Coal tar has already been noticed as one of the products resulting from the distillation of coal, and at the first establishment of public gas works, great profits were expected to be realized from the sale of this article ; but from the large quantities produced, and from its inapplicability to most of the purposes for which vegetable tar is employed, it was soon found difficult to find a market, and it became an object to utilize the material by converting it into gas. For this pure various processes were resorted to, nearly resembling that just described, for converting oil and other liquid matters into gas; but a serious inconvenience was found to result from the deposition of asphaltum in the pipes, (owing to the imperfect decomposition of the tar,) which quickly choked them up, and rendered them unserviceable, whilst the gas afforded but a feeble light, and emitted much smoke. Numerous plans have since been proposed for the remedy of these evils, of which we shall only notice the invention of Messrs. Vere and Crane ; the apparatus, it is stated in the specification, is also applicable to the distillation of all animal or vegetable solid or liquid matters, from which earburetted hydrogen may be obtained. The process consists in introducing into the retort a constant stream of water, or a current of steam into the exit pipe, which, mixing with the volatile matters arising from the substance under decomposition, causes them to fall down again into the retort without proceeding further to choke up the pipe, while the more gaseous products pass on through the steam in a purer state to their destination, to be afterwards treated in the usual way. Fig. 1 is a front elevation of the improved retort set in the brickwork of the furnace ; and Fig. 2 is a vertical section of the same ; the letters have a reference to the same parts in each figure. a is the ash-pit, h the furnace, c c the flue winding round the retort ; d the retort, with its lid fastened in the usual way by a cross bar and screw ; a the exit pipe, through which the gas escapes as it is generated ; f is a wrought-iron pan or tray, to hold tar or other liquid matter to be distilled; g a supply pipe to f, leading from the cistern or reservoir A ; i is a water pipe, and k a cistern of water.
When tar, for instance, is to be operated upon, the retort, partly filled with coke or broken bricks, is to be brought to a bright red heat, which may be ascertained by inspection through the holes o o, shown in Fig. 1, which are provided with stoppers ; the cock of the water pipe is then opened, to admit the water to flow in a slender stream into the retort, the heat of which immediately converts it into vapour. This done, tar is to be admitted from the reservoir h to flow through the pipe g into the pan f, where it is quickly decomposed ; the gas, as it ascends, enters the exit pipe, and necessarily passes through a large volume of steam, which, the patentees state, causes an instant precipitation of the car bonaceous matters, which would otherwise lodge in the pipes, and ultimately obstruct the passage of the gas through them. The gas thus relieved in the earliest stage from the principal contaminating matters, has then to pass through the ordinary purifications, by which it is ultimately delivered to the burners, in a state of great purity, for consumption. When coal or other solid matters are to be decomposed to obtain the gas, the pan f, the pipe g, and the reservoir h, are to be removed, and the operation conducted without them, retaining however the use of a current of steam as before.
The subjoined engraving represents Mr. Gordon's portable gas lamp. The idea of employing gas as a substitute for lamps and candles, occurred to Mr. Murdoch, as we have seen, so early as the year 1792, when he made some experiments with that view, but seems to have subsequently aban doned the idea ; but to Mr. Gordon is due the merit of realising it, and thus at length rendering inflammable gas applicable to every purpose of arti ficial illumination. These lamps consist of strong wrought-iron vessels of various dimensions and forms, in which the gas is compressed into one thirtieth of its bulk at the ordinary atmospheric pressure; the flow of the gas being regulated with the utmost exactness, according to the degree of light required, by a valve, which we shall subsequently describe.
A company having been formed to carry Mr. Gordon's invention into effect upon an extensive scale, works have been established in London, and at some of the principal country towns, at which the gas is manufactured; and the lamps being charged therewith, are furnished to the consumer as occasion requires. The gas with which the lamps are charged is usually procured from oil, on account of its greater purity, and of its occupying less space than coal gas, one foot of oil gas being equivalent in illuminating powers to nearly three feet of coal gas. The gas is generated by the usual processes, and the following en graving represents the apparatus employed at the London Portable Gas Works for charging the lamps ; a is the main horizontal shaft of a steam engine, upon which are fixed two spur wheels b b ; the teeth of these take into the teeth of two similar wheels cc fixed on the axis of a three-throw crank, to which is thereby communicated a rotatory motion. The crank imparts (in the usual manner) an alternating motion to the rods e e e, which work three force pumps: for a description of the forcing pumps originally employed for this purpose, (which were of a singularly ingenious construction, although we believe they have since been replaced by others more nearly resembling the ordinary force pump,) we refer the reader to the article AIR PUMP. As the plunger of each pump is successively raised, a quantity of gas equal to the space pre viously occupied by the plunger flows from the gasholder into the chamber at the opposite end, by means of a pipe of communication, part of which is brought into view at j. The valve by which the gas enters, opens inward, so that it cannot return the way it came ; but there is another valve which opens outward, and this is kept closed by a spring of sufficientpower to prevent the escape of the gas in the uncompressedstate ; upon the descent of the plunger the strength of this spring is overcome, the gas is forced out, and the valve closes again. From the pumps the gas proceeds along the tube g, and enters by the jointed valve h, into a strong wrought iron recipient i; in this vessel it is evident the gas might be collected and condensed to any required number of atmospheres; but the valve j being opened (by the cross-handled key shown), the gas is suffered to flow through the pipe k k, which is extended along the upper side of the " filling table" m, and from thence into the reservoirs ("port able lamps") /114 by which arrangement the pressure of the gas becomes equalized in all the vessels, however great their number. The degree of con densation at which the gas has arrived by the continued action of the pumps, is shown throughout the process by a mercurial, applied in the following manner. The pipe n, which proceeds from i, conveys the gas under compression into the reservoir of mercury, at the bottom of the sage o o; the pressure of the gas upon the surface of the mercury causes the latter to rise in a long lass tube, hermetically sealed at top, and inclosing a portion of atmospheric air above the surface of the mercury ; this air becomes com pressed into a smaller space by the rise of the mercury, as the condensation of the gas advances, and the diminution of its volume indicates, upon a scale attached to the tube, the degree of condensation or pressure in the lamps; and when the mercury arrives at the line denoting thirty atmospheres, the valve j is shut by means of the cross handle. All the lamps attached to the pipe in con nexion with the closed valve being now filled, are taken away by unscrewing them from the sockets in the tube. The external pressure being removed by turning off the gas, the lower valves of the lamps close by the pressure of the gas within them, and the contents are further secured from escaping by a workman screwing a cap over the lower valve as he successively removes each of them from the tube. To ascertain whether there is any leakage, the lamps are im mersed one by one in a contiguous trough of water, where, if any leakage exists, it is immediately shown by the gas bubbling up. The perfect state of each lamp being thus ascertained, they are arranged, in extensive racks or stands, ready to be taken out to the consumers by the Company's carts, which are regularly dispatched to all parts of the town.