The above-described apparatus is use ful when the nitrous acid supply is derived from the decomposition of nitre in the kilns. It is, however, almost entirely gone out of fashion. A more common form of deni trator is the following, known in this conntty as a " steam tower." In Fig. 56, representing such a tower, part of tho side wall is removed, in order to show the interior of the apparatus. It consists of a cyliudrical-shaped tower, of strong sheet lead, put together in three pieces, which aro hurnt together at a and b, the whole being 12 ft. high and about 3 ft. in diameter, and standing on a solid foundation. The bottom A is also of sheet lead. Four strong iron bands c help to bold the struoture firmly togethor. In order to proteet the load from the effect of the hot acid, it is providod with a casing of hard burnt flre-bricks, so formed and arrangod that they lio quite close one upou another. The joints aro made with flue pipe-clay cement. On the top of the cylinder a basin C is fixed, resting close down upon tho uppermost tier of tho brickwork. The nitro - sulphuric mid flows through the leaden feed-pipe D, placed in the middle of the basin. The disengaged sulphurous acid finds its way to the chambers through tho stoneware pipe E, which is fixed tightly in the basin. Sometimes this pipe is covered with an outside coating of lead to prevent the mischief which may arise from a break ing of the pipe from any cause. The steam is admitted at the bottom of the cylinder by the pipe F at such a height that the mouth of the pipo remains higher than the level of the sulphuric acid which collects at the bottom. The pipe is supportt. d, sur rounded, and covered with fire-bricks in such a manner that spaces remain for the free passago of the steam and the acid. Above this flints arc packed, reaching nearly to the summit of the tower. Those at the bottom are about as large as a man's fist, and decrease in size as they rise till they are no larger than nuts. Instead of flints sometimes broken remains of hard burnt stoneware vessels are used.
The nitro-sulphuric acid flowing in from above, trickles down through the flints and is decom posed by the steam which it meets, while the liberated sulphurous acid streams in a gaseous form through the pipe E into the chamber. The sulphuric acid, weakened by the condensed water, flows away at the bottom of the cylinder through the pipe G into the cistern II. This outlet pipe is so bent that the acid lying in it shuts in the gases.
As we have before remarked, the steam should be so adjusted that the acid made in the chambers may contain a little more water than the tetrahydrate, or sp. gr. ; when much stronger it will hold chamber crystals in solution, when much weaker it will cause a decomposition of the nitrous acid. These facts teach is that the proportion of water present has an important influence upon the process. If, for instance, the gases from the last chamber are brought into contact with sulphuric acid containing so much water as to equal more than 4 equiva. of water to 1 equiv. of acid, no nitrous acid will be absorbed, and only a little nitric acid arising from the decomposition of the nitrous and hyponitric acids. With perfectly dry gas the acid may be tetrahydrate or 1•55 sp. gr. Generally acid of 150° Tw. is now used, because it can be concentrated
to that degree in leaden pans. As it has been found, however, that acid of 170° Tw. absorbs nitrous acid far more readily and to a greater extent, viz. three times as much as acid of 145° Tw., it becomes a question whether the extra cost of concentration to that strength in glass or platinum would not be repaid. At any rate it is advisable so to regulate the conduct of the manufacture when working with a Gay-Lussac tower that the gases from the last chamber shall be as dry as possible. In this case the steam to the last chamber should be so reduced that the acid made in it too will show 110° Tw. If, however, the steam be admitted to the last chamber in such a degree as to produce acid of only 52° Tw., the gases must be dried as much as possible before leading them into the tower. This may be effected by allowing them to circulate in a long channel J, as ahowu in Fig. 37, in which a great part of the moisture will condense as weak, somewhat nitrous, aulphuric acid, which may be run into the last chamber.
To perfectly carry out the idea of the Gay-Lussac tower, the amount of steam must be very carefully regulated, and further success depends greatly upon the proportion of oxygen in the chamber gases, which must be so great that the nitrous acid cannot possibly be reduced to lower oxide which is not absorbed by sulphuric acid. To fulfil these requirements the excess of air we have already indicated muat not be diminished. But even when both the preceding conditions are fulfilled the success of the process is not ensured ; in fact, so many small trifles need rigorous attention, that the process is extremely difficult of accurate adjustment, so much so that many manu facturers hesitate about erecting the expensive plant necessary.
We have already said that it is still doubtful whether the nitrogen combinations in the chambers are as nitrous or as hyponitric acid, as we know that either of them may be formed, and even both may exist iu the gaaea at the same moment, and their state probably depends upon the proportion of the sulphurous acid to the oxygen. Even if hyponitric acid be absorbed by the sulphuric acid, still we have seen that it forms a very weak chemical combination with it, and that the hyponitric acid is given up very freely on subjection to heat, and even at ordinary temperatures it escapes rapidly in red fumes. The affinity between nitrous acid and sulphuric acid is, on the other hand, very great, and these two acids form, as we have seen, a definite and fixed form. It is therefore easy to understand that the Gay-Lussac process will not succeed when the gases contain only hyponitric acid, which does not admit of reduction to nitrous acid. This may happen when, in the desire to work the chambers well, the escaping gases having only the normal excess of oxygen are quite free from sulphurous acid. In proof of this it is found that in each case when the Gay-Lussac tower works well, the escaping gases still contain a small proportion of sulphurous acid, which either prevents the oxidation of the nitrous acid to hyponitric acid, or tho already existing hyponitric acid is reduced to nitrous acid by the action of the sulphuric acid despite the presence of oxygen.