It is essential in dissolving the recovered soda that a high temperature should be employed, as otherwise a portion of the soda present as silicate of soda will be lost, as it is only with difficulty soluble, and requires rather prolonged heating with water. Whatever the form of apparatus employed it should be so arranged that, after running off the strong liquor, the insoluble residue may be further treated with water. In the case of the vats mentioned above, this process is made continuous, pure water being run in at one end, and strong liquor flowing from the other. If other forms are used, the liquor after settling, may be run off by means of a pipe passing through the bottom or side of the vessel, and near the bottom, and consisting of two parts, one long, and one short. The short part is stationary, and is connected to the longer part by means of a movable knee joint, allowing it to be deflected. The liquor having settled sufficiently, the movable limb is lowered beneath the surface of the liquor which is then allowed to flow through. As the surface of the liquor falls, the pipe is gradually lowered. In this way the clear solution can be run off without disturbing the residue at the bottom. The open end of the pipe is usually covered with coarse wire gauze, to keep back insoluble impurities. With properly calcined recovered soda, the solution should be bright and almost colourless. If at all brown in colour, and if it has an empyreumatic odour, it indicates imperfect calcination. The residue in the dissolving tanks consists chiefly of carbonaceous matter, together with some soda, insoluble matter, &c. The liquor is now ready to be causticised. This should be done in a separate vessel, although it is the practice in many mills to perform this operation in the same vessel in which the solution of the soda has been conducted. A good form of causticiser can be made from an old egg-shaped boiler, by cutting it in two along its length. {189}