_Bleached Pulp._--The second portion of the moist pulp is mixed with a solution of bleach, the strength of which has been accurately determined by the usual methods. The amount of bleach added should be about 20 per cent. of the weight of air-dry fibre present in the moist sample of pulp. The pulp should be bleached at a temperature not exceeding 38° C., and when the colour has reached a maximum the amount of bleach remaining in solution is ascertained by titration with standard arsenic solution. In this way the amount of bleaching powder required to bleach the pulp is determined. The product is then made up into sheets of pulp which are dried by exposure to air and subsequently weighed. _Yield of Pulp._--The percentage yield of finished pulp obtained from the raw material is determined from the figures arrived at in the experiment described, and the weight of raw material necessary to produce one ton of bleached pulp is readily calculated. _Examination of Bleached Fibre._--The fibre should be carefully examined under the microscope and a record made of general microscopic features, especially with reference to the length and diameter of the fibres, and the proportion of cellular matter present, if any. _Sample of Paper._--It is only in the case of short-fibred material similar to esparto and straw that sheets of paper capable of giving comparative results as to strength can be made. The figures obtained with fibrous materials of this kind are only comparative, because it is possible in practice to make a much stronger sheet of paper when the material is beaten properly under normal conditions. A similar investigation should be made by submitting the fibre to treatment with bisulphite of lime, that is to say, if the fibre lends itself to such a process. A lead-lined digester is necessary, and the solution employed is bisulphite of lime prepared according to the directions given on page 160. The preparation of sulphite pulp requires more attention than the manufacture of soda pulp. It is most important that the digester should be absolutely tight in order to prevent the escape of any free sulphurous acid gas, and the contents of the digester must be heated slowly until the maximum pressure has been reached. CHAPTER III THE MANUFACTURE OF PAPER FROM RAGS [Illustration: FIG. 6.--A Rag Sorting House.] The word rag is used to designate a very wide range of raw material suitable for conversion into paper. In the case of high-class hand-made writing papers only the best qualities are employed, such as new linen and cotton cuttings from factories, or well-sorted rags of domestic origin. The usual classification adopted by merchants who supply the paper mills is somewhat as follows:--