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 attacked by insects. _Tawed_ leather, used for
gloves, is made by impregnating the skin with a liquor containing alum
and salt, and afterwards washed in a mixture of yolks of eggs and water;
the saline and animal matters combine, and give it that peculiar
softness, and such leather is afterwards coloured as may be required;
having been rolled over wooden rollers, in which are grooves, it is