_Dusting._--The rags are next passed through a machine which removes dirt. This is a hollow cylindrical or conical drum having an external covering of coarse wire cloth, which rotates inside a wooden box. The shaft is provided with projecting spikes, so that the rags are violently agitated in their passage through the machine. The dirt and other impurities fall through the wire on to the floor of the room, while the clean rags are discharged from the lower end of the drum. The loss in weight varies according to the condition of the rags. With good materials the loss may only be 1-2 per cent., while with dirty common rags the loss during cleaning and dusting may amount to 10 per cent. [Illustration: FIG. 8.--A Rag Cutter.] _Boiling._--The further purification of the rags is effected by a chemical treatment, viz., boiling at a high temperature with alkaline substances, which process removes fatty, glutinous, and starchy matter from the material. [Illustration: FIG. 9.--Interior of Paper Mill for Hand-made Paper (R. Batchelor & Sons).] For this purpose a spherical digester is used, generally 7-9 feet diameter, and capable of holding 2-2½ tons of rag. The boiler or digester is filled with dusted rags, and the requisite amount of alkaline solution added. The manhole is then closed, and steam admitted through the hollow trunnions until the pressure reaches 20 or 30 lbs., at which pressure the boiling is continued for three to six hours according to requirements, the digester rotating slowly the whole time in order that the rags may be evenly and thoroughly boiled. [Illustration: FIG. 10.--View of a Rag Boiler, showing connections.] The liquor employed for boiling is a solution of caustic soda, carbonate of soda, or milk of lime. In the case of caustic soda the amount required varies from 5 to 10 per cent. of the weight of rag. Caustic soda is preferable to lime, because it acts upon the grease and other fatty matters, forming a soluble compound which is freely removed in the subsequent process of washing. Many paper-makers, however, use milk of lime, carefully strained through fine cloth, almost exclusively. Considerable experience and skill are necessary in this operation in order to avoid injury to the fibre not only as regards its strength, but also its colour.