The small residual portion of the gas leaves this chamber by the exit flue near the bottom, and passing through the vessel J, in which a small proportion of sulphuric acid is still formed, escapes finally by the vertical chimney K. This conducts the vapours, consisting chiefly of nitrogen, from the air, the superfluous oxygen, some sulphurous acid, and, when the operation is well managed, a considerable portion of nitric or hyponitric acid, into the atmosphere. In order to watch the colour of the gases, windows are placed in all the flues or connection pipes, either in the form of glass panes let into the side and top, or in the shape of a glass cylinder the full size of the pipe.
The chimney K is provided with a slit, into which a leaden damper h can be introduced, and by opening and shutting this the draught can be regulated as desired. Sometimes, instead of a simple daMper, the flue is enlarged and a perforated damper is introduced, the holes in which are, collectively, just equal to the diameter of the flue. By closing some of the holes the draught may be lessened. In order to prevent the wind from interfering with the draught, the top of the chimney is provided with a cylinder L, open above and below, and of larger dimeAsions than the chimney pipe. In many works the gases are conducted to the main chimney, instead of into this special substitute.
The roof of each chamber has an opening i, closed by a cover, which stands in a rim filled with water, or usele.,s acid. Figure 40 shows the arrangement. This serves to show the character of the gas, which is necessary to be studied in regulating the process.
They are often of glass for admitting light into the chamber, which latter is then furnished with corresponding side windows, at a convenient height for seeing into the chamber. On suspending the clambers their ventilation is much hastened by *removing these covers. The roof of the first chamber, being necessarily of greater length, may have two such covers with advantage. Besides these, in some works there are openings in the side walls of the chambers, near the bottom and about 3 ft. square, serving as man holes. Workmen entering the chambers wear boots of thin sheet-lead, as a protection against the acid. During the working these openings are closed by wooden lead-covered doors, which are fastened tight up to the leaden wall of the chamber by supports from the chamber framework. The joints between the door and the chamber wall are made tight with putty, generally prepared from pipe-clay and the of rapeseed oil refineries. These holes are seldom or never met with
in this country.
Further, it is necessary to provide a small bole in the side of the first chamber, just opposite the point where the gases enter, with a diameter of about au inch, which is stopped by a stoneware stopper and used as we 6;111 inilieate presently. The connection pipes also are provided with holes for watehin;z the process os 61..C1 I at 1. 1Vhen it is not possible to lute the covers with water they niust bo made secure with putty. Each connection pipe, as well as the chimney K, is provided at its lowest point with a small double bent tube, as seen at 01, Fig. 37, which is so mado that no gas can issue from it, but that the acid which condenses within may escape, drop by drop, and be collected in a vessel for testing; this serving as another guide in the conduct of the process.
Each chamber, as seen at n, Fig. :is, has an overflow pipe, so that the acid runs continually from the third to the eecond chamber, thence to the first, and from it into the reservoir M, so that, however fast it may condense in the chambers, it shall remain at a constant level in them.
The reservoir 51, which is a wooden tank lined with sheet lead, serves to show the exact quantity of acid made. In order to do this, it is so arranged that the aeid may be led into either of the two reservoirs. When one is full, it is run into the other. They are of exactly similar dimensions, and furnished with a scale and a float, so that it is easy to calculate the volume of acid made daily from the figures indicated. By noting the epecifie gravity, the exact quantity obtained can be reckoned.
With this arrangement, it is not advisable to draw the acid direct from the reservoir by taps, as through carelessness, or their not fitting tightly, to acid may coutinually escape iu drops, and tiles the correct make cannot be ascertained. It is better to draw it off by means of siphons. This is shown in Fig. 41.
The siphon a, of bent leaden pipe, hangs from a light iron chain b, which runs over the pulley o.
It gun thus bo raised or lowered at will, and may be temporarily suspetuled at any height by fastening tho string at d. Both logs of the siphon, ene inside tho reservoir, the other outside, ter minate iu little leaden vessels e, hanging on knobs with which the siphon is furnished at f.