_Washing._--When the rags have been sufficiently boiled, the steam is turned off and the pressure allowed to fall. This can be effected quickly by blowing off from a valve fixed at the bottom of the boiler opposite to the manhole. The cover is removed from the boiler and the boiler slowly rotated in order that the contents may be discharged into a tank placed below. The "black liquor," as it is called, is then drained away from the rags, which are immediately subjected to a preliminary washing. The process of washing must be carried out in a thorough manner in order to remove all soluble compounds, which if left would cause an unnecessary waste of bleach in the subsequent stages of purification. There are many schemes employed for washing, most of them being devised with the idea of using a minimum quantity of water. The most general practice, in the absence of special machinery, is the preliminary treatment in the tank below the digester, followed by a more complete washing process in a machine known as a breaking engine. This apparatus is a shallow oval-shaped vessel with circular ends, divided lengthwise by a partition called a mid-feather, which, however, does not extend the full length of the apparatus. In one of the two channels into which the vessel is thus divided a heavy roll is fitted, which is provided with a number of steel knives. On the floor of this channel there is fixed a "bed-plate," also provided with projecting knives which are parallel with the knives in the roll. The distance between the knives in the roll and those in the "bed-plate" may be altered as required by means of an adjusting screw. In the other channel of the breaking engine there is fitted a "drum-washer," which serves for the removal of the dirty water from the machine. This drum is divided into sections by means of partitions which reach from the centre to the circumference. The surface of the "drum-washer" consists of a fine brass wire cloth supported by a coarser material placed underneath. [Illustration: FIG. 11.--A Breaking and Washing Engine.] The breaking engine is half filled with clean water, and the rags are thrown into the engine until it is suitably filled. The rotation of the heavy roll causes the mixture of rags and water to circulate round the vessel, the floor of which is so constructed that the pulp is drawn between the roll and "bed-plate" and discharged over the "backfall," which is that portion of the sloping floor behind the "bed-plate."