In Fig. 81 is shown the Stanhope purifier, which is largely used for the purpose of softening water. The following details will render its action clear: A is a store tank containing caustic soda solution. B is a tank into which the water to be treated flows, and which is maintained at a constant level by means of a valve and float. C and C are two tanks which are used alternately, and in which lime water is prepared. A definite quantity of caustic soda solution is added from B; this mixture forms the reagent by which the softening of the water is accomplished. D is a small tank for the purpose of maintaining a constant head of reagent. The reagent and the water are mixed together in the pipe F, the quantities being regulated by the cocks {216} shown at H. The pipe F leads into the vessel E, which is fitted with a number of V-shaped shelves, placed at an angle of 45°, and riveted alternately to opposite sides of the vessel. This arrangement causes the water to take a serpentine course. The position of the shelves is indicated by the dotted lines. As soon as the mixture of water and reagent flows down the pipe F, and enters the vessel E, the calcium carbonate commences to precipitate and settles upon the V-shaped shelves. At the top of the tank a layer of wood shavings, inclosed in wire-netting, is placed as shown by the dotted lines. This acts as a filter, and intercepts any particles that may not have subsided. The clear purified water passes away by the pipe I. The precipitate that collects on the V-shaped plates is from time to time drawn off by the cocks shown at G; this may be done without interfering with the working of the apparatus. While there can be no doubt that the use of soft water is advantageous in the boiling, washing, and bleaching processes, and also of course for supplying steam boilers, it is probably an advantage to use a hard water for diluting the pulp before running it into the machine. In the case of papers which are loaded with any of the forms of calcium sulphate (pearl-hardening, crystal-hardening, &c.) the use of very soft water is objectionable from the fact that a certain quantity of calcium sulphate is dissolved. This would not take place to the same extent with water which is already charged with sulphate and carbonate of calcium. The removal of soluble iron from a water is effected by the softening processes described. {217} CHAPTER XVI. ACTION OF CUPRAMMONIUM ON CELLULOSE. PREPARATION OF WILLESDEN PAPER. The action of a solution of copper oxide in ammonia upon cellulose has been already referred to (see p. 5). Celluloses, and also lignocelluloses, when treated with such a solution, gradually gelatinise and finally dissolve. On evaporating the solution to dryness, a gummy amorphous mass is obtained, containing the cellulose intermixed with copper oxide.