After the digester has been loaded with chips, the chemicals are introduced and the “cook” is carried on by means of high steam pressure. The strength of the chemicals, pressure of steam, and duration of cooking, are the principal factors in determining the result from any particular wood. Slow cooking at low temperatures yields the best results. [Illustration: WET MACHINES WHERE THE PULP IS CUT OFF IN SHEETS, THE BROWN CO. To the right are the hydraulic presses for removing moisture from the pulp. The pulp is shipped about seventy per cent moist.] SODA PULP.--Soda pulp takes its name from the caustic soda which is used as a solvent. Rotary digesters are employed in its manufacture. The principal wood used for making soda pulp is poplar, though chestnut and aspen are also used. Soda pulp is soft in texture and of no great strength, but in combination with harder stocks it lends mellowness to the sheet. It is almost one-third cheaper than bleached sulphite pulp, quotations for February, 1915, being $2.20 to $2.35 per hundredweight, whereas bleached sulphite was quoted at $2.80 to $2.95 per hundredweight. The prices since the war have risen over 100% and were quoted in September 1919 at $4.75 to $5.00 and $5.75 to $6.25, respectively. One reason for the difference in price between soda and sulphite pulps, is that the soda is recovered from the spent liquor, whereas in the sulphite process the liquors go to waste. SULPHATE PULP.--The solvent used in making sulphate pulp is a mixture of caustic soda, sulphide of soda and sulphate of soda. Sprucewood, largely, is used and the pulp produced is exceedingly strong. Unbleached sulphate pulp is used, notably, in the making of Kraft wrapping-paper. The soda is recovered from the spent liquors. SULPHITE PULP.--Sulphite pulp is produced by the use of bisulphite of lime; this, being acid, necessitates a special brick lining in the digesters, which are of the vertical type. Sprucewood is the best raw material and yields a strong, fairly long fiber, capable of being bleached to a good white color. MITSCHERLICH PULP.--A special method for making sulphite pulp was invented in Germany by Professor Mitscherlich. It varies from the ordinary process in that the cook is continued over four times as long under lower steam pressure, and yields a fiber of greater strength.