When steam is utilised to extract dye-woods these defects are removed. A burnt extract is not to be feared, and a small quantity of water is sufficient to extract the colouring matter completely. The apparatus illustrated in Fig. 37 is very suitable for the extraction of dye-woods, and in general for obtaining vegetable extracts. The extraction vessel is pear-shaped, its two hollow axes move in hollow bearings. Thus it may be turned upside down, and steam and water can be introduced into the interior through the axes, which are connected with the pipes, E, W, and R. The opening at the pointed end of the pear is closed by the screw, S; through it the substances to be extracted are introduced. In the lower portion of the vessel is a sieve upon which the materials are spread out. The opening of the pipe connected with E and W is below the sieve. [Illustration: FIG. 37.] The process is commenced by introducing the materials through the upper opening, fastening the cover down steam-tight by the lever, B, and the screw, S, and then running in water through W until it flows out through the lower of the two narrow pipes which are shown somewhat paler in the shaded portion of the drawing. All the taps are then closed with the exception of that on the upper of the two small pipes. By opening the cock on R steam is led in. As a rule, steam at a low pressure is used, not more than half an atmosphere. The contents begin to boil in 15 to 30 minutes, steam then issues from the open tap. According to the nature of the material to be extracted, boiling is continued from 40 to 60 minutes. The side tap is then closed, and the tap on the pipe E opened. The pressure of the steam now forces the liquid up the pipe E; the pressure of half an atmosphere is sufficient to raise it about 4 metres. In this way it can be forced into a tank above. The rest of the liquid can be run off through the cock, _h_, at the bottom of the vessel. When quite empty the cover is taken off, the vessel turned over, and the solid residue removed. One extraction of dye-woods is not sufficient to remove the whole of the colouring matter; in most cases the wood is treated a second time before the apparatus is emptied. Even then some quantity of colouring matter remains in the wood. The twice extracted wood is brought into a tub filled with water, and the solution resulting from the long contact of the water with the wood is used in the next operation to extract new quantities of wood instead of fresh water. The dimensions of the extraction vessel vary according to the size of the works. A large apparatus costs not much more than a smaller, since in both cases the labour is the same, so that it is advisable to use a large apparatus. When the copper extraction vessel is made to have its greatest diameter about 1 metre, it can be charged with about 50 kilogrammes of rasped wood at once. [Illustration: FIG. 38.]