The gas is mingled with various volatile products as it issues from the retort, and requires to be purified before it is fitted for illumination. The most abundant matter that passes over with it is tar. The vapour of that substance, however, condenses when cooled, and it may thus be separated from the gas very effectually. For that purpose the gas, after having deposited a large portion of the tar in the hydraulic main, is made to traverse through a number of vertical pipes, and in passing through them a further quantity of tar, accompanied by ammoniacal liquor, is deposited, and collected in a reservoir at the bottom. The next process is the purification of the gas from carbonic acid and sulphuretted hydrogen. This is commonly done by passing it through water and lime; the combination of the carbonic acid with the lime being facilitated by agitation. The method of purifying by lime was introduced by Mr. Clegg; and by a later process, oxide of iron is used to absorb the sulphuretted hydrogen. The gas, when purified, is conveyed to the gas-holder, whence it is forced by pressure into the mains and pipes. An apparatus for generating coal gas on a small scale for private establishments, remote from sources of ordinary supply, is represented in the accompanying woodcut. The retort, A, is fitted in a small furnace. The coal is put in at F, and the products of distillation pass through the bent pipe, E. The more liquid portions of the tar pass at once through the tube, B, into the receiver, G; and as the gas passes along the bent tube, C, it becomes cooled, and a further deposit of tar and ammoniacal liquor is made. The gas is then conveyed along another tube into the purifier, H, filled with lime and water, and it thence passes into the gas-holder. Tubes are inserted into the latter for conveying the gas to the burners. [Illustration] The quantity and the quality of the gas yielded by coal differ materially according to the kind employed. One ton of good Newcastle coal will yield 9,500 cubic feet of gas, which, when burnt in the best manner, gives a light equal to that of 422 lbs. of spermaceti candles. One ton of Wigan cannel coal yields 10,000 cubic feet, and gives a light equal to 747 lbs. of spermaceti candles.[13] The price, in London, of good gas from Newcastle coal, is 4s. 6d. per thousand cubic feet, which gives a light equal to 74½ lbs. of spermaceti, and equal to 89 lbs. of mould candles; therefore, when the latter are 8d. a pound, the burning of gas is twelve times more economical than the burning of candles. In Liverpool, gas from cannel coal is supplied at the low price of 3s. 9d. per thousand feet; and that gas gives at least one-third more light than the ordinary London gas.