Aecordiugly we may reckon that I litre of tho gas has the following composition :— 0.1123 sulphurous acid 7 oxygen 0 7000 nitrogen Total 1.0(100 litre.
In conformity with the previously given table of the weights of the various gases at 0° C. and 760 mme. pressure, we have in one litre of the gas: 0.1123 ; 2'8731 + 0'0977 ; 1.4298 + 0.7900; 1.2562 = 1.4547 gramme.
The volume of the gases increases, however, with the rise of temperature in the ratio of about 1 273 of the volume for every 1° C. Therefore a litre of gas at 0° C. becomes at t° C. (the pressure remaining the same) 1 + t 273 = 273 273 t litre.
If we take the temperature of the gases in the vertical shaft at 100° C., which is certainly too low, we have 273 100 = litre from each litre at 0° C. and the same pressure, which 7 will weigh, according to the above calculation, = 1.0647 gramme.
For comparison we will now take the weight of one litre of air at 760 mme. pressure and a temperature of 20° C. One litre of air at 0° C. and 760 mme. weighs 1.2932 grm., and will measure 3 + 20 at the same pressure, and a temperature of 20° 27 273 1.0733 litres. Hence 1 litre of .
air at 20° C. and 760 mme. pressure gives a weight of 1 29321.0733= 1.2049 gramme.
Atmospheric air is therefore much heavier than the hot gases in the kilo. Even if we take the temperature of the air at an unusually high figure, for example, 35° C. we still find that the kiln gases are much lighter than the air. The weight of the latter may be taken ns 273 + 35 = 1.1282 for 1 litre, or 1.2932 gramme.273 1.1282 In this calculation it is not necessary to take into consideration the varying proportion of moisture contained in the atmosphere, because, by its great expansion in the hot kiln, it can only increase the difference between the weight of the kiln gases and that of the atmospheric air.
In consequence of the fact that the gases in the vertical channel are lighter than the air outside, this air will rush into the kilns at a speed corresponding with the pressure exercised from below. This speed or draught increases in proportion to the height of tho vertical shaft or channel, and it is thus advantageous to allow the latter to emboueh into the chamber at as great a height as possible. As a superabundant draught is thus secured, the amount of air admitted to the kilu is regulated by suitable ventilators according to need.
A second cause producing a draught is the formation of the sulphuric acid itself, for the space but lately occupied by the gases forming the acid cannot remain a vacuum, but will be immediately refilled with new gases. The condensation of the sulphuric acid takes place during the circulation
of the gases.
A third promoter of the draught is the chimney or stack-pipe, through which the uncondensed gases from the last chamber escape into the air. As these gases contain the nitrogen collected in the chamber, and only 5 per cent. by volume of the heavy oxygen, and as these are saturated with steam, which lessens their specific gravity, and are generally much warmer and can, at any rate, never be colder than the atmospheric air, they are necessarily much lighter than the latter.
If none of the processes hereafter to be described be employed for recovering the nitric acid, then that gas will form an impoi taut constituent of the escaping volume, besides a small proportion of sulphurous acid, by which the specific gravity of the mass will be somewhat increased. Their influence is, however, exceedingly slight, and may be altogether disregarded.
A chimney or stack-pipe, as shown in Fig. 38 (K), with a height of 50 ft., will give a more than sufficient draught. Iu cold regions it is advisable to wrap non-conducting materials about that portion of the pipe which reaches above the roof of the building, so as to check, as much as possible, the evil effect of the cold. In some works the above-described chimney is replaced by putting the chamber-flue into connection with the main chimncy-stalk of the works. But many manufacturers prefer to have a distinct flue-pipe opening into the free atmosphere, and furnished at the top with an open-ended cylinder for protection against the influence of the winds, because in this way it is much easier to regulate and measure the draught than is the case when the chamber exit leads into the common stalk. At one time the gases will rush through the chamber system far too fast, and thus create an enormous waste. Besides, the draught is subject to much greater vacillation in the common chimney than in a special pipe, because the temperature in a chimney can never be maintained at oue degree. When, for any reason, the draught, which means the amount of air admitted, is not sufficient, it may be increased by opening the dampers which regulate the diameter of the passages through which the air has to pass.