TEMPERATURE, Sec.

95

diminution, even though considerable, in the quan-tity of gas obtained; whilst, according to others,it is more economical to obtain the largest possiblequantity of gas at the expence of any consequentinjury to the distillatory vessel.

The truth appears to be wholly with neither ofthese extremes, nor indeed in any absolute generalrule which can be ventured on the subject.

The degree of temperature proper to be adoptedin gas works, where the method of decomposingcoal in masses, or layers from four to eight inches inthickness, and upw ards, is practised by means of thecast-iron retorts, of w hich a description has beengiven, p. 53, chiefly depends on circumstances of alocal nature, with regard to the price of coal and la-bour, so that where in one place it may be more pro-fitable to employ a very high temperature for theproduction of the gas, it may be in others quite thereverse.

The utmost therefore that can be done on thishead, is to state w hat these circumstances are, andto shew' the value which belongs to them underevery supposable situation.

In this metropolis, and in all other places wherecoal and labour bear a higher price than probably