Besides the connective-tissue fibres, the skin contains a small proportion of fine yellow “elastic” fibres. If a thin section of hide be soaked for a few minutes in a mixture of equal parts of water, glycerine, and strong acetic acid, and then examined under the microscope, the white connective-tissue fibres become swollen and transparent, and the yellow “elastic” fibres may be seen, as they are scarcely affected by the acid. The hair-bulbs and sweat- and fat-glands are also rendered distinctly visible by this treatment. On the other hand, the white gelatinous fibres are most easily seen by examining the section in a strong solution of common salt, or in one of ammonium sulphate; or by staining with some aniline dyes such as safranine. Sections are most readily cut for these purposes by the use of the freezing microtome, or after previous hardening in alcohol. For further details see L.I.L.B., p. 254. Ordinarily in the production of leather only the corium, or true skin is used, and in order to obtain it in a suitable condition for the various tanning processes, the hair or wool, together with the epithelium, must be completely removed without damaging the skin itself; and especial care must be taken that the grain, or portion next to the epidermis, does not suffer any injury during the treatment. All the methods employed depend upon the fact that the epidermis cells, especially the soft growing ones next to the corium, and those of the epidermis layer which surround the hair-roots, are more easily destroyed than the corium itself owing to their different chemical character. The “unhairing” process consists essentially in breaking down these cells by chemical or putrefactive agents, and removing the hair together with the rest of the epidermis by mechanical means. Of the various substances which may be used for this purpose, lime is one of the most convenient, as its solubility in water is so slight, that a solution of such a strength as to injure the hide cannot be easily made. Caustic alkalies, on the other hand, are much more soluble, and unless care be taken to use only the proper quantity, a dangerously strong solution may be made with consequent damage to the skin. The addition of small amounts of sulphides to the lime-solution accelerates the unhairing owing to their special solvent action on the epidermis-structures, and also in the case of alkaline sulphides, by the caustic alkali which is produced by their reaction with the lime. Even if used alone, strong solutions of alkaline sulphides rapidly destroy both hair and epidermis, converting them into a mass which may be swept off the skin like wet pulp, and yet they have practically no injurious action on the true skin.