Cannel coal, and the most bituminous of the Newcastle coal, and many others in the country, when exposed to dis tillation at a red heat, furnish several gaseous products, the principal part of which is the carburetted hydrogen, a quan tity of tar, and an aqueous fluid charged with carbonate of ammonia. The separation of the carburetted hydrogen, which when pure burns with great brightness, and without smell, is now effected with great success, on the largest scale, and the other products, part of which have only been applied to useful purposes, will in time find beneficial sources of consumption.
We shall decline entering minutely into the history of the discovery and progress of the art of lighting with gas. With respect to the discovery, it would be difficult to fix any period to it, or with justice to give it to any particular person. Dr Hales and others, among their numerous ex periments in obtaining gases from different bodies by heat, cannot have failed to witness the perseverance of their in flammability.
An account will be found in the Philosophical Transac tions, vol xli. of some experiments upon the distillation of coal by Dr Clayton. He collected the gas in bladders, and burnt it. In Lord Dundonald's attempts to extract tar from coal, the gas evolved at the time was fired. It is singular to remark, that coal is now distilled for its gas and coak, the tar being of no value ; and that this invaluable substance should have been the only thing which his Lordship sought for. We do not hear of any attempt to apply the coal gas to the economy of producing artifical light, before the ex periments of Mr Murdoch, made at Redruth, in Cornwall. He distilled coal and other inflammable bodies from an iron retort, conveyed the gas to a distance through tinned cop per pipes, and burned it as it escaped through small aper tures. These experiments were made in the year 1792. After leaving Cornwall, he resumed his experiments at Old Cumnock in Ayrshire, in 1796, where lie exhibited the new phenomena to numerous friends. He after this, in 1798, went to the Soho Foundery, where he constructed an apparatus for lighting the building, in which he made some improvements in the means of purifying the gas, to get rid of the smell. In the spring of 1802, on the event of the general peace, he made a grand display of these new lights at the Soho works, which excited much curiosity.
Although Mr Murdoch was certainly not the first ob server of the inflammability of the coal gas, he may lay just claim to the application of it to economical purposes; and doubtless first pointed out the practical means of bring ing it into use. The manufactory of Messrs Lee and Phi lips of Manchester was lighted with gas, under the direc tion of Mr Murdoch, as early as 1805, which, in the pre sent state of gas lighting, is considered as very complete. At that period Mr Lee had his house lighted with the gas.
Mr Clegg, late of Manchester, and originally with Boul ton and Watt of Soho, has given much attention to the sub ject of gas lights, and has contributed many useful im 'movements in the apparatus used for preparing it. He has given a description, with the aid of a drawing of a gas apparatus, to the Society for the encouragement of Arts and Manufactures, which is published in their transactions for 1808. This apparatus, with some few improvements,
is at present considered the best. As we shall describe one embracing several improvements upon Mr Clegg's, it will not, in this limited article, be possible to give a copy of his in addition.
The apparatus generally consists of an iron retort of a cylindrical form, its length being from seven to eight times its diameter. It has an opening at one end, which can be closed by a lid with a conical edge, and pushed up to its place by a catch or wedge. The retort is placed horizon tally in a furnace, which allows the flame to pass under it in the direction of its length, and over it on its return, after which it enters the chimney.
The fire should be so intense as to be capable of heating the retort to a bright red heat, and of keeping it up at the same time ; but should not be capable of heating it very hot, as the retort itself might be melted, or at least much injured, and the gas less perfect.
At some distance from the mouth of the retort, which projects a little way beyond the brick work in front, a pipe ascends from the upper side, and at right angles to the length of the retort. Through this the gas escapes, as cending first, and then, the pipe turning, it descends into a wide pipe, placed in a horizontal position. This last is called the condenser. From the end of this pipe an in clining pipe proceeds, which conveys the gas, and also the tar, after it has accumulated in the condenser, till this last admits it to run off. The gas and tar now proceed together, the former occupying the upper half of a longitudinal sec tion of the pipe, and the latter the lower half. The tar is at length deposited in a vessel, from whence it can be drawn at pleasure. The gas first ascends from the pipe in which it was accompanied by the tar, and then descends into a vessel containing a mixture of lime and water, by which it is purified. It then passes into an apparatus call ed a Gazometer. This consists of an outer vessel filled with water. A second vessel, less in diameter, is invert ed and immersed into the first. If the common air be al lowed to escape from the inner vessel, it will freely descend, and water will occupy the place of the air. II' now the source for the escape of air be stopped, and the inner ves sel counterpoised by a weight, the inflammable gas, puri fied as above, may be admitted under the inner vessel, which will ascend to make room for the gas. The sus pended vessel is a little heavier than the weight, so that if the force of the entering gas were withdrawn, and an open ing made to permit the air to escape, the vessel would de scend. This apparatus is not only a reservoir for the gas while its production is going on, but it serves to force out the gas to he burnt, with a gradual and uniform pressure, which gives steadiness to the flame. The gas is set on fire when it is escaping through one or more small apertures, about one thirtieth of an inch in diameter. These are some times disposed in a circle, about the size of the circular wick of the Argand lamp, and hence have been called Ar gand burners. A glass is placed over them, similar to the Argand lamp. The gas tube sometimes terminates in a spherical head, perforated with different numbers of holes.