Paper is sold in sheets of definite sizes, and is made up into reams containing from 480 to 516 sheets. These sizes {176} correspond to different trade names—such, for example, as crown, demy, royal, imperial, &c. The weight per ream is usually expressed in addition to the name, thus: 14-lb. demy, 18-lb. double crown, &c. In this way the consumer is enabled to make a rough comparison of the thickness of the different kinds of paper. In making paper it is the duty of the machineman to examine and weigh from time to time a sheet of the desired size, in order to ensure uniformity. Special lever balances can be obtained, showing at a glance the weight per ream of different numbers of sheets. {177} CHAPTER XII. CAUSTIC SODA, RECOVERED SODA, ETC. As we have already pointed out, caustic soda and lime are the only alkalis employed by the paper-maker for boiling fibres; the special cases in which carbonate of soda is used have been mentioned in their proper place. The preparation of milk of lime requires little or no description, as the operation is a simple and tolerably familiar one. Before using, it should be passed through a fine wire sieve, to keep back sand, coal, and similar impurities which the lime invariably contains. Caustic soda is met with in commerce in four forms, differing from each other in alkaline strength and colour, and of course in price. The lowest quality is what is known as caustic “bottoms”: it consists of that portion remaining at the bottom of the caustic-pot after the clear fused caustic soda has been ladled out, and it contains a considerable quantity of ferric oxide and other insoluble impurities. It is of a dark reddish brown colour, and contains 50–60 per cent. of alkali (Na_{2}O). Its use in paper-making cannot be recommended, except for the preparation of the very lowest grade of pulp. The solution should be allowed to remain at rest, in order that the insoluble matter may subside. The next in quality is known as cream caustic, so called from its slightly brown colour. It is usually sold containing 60 per cent. of alkali, in the form of a tolerably friable mass, having a crystalline structure. It is a very suitable form of alkali for the paper-maker.