We now proceed to describe the conduct of the concentration, and the apparatus in which it is effected. We have already referred to the cisterns into which the chamber acid may be run in order to gauge the quantity made, and which are marked M in Figs. 37 and 38. The acid is run from hore by means of a leaden channel or pipe, either into the concentrating apparatus or first into the appamtus where it is to be treated with sulphurous acid. In order that the cistern M may be emptied as rapidly as possible, it is better to have a second cistern intervening from which the flow of acid is made continuous and in exact accordance with the rate of evaporation.
Fig. 78 is a plan, and Fig. 79 a longitudinal section, of a concentrating apparatus, with platinum still suited to the prepamtion of about ton daily of acid at 1.850 sp. gr., which is about the quantity produced by the chamber system we have described. The apparatus consists of five leaden pans A, one below the other, and a platinum still B, besides a hood and other accessories. The leaden pans are uncovered, except when the acid is to be treated with sulphurous acid in the first pan, which is then furnished with the apparatus H already described. The aoid flows from one pan to the next, through the siphons g. These are similar to those already described in connection with the cistern M. Each pan is 5 ft. square. The first, into which the fresh acid flows, is about 15 in. deep, each lower one being a little shallower than the preceding, so that the last which feeds the platinum still is only about 11 in. deep. The pans are made of sheet lead about in. thick. They rest on cast-iron plates a supported by brickwork, which are generally about 2 in. thick where they lie just over the fire, decreasing to about in. as they recede from the point of greatest heat. The four first pans are heated by a special furnace, the door b of which is seen in Fig. 79 and the fire-bars c in Fig. 78. The flame spreads under the whole of the bottom of the pan lying just above the furnace, and is then conducted by the wall d under one half of the three upper pans, turns at e, Fig. 79, round the end of the wall d, traverses the other half of the three upper pans, and escapes finally by the flue f into the chimney. These four pans lie quite close to each other, wall to wall, without being separated by brickwork. Each of the three upper pans has a lip hammered in the edge, by which the acid can run from one to another if by any accident the siphons should become stopped, or the supply of acid be admitted too fast.
From the last pan the acid flows to the platinum still B, whose greatest diameter is 33 in., and the height to the rim i on which the hood rests is 20 in. This size will hold about 320 gallons. The lower part is made entirely of one sheet of platinum for greater strength. The upper parts
are joined by pure gold soldering with a hydrogen flame. The still is heated by the furnace k. Formerly the bottom was allowed to rest on brickwork, so that the flame acted only on the sides. In this manner, however, the consumption of fuel was very great and iron rods were substituted, but these were very quickly destroyed, so that finally the fire was allowed to play direct on the bottom of the still. It rests with the bottom edge on brickwork / and is supported in the middle by an additional wall m. The flame passes under the bottom of the still, round its sides by the flue n, and thence under the fifth pan. From there it passes by the flue o into the chimney. The projecting portion of the brickwork covering the flue n, is covered with an iron plate reaching about 4 in. over the still. This plate, and that part of the still which does not lie in the brick work, is plastered with clay to lessen the cooling and condensation of the vapour formed. By this plan the fuel is economized and the production increased.
The still is fed with acid through the siphon p, one end of which dips into the last pan and the other into a little leaden vessel q, furnished with a spout. In order that the siphon should regulate the outflow according to need, it hangs upon a chain running over the pulley r, which can he raised or lowered. From tho vessel q, the acid flows into a platinum funnel-mouthed pipe s, and thence iuto the still whose neck is fitted with a little hole for receiving the pipe s. The pipe itself is closed hy the acid lying in the still. Besides this, a little box between the pipe and the funnel mouth through which the acid has to pass, serves to make the arrangement air-tight. This box has two partitions, one fastened to the top, and allowing communication at the bottom only, the other fastened to the bottom, and permitting communication only at the top. The acid flowing through these, cuts off the passage of gases. A platinum float indicates the level of the acid in the still. It plays in a pipe which is fixed to the still in the same way as the feed pipe: The hood C is thrust into the wide neck of the still in such a way that a strong rim on it fits on to the riru of the still. Between the two flanges is placed a thin layer of asbestos, and the two are then brought into close contact by iron bolts. The arm of the hood opens into leaden ball D, to which it is fastened by screw bolts on the flanges, and the joint made with putty. To this ball is secured a leaden worm, in which the weak evaporated acid is condensed, and collects in the cistern E. The worm is contained in the lead-lined vessel F, into which cold water constantly flows from below by the pipe u, and escapes above in a warmed state at the lip v.