There are a few points bearing on the above ealculatione which must not be overlooked. It has been taken for granted that the gaseous mixture leaving the chambers contains no longer any sulphurous acid, whereas in practice that gas is never completely converted, and some small proportion alwaye romaine uncondensed. This fact, the influence of which is trifling, can only increase the dousity of the escaping gases, for each of the gases forming the compound is far lighter than sulphurous acid. This cannot therefore in any way disprove the foregoing conclusion.
Further, the nitrogen compounds cannot be correctly estimated, because as yet we do not know for certain whether nitrous or hyponitric acid is contained in the chamber gases, and because no reliable examination of the density, &c., of the vapours of these acids has hitherto been made. But it must be observed that, theoretically, the nitrogen compounds aro not consumed, but should escape unchanged from the chamber process ; in that case, in consequence of the cooling to which they are subjected in their passage through the chambers, they could certainly only effect an increase of the difference between the densities of the entering and escaping gases. It is an absolute certainty, however, that one portion of the nitrogen compounds is lost and withdrawn from the gaseous mixture during the process, and this may be thought to exercise an opposite influence upon the conclusion arrived at. The following remarks will, however, show that the maximum effect possible from that source is much too small to throw any doubt upon the correct ness of the conclusion stated.
In good working. the proportion of nitrate of soda used per 100 parts of sulphur burnt never exceeds 7 parts. This means parte of hyponitric acid per 100 parts sulphur. For each kilogramme of sulphur there are then 37.9 grms. hyponitric acid, which, as vapour at 0° C. and 760 mm., make 18.3 litres, because the ascertained weight of one litre of this vapour is 2.0715 grms. Now we have seen that for each kilogramme of sulphur burnt, the mass of gas produced at 50°C. and 760 nim., and saturated with steam, is 8345 litres, and in this volume there are 18.3 litres of hyponitrio acid. It is evident therefore that no contradiction nor modifica tion of tho above conclusions can be produced by the action of the nitrogen compounds.
Division of the Labour.—As it ie most important that the conduct of the process should be made as constant and regular ae possible, the daily consumption of sulphur is divided into 24 portions, one of which is added every hour. If the nitric acid be derived from the decomposition of nitre with sulphuric acid in the sulphur burner, this is apportioned in the same way.
The sulphur and nitre should be weighed out by the foreman daily, and be placed in suitable vessels, within handy reach of the kilns ready for use, but the sulphuric acid may be left to the judgment of the "kiln man," to be added as required by means of a leaden cup of the proper capacity. It is advisable that the acid used should be about 110° Tw., in order that it may
thoroughly liberate the gas.
We have already suggested that the sulphur should be damped a little, so as to prevent its volatilization. The workman spreads the charge of sulphur next to be used on a board floor and then sprinkles over it an amount of water in proportion to its weight. It should be turned over once or twice so as to become well moistened. This is done just before charging, and then the nitre pot is prepared as already described, so that everything is in readiness for charging.
As one hour is not always enough to completely liberate the nitric acid from the nitre, the nitre-pot is left in the burner during the combustion of two charges of sulphur, so that there are constantly two nitre-pots in the oven. Before the introduction of each charge the nitre-pot which has been longest in the oven is withdrawn, and the sulphur ashes are cleaned out with a rake from the bed of the furnace. As soon as the kiln is cleaned a new portion of sulphur is introduced on a broad iron shovel. Then the fresh pot containing nitre and sulphuric acid is put in and the door closed. The sulphate of soda formed is taken out of the pot, while yet warm and soft, with an iron spatula or spoon.
When ready-made nitric acid is used, and, as described, is allowed to run directly into the chambers, the labour at the kiln is reduced to cleaning out the kiln, preparing the sulphur, and charging it.
In the works we have particularly described, the "kiln man" had also to look after the steam boiler. Care must be taken that the pressure of steam be not allowed to get down. The augmen tation or diminution of the steam admitted to the chambers may be easily managed by simply varying the pressure in the boiler, instead of regulating the taps throughout the whole series. When, however, it is necessary to alter the steam admitted to an individual chamber, the tap leading to it must be the means employed, while the pressure is allowed to remain unchanged. The work is carried on day and night, two workmen changing shifts with each other, and each working 12 hours. Certain modifications of the process will be necessary until such time as the chambers have reached the correct temperature and are working well. In order to bring things into their normal condition as soon as possible, the sulphur-burner is first heated by fire, which is easily done by the fireplace z, shown in Fig. 39. When no such fireplace is provided, the fire is placed in the burlier itself, and the smoke is conducted away by an opening made for the purpose, as during the heating the vertical connection between the kilns and the chambers is discontinued.