From these observations it will readily be believed that the saving of nitre effected by the Gay-Lussac towers varies considerably in different works. With first-rate manipulation the nitrous sulphuric acid should contain 3i per cent. of nitrous acid. Winkler found at one works, when the acid was used at 145° Tw., that it contained about per cent. of nitrous acid. The analysis shows the following composition : Sulphuric acid .. 60.200 Water .. 37.191 Nitrous acid Nitric acid .. .. • 256 Organic colouring matter .. .. • 022 100.219 In order to find out with case and approximate correctness the amount of nitrous acid contained in the nitro-sulphuric acid, according to one plan the nitro-sulplinric acid is poured from a burette into a titrated solution of ohromate of potash, until the pure green colour of chromium oxide is produced by the decomposition of the chromie acid. The known quantity of oxygen which the chromic acid thus gives up serves to convert the sulphurous into sulphuric acid, and at the same time, from the amount of nitro-sulphuric acid used, the proportion of nitrous acid can be readily calculated.
Many of the drawbacks attending the use of the Gay-Lussac absorbing tower are removed by having a Glover's denitrating tower working in conjunction with the Gay-Lussac tower.
Figs. 57 and 58 show such a tower of the smallest size for which plans are furnished, the dimensions being increased in accordance with the amount of work required to be done. This tower is the invention of Mr. John Glover, Newcastle-on-Tyne, who first tried to denitrate, and at the same time concentrate, the acid from the Gay-Lussac tower along with the acid from the chambers, in the year 1859. It has been gradually perfected until it has reached its present state. It has been generally adopted since 1869 and 1870, and no acid works can now be considered complete without it. The Gay Lussac tower was of course in use long before tho above dates, and the acid from that tower, after having absorbod the nitrogen compounds leaving the chambers, was denitrated by being run down the small leaden towers packed with coke, and into the bottom of which steam was injected, as we have already described. This of course at once reduced the acid in strength, consequently the nitrogen compounds were given off and taken into the chambers. This plan compelled the reconcentration of the acid, which was done in leaden pans by surface heat at a large expenditure of fuel and heavy wear and tear of the pans, to say nothing of the annoyance of the acid fumes given off during the process. This plan be oamo so wetly, and suoh a nuisance, that the Gay Lussao tower fell into disuse, the expense of uphold ing the pans and oost of fuel far exceeding any benefit derived from the saving of nitre. With the Glover tower, however, all the acid is easily and economically concentrated to a density of 145°-155° Tw., and at the same time all thoroughly denitrated, thus saving the whole of the fuel formerly required for eoncentrating acid for the Gay-Lussac tower, besides that used in the decomposition of salt for making sulphate of soda, as hot and strong acid is thus always at hand for the decomposing pans. All fumes given
off in the tower of eourse go into the chambers. Objection is taken to the tower, in that some of the nitrogen compounds are reduced in it by the hot gases from pyrites burners to the lower oxides, and even to nitrogen. The faot, however, remains that no works having any pretensions to being worked aeientifically are without the towers. Works in the Tyne district are working with 2 and 3 per cant. of nitro on the sulphur actually converted into oil of vitriol 1.845 sp. gr. ; the latter is the outside quantity. Wear and tear of chambers is also saved by the gases entering so much cooler. The gas from the pyrites burners enters the tower at about 700° Fehr., and leaves the tower at 160°-180° Fehr., taking with it all the steam due to the concentration which has taken place in the tower. The figure shows a leaden tower of oblong shape, lined with hard silicioua fire-brick, and packed with flints or flints and coke. It is placed, of course, at the end of the pyrites burners, between them and the chambers. The hot sulphurous acid from the kilns enters, as shown, below a perforated arch, which carries the packing and meets the descending current of mixed nitro-sulphuric acid from the Gay-Lussac tower, and chamber acid which had previously been introduced at the top of the tower ; the gaseous constituents of the pyrites and sulphuric acids passing through a pipe to the chambers, along with all the steam and volatilized acid which are formed during concentration. The acid issuing from the tower is sufficiently strong to be used again on the Gay-Lussac tower, and so on. The towers give no trouble, and are easily worked. Cases have, however, occurred where they have not done so, but whenever this has happened it has been owing to ignorance in proportioning the apparatus to the work required, and as the invention is not patented, this has sometimes happened when towers are erected by incompetent persons..
Under certain conditions, it may be convenient to use other bodies than concentrated sulphuric acid for absorbing the nitrogen compounds. Kuhlmann uses the ammoniacal liquor from gas works, which is allowed to flow down a tower filled with coke, such as we have already described. The ammonia combines with the acid contained in the gases, and by allowing the solution which escapes from the tower to stand, the salt will crystallize out. The same manufacturer also uses carbonate of barium in jars, such as we have already described, by which is produced a white paint, known as " permanent white," sulphate of barium ; the nitrogen acids combine with the barium as s. soluble salt, whence they can be recovered and the residue can be re-utilized.