the clearness and brilliancy of the light materially depend upon its purity; besides, the lower the temperature of the mixture is reduced, the more completely is the gas deprived of the ammonia. It may be proper to notice, that the injurious substances which are generally mixed with coal gas in the greatest quantities are sulphuretted hydrogen and ammonia: both these are very offensive to the smell, but particularly the sulphuretted hydrogen. When the purification of the gas has been completely effected, it passes into the gasholder.

The method of purifying coal gas which is described above, is that most generally practised; but other plans are pursued to effect the same purpose. One of these is denominated the drylime method, and the process consists in passing the gas through several layers of that substance. The lime is slacked, and, when finely sifted, a sufficient quantity of water is put to it, that by rendering it moist its particles may adhere slightly together. The lime is contained in several vessels,

under the fires of the furnaces that heat the retorts, so as to evaporate or destroy its noxious qualities. At some of the Gas-light works it is also exposed to a second burning, to restore it and render it fit for use. According to the statement of Mr. G. Lowe and Mr. J. T. Cooper, a bushel of lime will purify from 11,000 to 12,000 cubic feet of gas.Evidence before the House of Commons, 1825.

which are connected together at their tops by means of bent pipes, and each is fitted up with one or two shelves, which are perforated with a great number of holes. The purpose of the shelves is to hold the lime, which is spread upon them several inches thick, and as the gas enters by a pipe fitted to the lower part of the vessels, it has to force its way through all the layers of lime contained in the whole range before it can arrive at the gasometer. The vessels have not any bottoms, and are placed in a cistern of water, or other fluid, to the depth of about six inches, in order to prevent any gas from escaping. At some places this process is used, and found very effective.*

The fact being known that sulphuretted hydrogen and ammoniacal gas will combine in certain proportions, and form a soluble mixture, occasioned

* This plan is a contrivance of Mr. Reuben Phillips's, of Exeter, and he obtained a patent for it in 1817. In the course of the evidence which Mr. Adam Anderson gave to a committee of the House of Commons in 1825, on the London and Westminster Oil-Gas Bill, he stated, that the method of purifying coal gas by dry lime was practised at the Perth Gas Works with extraordinary effect, as respected both the purity and illuminating power of the gas. He also stated, that the knowledge of the peculiar process practised there was confined to that establishment, and therefore declined to disclose it.-Minutes of Evidence..

its application to the purification of coal gas. To effect this, the two gases are mixed together as they are produced, and the compound afterwards passed through water. In this process the ammoniacal gas is obtained from lime saturated with ammoniacal liquor. The distillation is carried on in a separate apparatus, and the gas is collected in a large vessel, which is fixed directly over the brick-work of the fire-place. The connexion between this vessel and the retorts is formed by á large pipe, through which the coal gas passes as the heat evolves it from the coal. The sulphuretted hydrogen and animoniacal gas thus unite, and form a soluble compound, which is dissolved in passing through a large cistern of water, and the carburetted hydrogen is rendered fit for burning.*

Mr. G. Holworthy Palmer introduced a method of purifying coal gas, which is essentially different from any of the preceding. The operation consisted in passing the gas through red-hot pipes,

* For this mode of purification Mr. Daniel Wilson, of Dublin, obtained a patent in 1817. A patent has recently been taken out by J. F. Ledsam, of Birmingham, to purify coal gas by a similar process; but I presume that either his apparatus, or mode of applying the principle, is different from Mr. Wilson's. Mr. Ledsam states his method to be very effective, and also that it has been practised to a considerable extent at the Birmingham and Staffordshire Gas Works.

which contained iron turnings, iron ores, black oxide of iron, and various other substances, capable of being oxidized. The sulphuretted hydrogen and carbonic acid were thus acted upon, and separated from the carburetted hydrogen; but as some inconveniences attend this process, its use has been very limited, though a patent was obtained for it. Various other machines and processes have been suggested, but they are unimportant, and therefore any description of them is unnecessary.

OF THE GASOMETER, &c.

The coal gas having passed through all the various processes which render it fit for burning, it is conveyed, by means of a pipe, from the purifying apparatus into a large vessel denominated the gasometer, where it is stored up for use, till it is

* Mr. Palmer took out his patent in 1818; and Mr. G. Lowe had purified coal gas by a similar process about the same time.-Philosophical Magazine, Vol. LII. page 371. Mr. S. Palmer, of Liverpool, also purified the gas by passing it through iron tubes, heated to a dark red, but he employed no oxidizeable substances.-Philosophical Magazine, Vol. LII. page 292.

forced into the mains to supply the lights. The form of the gasometer is generally circular, but the dimensions are regulated by the magnitude of the establishment for which it may be required. Where the works are large, the size is frequently from thirty to forty feet in diameter, with a height proportional, so that it may be sufficiently capacious to contain from twenty to thirty thousand cubic feet of gas, or even more.t Vessels of this kind are composed of two separate parts, a lower and an upper one, but the lower is the more capacious of the two, and consists of a cistern or vat, which is firmly fixed in the ground. The materials of which it is constructed are either iron, wood, or brick-work; its diameter is several inches larger than the other, and its purpose is to hold water.

* M. Lavoisier was the inventor of the gasometer, of which he published an account in 1789, and has given a representation of it in his Elements of Chemistry. Some of those employed by Mr. Clegg were square, and the Portable Gas Company still use that form, but the dimensions are not very large.

+ When gasometers were constructed of sufficient capacity to hold from twenty to forty thousand cubic feet of gas, considerable alarm was excited in London; but it was asserted at the time, that an idea was entertained at Paris for making one to contain two hundred and forty thousand feet; this has since been accomplished, and is stated to comprise two hundred and fifty-six thousand cubic feet!