If the acid in the small chamber also be made at 112° Tw., then it is evident that the acid formed in the connection of these chambers will be some degrees stronger, and, like the acid drip ping from the connection of Nos. 1 and 2, will contain dissolved cham ber crystals.
With large chambers it is not sufficient to have drips only in the connection pipes, but the ehambers themselves must be provided with what are oalled " drip-trays," as shown in Fig. 48. A is a leaden vessel inside the chamber, 2 ft. 6 in.
from the bottom, aud " burnt " on to the chamber wall. The acid caught in it flows by the pipe a, which pierces the chamber wall and is burnt to it, into the cylin drical leaden vessel B, where a hydrometer is floating. The vessel is fitted with a side pipe b, entering near the bottom, rising higher than B, and provided at the top with a leaden funnel for catching the acid. In accordance with the laws of hydrostatic pressure the acid flows continuously in at the bottom of B, and away again at the top by a little spout, falling into the cistern C, from whence it returns to the chamber through a small tube.
These drip-tests show the strength of the acid actually being formed in the chatnber itself to which they are attached, and in good working their degree of strength ought to differ only to a trifling extent from that of the acid on the floor. The usual small differences noticed generally occur in proportion to the distance from the gases inlet, and as a matter of course vary in every different works, therefore it is necessary to find out in each case what is the proper strength for each individual drip, in order to keep the working process at the best possible grade.
2. The Nitric Acid in the Sulphuric Acid.—Besides trying the strength of the acid in the drips and in the chambers, it is necessary to observe how much nitric acid it contains. This may be done by a solution of indigo in sulphuric acid, the blue colour of which is destroyed if the amount of nitric acid present be large. The test is not very sensitive, and a very small proportion of nitric acid will not show itself at all, or only after a long time ; still it is sufficiently good for the purpose. When, however, it is desired to estimate very trifling proportions of nitric acid or nitrogen compounds, as for instance in connection with concentration in platinum vessels, to which we shall presently come, a much more delicate test must be used, such as the following ;—Some of the acid to be tested is poured into a glass test-tube held sloping, and then a little of solution of sulphate of iron is added. Concentrated sulphuric acid is now carefully and slowly poured down
the inside of the vessel so that it shall float. Its great specific gravity soon causes it to sink bodily to the bottom of the vessel. If the acid contain nitrogen compounds, a hyacinth-red layer will be formed at the point of contact between the acid to be tested and the concentrated acid, and will become brown-red, and even black, according to the quantity of nitrogen compounds present.
Under proper working, the acid formed in the first chamber should contain no nitric acid, but rather an excess of sulphurous acid, as may be ascertained by its smell. On the other hand, the acid in the small chamber, especially in the last, should hold nitric acid. If the acid on the floor of the large chamber smell strongly of sulphurous acid, the consumption of nitric acid must be increased, and it should only be diminished when the acid contains nitric acid in such quantities that the indigo solution is discoloured after some time.
3. Examining the Gases.—A valuable means of controlling the operation is presented in the observation of the gases, which may be done by allowing small portions of them to escape at intervals through the luted openings made in the chambers for the purpose. The colour of the gases also may be distinctly seen through windows or glass cylinders in the chambers and connection pipe& From the appearance and smell of the gases, presence of nitrous and hyponitric acids may be judged as easily as the sulphurous acid. The gases of the first chamber must be sulphurous, while in the others a relative proportion of nitrogen compounds will be easily recognized by the red colour. When the gases are colourless in the exit pipe, but appear red on escaping into the atmosphere, they contain nitrogen oxide, and oxygen is wanting in the apparatus. If, however, they look colourless or white on escaping to the air, the nitric acid admitted is not sufficient or there is some disturbance taking place in the process.