The reader will thus observe, that of all the forms of retorts which have been hitherto fairly tried, upon a large scale, it has been satisfactorily ascertained, (excepting only as to the ellipsoidal retorts), that the cylinder is the best form for decomposing coal in masses, from five to eight or ten inches in thickness.
It is perhaps needless to state that in making experiments on the comparative value of the best form of cast-iron retorts, it is obvious that the operations should be continued for some months uninterruptedly; no conclusion can be drawn that may become practically useful in the large way, from processes carried on for a few weeks only. It is absolutely essential that the comparative trials be continued for months together, and that the inferences be taken from the total quantity of coal used during that period, compared with the total quantity of gas obtained, the deterioration of the retorts, and the time and labour expended.
Proceeding on erroneous data, many have persuaded themselves of having noticed that parallelopipedal and semi-cylindrical retorts last longer fit for use than those of a cylindrical shape, an assertion of which subsequent trials, conducted in the manner just stated, has clearly shown the fallacy. Enough has been done at the different gas works in the capital to settle this point, and there is now but one opinion amongst those who are best qualified to judge of the subject. Every body who has made the trial on a large scale, is convinced as already stated, that the best form of the retort for manufacturing coal gas where the process is conducted on the plan of decomposing coal in masses or layers of from four to eight inches in thickness, is a cylinder six and a half feet long, and one foot in diameter, and accordingly retorts of this shape and dimensions are now used in all the best regulated gas establishments in the metropolis.
A cylindrical retort of the description before named, weighs about nine and a half to ten hundred weight. These and all other shaped retorts are furnished with a moveable lid or cover having a conical edge to fit the mouth-piece; the cover is rendered air-tight, not as formerly by grinding, a mode which was costly, but by the interposition of a thin coat of loom, between the lid and the mouth of the retort.
The mouth-piece forms a separate part of the retort. It is bolted and screwed to a flanch which terminates the mouth of the retort, so that when the retort is worn out, the mouth-piece may be detached and applied to new retorts.
There are now in action 620 cylindrical retorts, at the two chartered Gas Works[12] in the metropolis; and the total number of retorts at all the London gas establishments amounts to 960.
| Westminster Gas Works, | - | Westminster Station | 250 | Retorts. | |
| Brick Lane ditto | 190 | ditto | |||
| Norton Falgate ditto | 50 | ditto | |||
| City of London Gas Works, | Dorset Street, | 130 | ditto | ||
| 620 | |||||
Application of heat.—Flue Plan originally adopted.
It must be obvious that the durability of the distillatory apparatus, greatly depends on the manner in which the heat is applied, to effect the decomposition of the coal contained within the retort. If the heat be very intense the whole vessel is rapidly destroyed. If it be too languid, the distillatory process is protracted, and much fuel, time, and labour wasted to no purpose; and the retort is speedily deteriorated, if the heat acts upon one part of it more than upon another.