When the kiln is hot enough, the proper communication is remade, the hole walled up, and the operation commenced. It is as well, at first, to give the kiln double charges of dry sulphur, in order to raise the temperature more quickly. The process is also brought much more rapidly into good order if a double quantity of nitre be used for the first day or so. At the commencement of the process, the moisture contained in the ordinary atmospheric air admitted is allowed to suffice for assisting the formation of the acid, and no steam is admitted, as while the temperature of the chambers is so low it will condense very fast without forming any corresponding amount of acid, and will at the same time cause the condensation of a certain amount of nitric acid. For this reason steam is not admitted until the commencement of the formation of chamber crystals. At first only very little is introduced, and gradually more and more until the correct limit is arrived at. Whilst expecting the formation of chamber crystals, the little stoneware plug placed in the wall of the first chamber opposite the gases entrance, is taken out and examined from time to time, as the crystals settle on it immediately they begiu to be produced.
Manufacturers are agreed that at the commencement of the process the bottoms of the chambers should be covered with acid of a certain density in order to bring the working into good order. This holds good especially when it is neoessary to have a certain amount of liquid on the floors of the ohambers in order to cut off their communication with the outer air, for which purpose water 8bould not be used, but acid of such a strength that it will not decompose the nitrous and hypo nitric acids. When the sides of the chamber are "burnt," or fastened to the bottom, there is no need for covering the floor with any liquid at all, but we strongly disapprove of this plan.
Until the apparatus has reached the proper temperature, the formation of the acid and conden sation of the gas proceed slowly, and in consequence the draught is slow and the sulphur requires longer to burn than under the normal oonditions. At first, therefore, the outflow damper and the ventilators which admit air are put wide open, and only reduced to the proper adjustment as the process of the burning improves.
It is of the highest importance that in each of the working processes—burning the pyrites, the introduction of the steam and nitrous acid into the chambers, the regulation of the draughts and the working of the towera—the utmost possible uniformity be attained. All defects and irregularities of workmanship should therefore be detected and remedied at once, and in order to effect this, the flve following points require careful and unremitting attention ;— 1. The strength of the acid produced.
2. Tht amount of nitric acid contained in it.
3. The escaping gases.
4. Tho amount of sulphurous acid in the gases.
5. The oxygen contained in the escaping gases.
The three flrst receive attention in almost every works, while the two last are only attended to in the best managed establishments.
1. The Strength of the Chamber Acid.—The strength of the acid on the chamber floor is tried daily in order to see whether it remains constant. Changes taking place in the process, however, cannot be noticed in this way until long afterwards, since the amount of the newly-formed acid is so small as compared with that already lying there. In order to check variations as rapidly as possible, the strength of the acid dripping from the connection pipes is regularly noted four or six times daily. From the strength of these drips the changes taking place may be very readily seen. These are not due solely to alterations in the steam, but also to such causes as the want of air or nitric acid.
In the ordinary manner of working, when the amount of steam admitted to the large ehamber is only sufficient to form tetrahydrate, an acid will condense in the pipe connecting it with the second chamber containing less water and more nitrogen compounds, and consequently several degrees stronger than the acid formed in the ehamber. If the acid made in the first chamber is at 112° Tw., in the second at 52° Tw., and in the third at 32° Tw., the liquids condensed in the pipe conneeting chambers 1 and 2 will, ail a rule, be at 133° Tw., or 21° higher than in the first chamber. The acid formed in the connection between chambers 2 and 3 will show about 63° Tw., or 11° stronger than in the second chamber. The strength of the acid flowing from chamber 2 back to chamber 1 has something to do with the difference observable between the strength in No. 1 and in the conneetion between Nos. 1 and 2. So also the aeid in the second chamber is weakened by the acid flowing in from No. 3. Tho acid formed in the pipe leading from the third charnber is never stronger, and is often several degrees weaker, than the acid in that chamber, as it is not thinned by the influx of weaker acid from another chamber. The strength of the acid condensed in the escape flue ean only be etronger than that formed in the last chamber when a great quantity of uncondensed sulphurous acid is escaping, and thus maintaining the forrnation of acid in the flue.