The truth appears to be wholly with neither of these extremes, nor indeed in any absolute general rule which can be ventured on the subject.
The degree of temperature proper to be adopted in gas works, where the method of decomposing coal in masses, or layers from four to eight inches in thickness, and upwards, is practised by means of the cast-iron retorts, of which a description has been given, [p. 53], chiefly depends on circumstances of a local nature, with regard to the price of coal and labour, so that where in one place it may be more profitable to employ a very high temperature for the production of the gas, it may be in others quite the reverse.
The utmost therefore that can be done on this head, is to state what these circumstances are, and to shew the value which belongs to them under every supposable situation.
In this metropolis, and in all other places where coal and labour bear a higher price than probably elsewhere in this country, and where saving of time is also an object of primary importance, it is clearly established, that the manufacturer who pursues the method of decomposing coal in masses from five to eight inches and upwards in thickness, by means of cast-iron retorts,[24] will consult his interest best, by employing such a high temperature for the decomposition of the coal, as will produce in the shortest time the greatest possible quantity of gas, from a given quantity of coal, without regarding the unavoidable deterioration of the retorts. But in places where coal and labour is cheap, it will be his interest to save the retorts at the expence of the coal. But that this fact may not rest on mere general assertion, I shall subjoin for the satisfaction of the reader a few statements of experiments made upon a large scale for the purpose of ascertaining these facts.
[24] The Retorts should be manufactured of what is called in commerce, iron of the second process. The best cast-iron of this kind, is of a light grey colour, its fracture is granulated and dull, it receives a dent from the blow of a hammer. The cast-iron which exhibits a dark grey or black colour inclining to blue, and presents granular concretions, readily friable, and therefore unfit for vessels intended to stand a long continued heat.
Annual Creditor and Debtor Account of manufacturing daily from 50,000 to 102,000 cubic feet of gas at the price which coal bears in the metropolis.—The operation being commenced with new Retorts, and the retorts being left in a fit working state.
The first of the following processes was conducted on the principle that coal and labour, being of an high price, as in London, it is most economical to obtain the greatest possible quantity of gas from a given quantity of coal in the least possible time, without any regard to the injury done to the distillatory vessel.
The second process is intended to illustrate the correctness of that principle, by shewing that where coal and labour are at the high prices stated in the first process, it is a losing system to work the retorts at a lower temperature, in order to make them last longer.
In some respects a similarity will be observed between these experiments, and those already given in [page 85], but besides their having reference to the separate and distinct circumstance of the high prices of coal and labour in London, it will be found that they also differ from the former statements, in exhibiting, not merely the expence of working, but the original cost of erecting the retorts, as well as the expence of replacing them.