In the case of sole leather, these processes may require from two to twelve months for completion; after which the leather is dried, smoothed, and compressed by mechanical means, and is then ready for use. Dressing-leathers, ranging from calf-skins to harness-hides, receive a much shorter tannage, and the subsequent treatment with fats and oils, which, together with mechanical manipulations, constitute “currying.” The thin film of grease distributed over the surface of the fibres renders them supple, and to some extent waterproof. The lighter fancy leathers, such as morocco, are dyed, and undergo many complex processes to fit them for their required purposes and improve their appearance. Many skins such as calf, glove, and glacé kid, are not tanned, but “tawed” by a solution of alum and salt, which is often supplemented with mixtures of flour and egg-yolk to fill and soften the leather. Salts of chromium are also employed in place of alum and salt, and produce an equally soft, but more permanent and enduring leather. Lastly, wash-leather, or so-called “chamois,” and buff-leather are produced by fulling the prepared pelt with fish or whale oil, which converts the skin into leather by subsequent oxidation, during which aldehydes are evolved. CHAPTER III. _THE LIVING CELL._ The larger part of the materials employed in leather manufacture are organic in their origin, and the skin itself is an organised structure, while the life-processes of putrefaction and fermentation play a large part in the tannery. Some knowledge, therefore, of biological structures and processes is necessary to a full understanding of much which follows, and a few words are not out of place with regard to the foundations of life itself. The bricks of which all living structures are built are the living “cells” and their products, and these first elements differ little, if at all, whether the life is animal or vegetable, the distinction being produced rather by the way in which they are put together, than by differences in the cells themselves. This is so much the case that it is often difficult to decide in which of the two classes to place the simplest organisms, since most of these forms are capable of active movement, and their modes of nutrition and reproduction are common to both kingdoms.