The neck of the retort is round, and closed with an iron door, kept in its place by means of a screw. The retorts are about 7 ft. long and 1 ft. deep. Three are set in brickwork in the form of a triangle, and heated by one fire. They communicate with a leaden chamber which is technically called a balloon. It is about 6 ft. high, 8 ft. long, and 2i ft.
wide. These balloons are supported upon scaffolding, so as to be on a line with the retorts, and are kept in their places by iron bands. At the end of each balloon is a small pipe, which is always kept open so as to allow the escape of steam and water, which is highly charged with carbonate ; this is collected for'resuhlimation. The retorts are heated cautiously. They are recharged every twenty-four hours with a fresh mixture of two parts carbonate of lime, and one part ammoniacal salt ; the chalk is well dried on an iron plate, which is set over the flue, so that the waste heat the fires economically desiccates it. All the retorts are not charged at the same time, for often there are five and six sets, and the times of charging are so arranged as to occupy as few hands as possible ; the residue in the retort being withdrawn, the fresh supply is thrown in, the door luted on and, with the exception of an occasional stirring with an iron rod which passes through the door of the retort, it is left for the usual twenty-four hours. When the retorts have been in work for about fourteen days, the balloons are opened, and the impure carbonate is found as a thick crust, lining all the sides ; it is more or less coloured from impurities. Each balloon has a small bole, closed with a plug, to enable the workmen to see how the sublimation is going on. The pipes leading from the retorts to the balloons are cleaned out after every charge, as they are liable to become choked up.
This crude carbonate is submitted to a second sublimation in iron tanks or pans, Fig. 198, about 16 ft. long and 2i ft. deep, 2 ft. 7 in. wide at the bottom, and tapering to 2 ft. at the top. These tanks are closed with two plates of iron with holes in each about 1 ft. iu diameter and 1 ft. apart from each other ; to each of these openings is kited a receiver formed by simply bending a piece of sheet lead into the form of a cylinder, its ends being kept together by iron straps ; the height of these n.evivere is about 2 ft., the top is closed either by soldering on a piste of lead or by luting.
A small fire is first lighted at each end of tho tank, and the heat gradually raised and regulated, preferably by inserting a thermometer through an opening into the pan. At the end of fourteen days tho leaden receivers arc lined with the carbonate which is carefully removed, the part next the load being scraped off, when the salt is ready for the market. In the figure, I is an imn plate, P the subliming-pots, C the fire-place, A the ash-pit, B the chimney, D fire-bricks to support the pots P, 8 spaces filled in with clay or sand, and 0 open spaces or holes in the plate I. Tho receivers are reshaped, and after cleaning are ready for another operation ; sometimes these receivers are mado in two halves in the form of domes ; 13 cwt. of the rough inuriate yields about 9 cwt. of the rough carbonate. The heat in subliming the refined salt must not be too high, as the colour is it ijured thereby.
The chloride of calcium obtained as a waste product in this manufacture may probably receive nn important application. M. Knob has found that this salt is capable of absorbing its own weight of ammonia gas, and giving up the &LIM again when it is heated ; it has been proposed to utilize this as a means of easy transport. It is quite possible that in small gas works, where the production of amrnoniacal liquor is too small to pay for its transport, an easy method of accumulating aud stowing it away must have been for some time a great desideratum. Of course, it will be necessary to obtain the gas as dry as pos,ible. The free ammonia in distilling could be more economically condensed with an acid, provided the acid could be cheaply procured, and obtained as a crude salt for transport, since it would be necessary to dehydrate the distillate; but where the dry salts can be acted upon with caustic (slaked) lime, the gae would be easily taken np by the chloride of calcium.