SPIRIT OF DISCOVERY.

Tanning.

The tanner steeps the skin at first in a weak infusion of bark, until it has acquired a nutmeg brown colour, and then he gradually increases the strength of the steeping liquors, and after a time he draws the skin out, and finds that it is converted into leather.  A thick piece of hide requires ten, twelve, or fourteen months, to be converted into good leather; and when you consider the length of time consumed in the process, and the great capital necessarily employed, you cannot feel surprised that various plans should have been proposed to lessen both.  It was proposed to tan with warm instead of cold liquors; and although the tan appeared to promote the skins in a shorter time, the quality of the leather was so much injured, that it was soon given up.  Then it was tried to force the tan through the pores of the skin, by employing great pressure; but this was not found to answer.  But you may ask why the tanner does not put the skins at once into a strong liquor?  The reason is, that the exterior surface of the skin would soon become tanned, and the central part would remain untanned, which, in a short time, would begin to rot and decay, and the leather so treated would soon fall to pieces.  The tanner, therefore, judges of the perfection of the tanning by cutting through the leather; and if he finds it of an uniform brown colour, without any white streak in the centre, he considers that the process has been successfully conducted.  It would require much time to describe all the operations of the tan-yard, but many of them are interesting, as regards the chemical agents employed.  I might have mentioned to you, that the mode of preparing the skin for tanning, is first to soak it in lime-water, by which the hair is easily detached; but the cuticle and under part of the skin, the cellular substance, are scraped off after it has been soaked in the lime water.  A great variety of substances have been used for tanning, as the acorn-cup of the oriental bark; catechu and sumach have been also used; but the oak bark is most generally used, as furnishing a large quantity of astringent matter.  It is not the business of the chemist to describe the different kinds of leather, but I may just mention, that the upper leather of shoes is called curried leather; the leather having been tanned, is rubbed over with oil before it is dried, and it is then very flexible, pliable, and durable; but if you take a piece of dry leather, and try to rub it over with oil or grease, you cannot make it enter the pores of the leather; the black colour is produced by rubbing it over with a solution of green vitriol, the sulphate of iron. Russian leather is tanned in an infusion of birch bark, and is said to be afterwards mixed with a quantity of birch tar, to give it that odour for which it is peculiar, which renders it valuable for book-binding, on account of preventing it from being