Besides the foregoing observations it is well to pay attention daily to the small stoneware plug in the chamber wall opposite the entrance of the kiln gases, in order to note their composi tion. On this plug any sublimation of sulphur or formation of chamber crystals will be immediately noticed.
4. The Sulphurous Acid in the Gases.—We have already seen that the gases entering the chamber should contain 0.1123 litre of sulphurous acid, litre oxygen, and 0.7900 litre nitro gen in each litre, or about 11 per cent. by volume of sulphurous acid. This holds good in practice, and should be daily con tolled. In some works a simple apparatus is used for examining the gases. In order to put it into communication with the interior of the chamber, the chamber wall is pierced and the orifice fitted with an indiarubber plug through which is passed glass tube.
Fig. 49 shows this apparatus as consisting of three principal parts fitted to an easily movable wooden frame.
An ordinary sugar-glass A acts as an absorbing vessel, a cylindrical metal vessel B, funnel-shaped at the bottom and furnished with a long narrow exit tube and tap a, serves for an aspirator, and a glass measure C permits the admeasurement of the water escaping from B.
The glass vessel A is tightly closed by a metal cover in which are two openings. Through one is inserted a brass tube b, bent towards the left at the top and furnished with a tap c, for the purpose of making communication between the interior of the vessel and the outer air or the other mouth of the pipe, or to disconnect altogether the interior of the vessel. Into the lower part of this brass tube a glass tube d is cemented, reaching almost to the bottom of the vessel, but somewhat bent on one side and drown to a lino point ut the end. The second opening e admits a short glass tube bent towards tho right at the top. It is very important so to arrange this that the stopper may be easily taken out and put iu, and it is better to make it air-tight by means of a screw, as this stepper must be withdrawn at earth test, and will very soon wear out if it fits so tightly as to make an airtight joiut of itself. The metal vessel B has a lateral tube, which may be connected with tho tube f by means of indiarubber tubing. In the upper part is an opening g, closed with a cork and
also with a acrew cover to make it air-tight.
When it is desired to make a test of the sulphurous acid contained in a volume of gas, a glass or other tube is inserted into the chamber and joined to the pipe b by indiarubber tubing. It is most important that these joints be air-tight, EO that no air may enter from without, for there is no means of discovering any accidental error in the test. The vessel A is 4 or filled with water through the opening e, and the vessel B is similarly filled moderately full through g. Froni an ordinary Mohr's burette some solution of iodine (containing 1'27 gr:n. iodine as iodide of potaah in litre) is added to the water in A, as well as some etarch solution by which the water is rendered of a deep blue colour. When all the openings have been closed and the tap c so regulated that uo air ean draw into A, the tap a is opened and the water flows out until the vessel B is emptied. The tap a ie then shut and the tap c opened, so that a communication is formed between b and d ; then a is opened, so that the water can only flow out very slowly, and the gas volume to be tested bubbles in through d, and rises throngh the coloured water. As soon as the sulphurous aeid mixes with the water it turns the freo iodine into hydriodic acid, and ia time the liquid will bo & colorized, which may be very well seen towards the end of the process when it proceeds with great rapidity. The tap a is then shut as well as the inlet of the decolorizing matter. The passago e is opened and a measured volume of the iodine solution is admitted to the vessi I A, which also assumes a blue colour. After closing e, a ie cautiously opened and so much water allowed to escape that the fluid in the pipe d, which had been reduced to the level of that outsido by tho opening of e, is drown to the point of the tube; a is then quickly shut, all tho liquid hitherto eaught in C is thrown away and the empty vessel replaced. Thereupon a is reopened, and the wator flowing sway causes the gas to be slowly sucked through A till decolorization again ensues, when the tap a is shut and the liolume water run into C is measured.