In many works the flow of nitric acid is re gulated by two cisterns, such as are shown at E, Fig. 43. These are filled alternately once every twelve hours with the requisite supply, so that each is filled once per twenty-four hours. The flow of acid to the chambers is conducted from both the cisterns into the same vessel ; when the one has delivered half its acid the other is become quite empty and is filled anew. In this way the changes of pressure are better compensated, and a more regular flow of acid is obtained than by one single open cistern, but it is unnecessary to say that the same regularity cannot be secured as by the Marriotte vessel.
The nitric acid introduced into the chamber must be spread over a great surface, so that the sulphurous acid gas may come as much as possible into contact with it. For this purpose a cascade apparatus is used, consisting of cylindrical vessels of pipe-clay, arranged as shown in the figure. At A is seen a vertical section of the whole system, B gives only the under part in section and a view of the upper part, C shows a view of the whole. Each system is composed of four pipe clay vessels, one iuside another. Tbe lowest vessel b is about 2 ft. 6 in. in diameter, and stands in the leaden saucer a, in which is a little powdered sulphur to a.ssist iu securing the perfect level of the vessel. The upper vessels decrease in size about 8 in. each, thus producing cascades over which the acid tumbles. The lowest vessel b is a simple saucer, each of the other three, on the other hand, is furnished with n bottom about * to / in. from the edge, and divided into two parts. Round about the under part are holes g, which, as shown at B, begin under the bottom and rtach under tho edge of the next lower vessel. Thus the nitric acid ns well as the chamber gases find frto circulation inside the apparatus, so that the greatest possible reaction may take place ou the whole surface of the acid. The nitric acid is conveyed to these apparatus by the pipes f, which aro also made of stoneware, and are introduced through the chamber wall from the jar D. These last are fed by the funnel i over the centre jar, either from the cisterns E, from the Marriotte vessel. The jars communicate with each other by the side siphons h, which maintain the acid at a constant level. Each jar supplies one cascade inside the chamber with acid by the taps lc. These
taps are placed at such a height as to prevent the level becoming so lowered as to onset the siphons.
The regular delivery of acid in the cascades can also be achieved by a pipe-clay rocking trough, Fig. 44. The acid is run from the cistern by a pipe a, fitted with a tap b, into the half of the treligh marked c. As soon as this is full, the trough falls over in consequence of the change of the centre gravity (as it turns on its axis at f) until the slop d holds it. Thus it empties itself. At the same moment the half e is raised and brought just under the mouth of Ilee pipe a, and is thus filled with acid in turn. Then the trough falls over until stopped by the knob g, and the aeid flows out on the opposite side. The acid del red thus intormittontly from the trough is caught in vessels standing underneath the flow pipes, and carried through corresponding cascade apparatus. The pipes must also be of equal diameter, and of such email bore that the volume of nitric acid which flows at each fall of the trough is sufficient to completely fill them all, so that each mny deliver the same proportion of the acid.
This arrangement is superior to the last in that the jars are done away with, and only the cistern tap needs regulating. On the other hand, it has the dis advantage that it would not be possible to notiee the accidental stoppage of any of the ttibe.q, and that the flow is not made continuous but intermittent. Also the tumbling of the trough may become interfered with by nn accumulation of dirt.
The above-described cascades are now altered to a simpler form in many works, with the advantage that the jars are dispensed with and the acid admitted to one series only.
Fig. 45 shows this arrrangemcnt. It consists of a pile of cylindrical tubes a, and flat round plates or saucers b, all of stoneware. The diameter of these pieces constantly decreases as the column rises, so that the nitrio acid flowing into the uppermost saucer overflows into the second, then into the third, and so on. The lowest saucer is about 2 ft. 6 in. diameter, and about 21j in. deep, and the highest is about in. wide, and 1* in. deep. The cylinders are provided with openings through which the nitric acid and the chamber gas can freely circulate.