In this way it is possible to determine alterations in length to within 0·005 per cent., and this variation can be directly seen under the microscope. Hohnel observes in his account of the experiments that all fibres become thicker when wetted, that vegetable fibres are more susceptible than animal fibres. Animal fibres expand about 10 to 14 per cent. in diameter, but vegetable fibres as much as 20 per cent., as shown in the following table:-- -------------+----------+------------------+---------- Animal Fibre.| Per Cent.| Vegetable Fibre. | Per Cent. -------------+----------+------------------+---------- Human hair | 10·67 | New Zealand flax | 20·0 Angora wool | 10·2 | Aloe hemp | 25·8 Alpaca wool | 13·7 | Hemp | 22·7 Tussah silk | 11·0 | Cotton | 27·5 -------------+----------+------------------+---------- The reverse is the case when the expansion of the fibres in regard to length is considered, since animal fibres expand 0·50 to 1·00 per cent. of their length, and vegetable fibres only 0·05 to 0·10 per cent. The maximum amount of expansion in the case of the vegetable fibres is obtained by gently breathing upon them rather than by the use of an excess of water. These figures are important as explaining many of the peculiar characteristics of vegetable and animal fibres. Advantage is taken of the greater expansion of the latter in the manufacture of instruments for the measurement of moisture, such as the hair hygrometer, in which the elongation of a stretched hair registers the variation in the moisture of the atmosphere. _Quality of Book Papers._--The Committee of the Society of Arts in dealing with the evidence as to the permanence of finished papers suggest the following classification as indicating the desired standards of quality:-- (A) CLASSIFICATION AS TO FIBRES. A. Cotton, flax, and hemp. B. Wood celluloses, (_a_) sulphite process, and (_b_) soda and sulphate process. C. Esparto and straw celluloses. D. Mechanical wood pulp. The Committee find little fault with the Principles which govern the trade in the manufacture of high-class papers, and limit the result of their investigation to the suggestion of a normal standard of quality for book papers required in documents of importance according to the following schedule:-- _Fibres._--Not less than 70 per cent. of fibres of Class A. _Sizing._--Not more than 2 per cent. rosin, and finished with the normal acidity of pure alum. _Loading._--Not more than 10 per cent. total mineral matter (ash). With regard to written documents, it must be evident that the proper materials are those of Class A, and that the paper should be pure, sized with gelatine and not with rosin. All imitations of high-class writing papers, which are in fact merely disguised printing papers, should be carefully avoided.