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 coating of the wax. This coating may be omitted; but it tends to facilitate the future removal of the tissue from the glass.

Now coat the glass with a plain collodion, giving a thick, tough, transparent film. The pyroxyline should be of the kind which yields a film free from opacity or opalescence. About ten grains in an ounce of solvent, consisting of equal parts of ether and alcohol, will answer the purpose. This film is, of course, suffered to dry before applying the tissue compound.

Next make a solution of gelatine and sugar, as follows:

The kind of pigment to be employed, and the proportion in which it is to be added, will depend much on circumstances, into the details of which we enter in another chapter; but it is especially important in the preparation of this kind of tissue, that the pigment employed be so finely divided that no subsidence will take place during the period the tissue compound remains in the fluid state upon the glass. The preparation in this state may be kept ready for use. It should be kept in a well-corked, wide-mouthed bottle; in hot weather it is apt to decompose if kept long. It may, if desired, be poured into a flat dish to the depth of about half an inch, and, when nearly dry, cut into shreds, and thoroughly dried; in which state it may be kept without risk of injury. When required for use after drying, it must be soaked again in eight parts of water.

The proportion of gelatine and of sugar given is that which is found to answer best under ordinary circumstances. But these proportions will be influenced by the