in gas at different 

 temperatures. As a general rule tho gas will contain tho maximum amount at the 

 lowest temperature to which it has been exposed in its passage from the retorts to tho 

 burners : 



Aqueous vapour has a specific gravity of '6201, and one cubic foot of it contains one 

 cubic foot of hydrogen and half a cubic foot of oxygen. In contact with ignited 

 carbon, or carbonaceous substances, it is decomposed; producing a mixture of hydrogen, 

 carbonic oxide, and carbonic acid gases. "When passed over ignited iron, it yields ita 

 own volume of nearly pure hydrogen. 



Having thus described the more important properties of the constituents of coal- 

 gas, we are now prepared to discuss the conditions involved in tho generation, 

 purification, and combustion of gas. 



On the generation of illuminating gas. The production of gas for illuminating pur- 

 poses whether derived from coal, peat, wood, or oil, depends, as we have seen, upon 

 a re-arrangement of the elements composing tho material employed. The nature 

 of this re -arrangement is dependent upon the temperature employed. The lower tho 

 heat at which it can be effected, the less the weight of coke or carboniferous residue left 

 in the retort, and, consequently, the greater the amount of carbon remaining combined 

 with the hydrogen ; the hydrocarbons thus formed being chiefly solids and liquids. 

 On the other hand, the higher the temperature employed, the greater is the weight of 

 carbonaceous residue, and, therefore, the smaller is the amount of carbon contained in 

 the volatilised matters, whilst the proportion of gases in these latter becomes larger as 

 the temperature increases. By employing a very low temperature for the destructive 

 distillation, the production of gas may be almost entirely prevented, whilst by the 

 employment of a very high temperature the three chief constituents of coal might 

 without doubt be completely converted into coke, carbonic oxide, and hydrogen. Now 

 the results produced by both these extremes of temperature are valueless to the gas 

 manufacturer, and it is therefore necessary to employ a heat sufficiently high to prevent 

 as much as possible the volatile substances from escaping in the form of conoensible 

 vapours, but not high enough to decompose the luminiferous constituents of the 

 evolved gas. If coal were a definite and single chemical compound, and could be so 

 exposed to heat as to suddenly raise the temperature of every particle to a uniform 

 and definite degree, it is highly probable that the results of the distillation would bo 

 far less complex than they are in the present mode of gas manufacture ; and it might 

 even be possible to find such a degree of temperature as would convert the whole of tho 

 hydrogen into one or more of tho higher gaseous compounds of carbon, thus giving 

 results of maximum value to the gas manufacturer. In the ordinary processes of 

 gas-making, where a charge of several cwts. of coal, often in largo lumps, is thrown 

 into an ignited retort, it is impossible to attain any such uniform temperature. Tho 

 heat is conducted very gradually to the interior of tho mass of coal, and therefore 

 various portions of tho charge are exposed to very unequal temperatures, especially 

 in tho earlier stages of tho distillation. Tho natural consequence of these conditions 

 is tho production, on the one hand, of products resulting from excessive temperature, 

 viz. hydrogen and light carburetted hydrogen, and, on tho other, of tar, which may 

 bo regarded as tho consequence of deficient heat. Notwithstanding several attempt?*. 

 these disadvantages have not yet been successfully overcome ; but tho importance of a 

 practical process which would secure a tolerably uniform temperature during tho 

 whole course of distillation, is seen from the remarkable results obtained with Clcgg's 

 revolving web retort : a form of apparatus undoubtedly tho most ingenious yet 



