(_b_) For the chemical identification of the fibres in a paper, the only available reactions are those with aniline sulphate solution. The majority of fibres (celluloses) give no reaction, straw and esparto celluloses and mechanical wood-pulp being the only ones that can be identified by its means. The authors have found that when a paper containing straw or esparto is treated for some time with a boiling 1 per cent. solution of aniline sulphate, a pink colour is produced. Esparto gives the reaction with greater intensity than straw. In this way the presence of a very small quantity of these pulps can be detected with certainty. Mechanical wood-pulp, when treated with a solution of aniline sulphate, develops, even in the cold, a deep yellow colour. If a paper containing mechanical wood-pulp so treated be examined under the microscope, the fragments of wood will be found to be deeply stained, whereas the other {202} fibres remain colourless or nearly so. It must be borne in mind that cellulose obtained from lignified fibres, if the boiling and bleaching processes have not been carried sufficiently far, will give with aniline sulphate a more or less intense yellow coloration. Various other reagents have been suggested for the identification of mechanical wood-pulp, all based upon the production of a colour with lignose. The reaction of lignose with chlorine and sodium sulphite solution already referred to (p. 19) may also be made available for the detection of mechanical wood-pulp in a paper. Imperfectly boiled or bleached pulps sometimes give this reaction faintly. _Quantitative Estimation of Mechanical Wood-pulp._—The determination of the amount of mechanical wood-pulp present in a paper is sometimes a matter of some importance, and it is also a matter of great difficulty. Some idea of the amount present can be obtained by observing the depth of the yellow colour produced with aniline sulphate or the intensity of the magenta reaction with chlorine and sodium sulphite. It is also possible to calculate approximately the percentage from the percentage of cellulose contained in the specimen. Mechanical wood-pulp (pine) may be taken to contain 60 per cent. of cellulose. If, therefore, a paper ascertained to contain a pure cellulose in addition to this constituent, yield 75 per cent. of cellulose on the ordinary test, it may be assumed that about 62·5 per cent. of mechanical wood-pulp is present.