The breaking length (strain) shows a mean increase of about 8 per cent.; the elongation under strain, on the other hand, a diminution of 6 per cent. For an interesting discussion of the question of the relative strengths of machine and hand-made paper see ‘Paper,’ by Richard Parkinson. [Illustration: FIG. 78.] The thickness of a paper may be determined by means of an ordinary micrometer, such as is shown in Fig. 78. The paper is placed in the jaws of the instrument, and the screw {199} advanced until it touches the paper. The thickness is then read off on the scale. Other forms of apparatus are sold for the same purpose. In making a determination of the thickness of a paper it is necessary to take the mean of a series of observations at different points of the sheet, as the thickness may vary somewhat. _Determination of Composition of Papers._ (II.) The analysis of a paper naturally divides itself into two parts:—(_a_) The determination of the nature of the fibrous material of which it is composed; and (_b_) the identification of such adventitious substances as size and filling material. (_a_) This again is divided into two sets of observations—microscopical and chemical. A fragment of the paper is soaked for some time in glycerine, and is then carefully teased out with a pair of needles, and the fragments laid on a glass slip with a drop of glycerine. A cover-glass is then laid on and lightly pressed down so as to spread the fibres in a thin layer. The microscopical features of the different fibres have been already described, and it is only necessary now to summarise the chief characteristics of the more important materials. _Cotton._—Flat riband-like fibres, frequently twisted upon themselves. The ends generally appear laminated. The fibres are frequently covered with numerous fine markings (see Frontispiece). _Linen._—Cylindrical fibres, similar to the typical bast fibre (see Fig. 6). The ends are frequently drawn out into numerous fibrillæ (see Frontispiece). _Esparto._—Esparto pulp consists of a complex of bast fibres and epidermal cells. These serrated cells are, as has been already pointed out, characteristic of esparto, straw, and similar fibres. Certain differences exist between those of esparto and straw, and even between the different species of straw, which enable the microscopist to identify their source. The most characteristic feature of esparto pulp is the presence {200} of a number of the fine hairs which line the inner surface of the leaf (_e_, Fig. 10), some of which invariably survive the boiling and washing processes, although the greater portion passes away through the wire-cloth of the washing engines. The presence of these hairs may be taken as conclusive evidence of the presence of esparto.