The disposition of the different divisions of a paper-mill depends of course upon the nature of the paper it is intended to make and to a large extent upon local circumstances. Fig. 79 will give some idea of the general arrangement of a mill for manufacturing esparto paper. {211} _Water Purification._—Arnot in his Cantor Lectures on the Technology of the Paper Trade states that from 30,000 to 40,000 gallons of water are required for the manufacture of each ton of paper. The importance of a pure and copious supply is therefore very evident. [Illustration: FIG. 79.] The impurities of water consist of two classes, insoluble and soluble. The former can be readily removed by processes of settling and filtration. For this purpose most paper-mills are provided with large ponds capable of holding several days’ supply. These are sometimes supplemented by filtering beds. Insoluble impurities can also be removed by passing the water through filter-presses. Fig. 80 is an illustration of Rœckner’s patent clarifier, which may also be applied to this purpose. The cylinders C dip below the surface of the water to be purified, which is contained in the reservoir A. They are open at their lower ends, but are closed at the top by the domes D. These are connected with the pump I by means of the pipes H. On starting the pump the water rises slowly in the cylinders. As soon as it reaches the level of the top, the action of the pump is stopped and the cocks K opened, when the water commences to flow down the pipes G, which together {212} with the cylinders form a kind of siphon. If the reservoir A be kept full, the siphons can be made to act continuously. The flow of water being slow, insoluble impurities have time to subside. The impurities may from time to time be removed by means of the small pump L connected with the bottom of the reservoir. The clarifier can also be used for the purification of the effluent water from paper-mills. [Illustration: FIG. 80.] Insoluble impurities can be removed from water by means of the Stanhope purifier, shown in Fig. 81. The soluble impurities of water consist mainly of carbonate and sulphate of calcium and iron, and occasionally organic matter. It is a moot point among paper-makers {213} whether or not the presence of the two former in a water is objectionable. For boiling and bleaching purposes it undoubtedly is, as when mixed with caustic soda or bleaching liquor they form a precipitate of carbonate of calcium, which lines the insides of boilers, breakers, and potchers as a hard scale, which is always liable to become detached, and to find its way into the finished paper. Moreover, carbonate of calcium is precipitated in the fibre, and carries with it a certain quantity of colouring matter, the subsequent removal of which is difficult.