The authors have proposed a method of estimating the amount of mechanical wood-pulp present in a paper, based upon the absorption of iodine in definite proportions by wood in a finely divided state, under strictly regulated conditions. The paper is carefully reduced to a fine pulp, and is then left in contact with a standard solution of iodine in potassium iodide. At the end of twenty-four hours the amount of free iodine is determined by titration with sodium thiosulphate and by deducting this from the amount originally taken, the amount absorbed is ascertained. As this amount, under {203} strictly comparative conditions, always corresponds to a definite amount of mechanical wood-pulp the amount present can be readily calculated. 2. _Loading, Sizing Materials, &c._—The determination of the amount of loading material in a paper has been already described (p. 134). The identification of the material can only be arrived at by a careful chemical analysis. The principal loading materials are china-clay and pearl-hardening (calcium sulphate). The ash from a paper containing china-clay is insoluble in boiling dilute hydrochloric acid; that from paper containing calcium sulphate is soluble: the solution on cooling deposits long needle-shaped crystals of CaSO_{4} 2 H_{2}O and gives with barium chloride a copious precipitate of BaSO_{4} (barium sulphate) insoluble in acids, and with ammonia and oxalate of ammonia a precipitate of calcium oxalate. The presence of starch in a paper can be readily ascertained by its behaviour with a solution of iodine. If starch be present the well-known blue colour of the compound of iodine and starch will be produced. The determination of the amount of starch present is a matter of some difficulty, the details of which are somewhat beyond the scope of the present work. It is based upon the conversion of the starch into sugar, and the estimation of the latter with Fehling’s solution. The nature of the material with which a paper is sized may be ascertained in the following way:— The sample, cut up into small fragments, is warmed for a few minutes with alcohol containing a few drops of acetic acid. The alcohol is allowed to cool, and is then poured into four or five times its bulk of distilled water. If any precipitate or cloudiness is produced, it indicates that the paper has been sized with rosin. The alcohol dissolves the rosin, which, being insoluble in water, is thrown down on dilution. The alcohol used should be previously purified by distillation, as some samples contain a small quantity of shellac, which would itself be precipitated with water. {204} The paper after treatment with alcohol should now be boiled for some minutes with water: the solution allowed to cool, and then filtered. To the filtrate a few drops of a solution of tannic acid are added, when, if the paper has been sized with gelatine, a white curdy precipitate will be formed.