Franz Kathreiner, who has made very elaborate researches on skin, and the changes which take place in it during the processes of tanning, employs a mixture of osmic and chromic acids for hardening, and at the same time staining the tissue. This mixture was first used by a German histologist with whose name I am not acquainted, in a research on the internal organs of hearing, and was applied by Kathreiner in 1879 to the investigation of skin, and communicated by him to the writer in the autumn of that year. His method is briefly as follows. The pieces of hide to be examined must, if salted, be well washed, or if dry, be thoroughly softened. For the study of hide in its unaltered and natural condition, it is essential that it be quite fresh, and taken from the animal as soon as possible after death. In any case the _Panniculus adiposus_ or fatty layer is, as far as possible, removed with scissors, the hair cut short, and the skin cut up into little pieces of 3-4 millimetres wide by 10-12 millimetres long (about 1/8 in. by 1/2 in.); the hair must lie exactly across these pieces. They are then placed for 4-8 days, according to the thickness of the hide, in about 12 times their volume of a solution consisting of 0·2 parts osmic acid.[B] 0·5 " chromic acid. 200·0 " water. [Footnote B: Solution of osmic acid is best preserved in sealed tubes in the dark. If obtained in solution it is rarely of full strength, for which allowance may have to be made. Care must be taken to avoid inhaling its fumes, which are very irritating to the eyes and to the respiratory organs, producing severe catarrh.] This solution must be kept from dust and light, in a glass stoppered bottle, and in a cool place. On removing the hide-pieces from this solution, they are placed in about 12 times their volume of absolute alcohol for 4-8 days, during which time the spirit must be at least 3 times renewed. The sections are cut with a razor flooded with absolute alcohol, so that the thin shavings float without friction upon it. The hide-pieces may be held either between soft cork, or, as is generally preferable, simply between the forefinger and thumb as shown in Fig. 7. The cut must be made exactly parallel with the direction of the hair roots, and from the grain towards the flesh; and the sections cannot possibly be too thin. After lying for 1/2-1 hour in absolute alcohol, the sections are soaked till quite clear in clove oil (which must be pale and of the purest kind), and may then be mounted in dammar varnish, or solution of Canada balsam.