CHEMICAL COMPOSITION OF HIDE.
The chemical composition of skin is very imperfectly understood. The
bulk of the skin is, as has long been known, converted by boiling
into gelatin or glue. The yellow fibres and cellular tissue remain
undissolved. Müntz, who made some interesting researches on the
subject, found that completely dried hide contained--3·086 per cent. of
cellular tissue insoluble in hot water, 1·058 of fat, 0·467 of mineral
matter, and 95·395 of matters soluble in hot water. Müntz counts the
whole of the tissue soluble in hot water as converted into glue; but
this is not strictly the case. Gelatin is not identical with the fibre
of the hide, which is only converted into it by boiling. The nature of
the change is not well understood; but it is either simply molecular,
or depends on the addition of one or more molecules of water. The
gelatin of bones seems identical with that of skin and connective
tissue, but that of cartilage differs slightly from it, and is called
chondrin. Raw hide, unhaired and purified, contains, according to
Müntz--carbon, 51·43 per cent.; hydrogen, 6·64; nitrogen, 18·16;
oxygen, 23·06; ash, 0·71; while gelatin has--carbon, 50·1 per cent.;
hydrogen, 6·6; nitrogen, 18·3 (Mulder); carbon, 50 per cent.; hydrogen,
6·5; nitrogen, 17·5 (Fremy). Probably, however, neither substance was
quite pure.
Gelatin is insoluble in alcohol, ether, and cold water, but swells in
the last, absorbing about 40 per cent. It is soluble in hot water,
but is reprecipitated on the addition of a sufficient quantity of
alcohol, resembling in this respect gum, dextrin, and many other
substances. It is soluble in glycerin, with the aid of heat, and in
concentrated sulphuric acid in the cold. Moist gelatin exposed to
the air rapidly putrefies. It first becomes very acid, from formation
of butyric (and perhaps other) acids, but afterwards alkaline, from
evolution of ammonia. Boiled with concentrated potash, it yields leucin
(amidocaproic acid, C_{6}H_{15}NO_{2}), glycocin (sugar of gelatin),
and other substances. The same products are obtained by boiling with
sulphuric acid, and probably also more gradually, and in greater or
less proportions, by the prolonged action of lime or barium hydrate,
by putrefaction, and by any other influence which tends to resolve the
gelatin molecule into its simpler parts. Gelatin is precipitated by all
tannins, even from very dilute solution. A solution containing 2/10000
parts is rendered turbid by infusion of gall-nuts or gallotannic acid.
The precipitate is soluble in excess of gelatin. Solution of gelatin
dissolves considerable quantities of lime phosphate, hence this is
always largely present in common glue. Gelatin is precipitated by
mercuric chloride, in this respect resembling peptones; but not by
potassium ferrocyanide, by which it is distinguished from albuminoids;
and it differs from albumen in not being coagulated by heat. On the
contrary, by prolonged boiling glue loses the property of gelatinising,
and becomes soluble in cold water, being split up into two peptones;
semi-glutin, which is insoluble in alcohol, and precipitated by
platinic chloride; and hemicollin, which is soluble in alcohol, and not
precipitated by platinic chloride. Both are precipitated by mercuric
chloride (see Hofmeister, abst. Chem. Soc. Jour. 1881, p. 294).
Gelatin or glue with about 3 per cent. of potassium dichromate becomes
insoluble when exposed to the light, from the formation of a chromium
compound. This reaction is the base of several modern photographic
processes, and has been used for waterproofing and for cementing glass,
&c.
The connective-tissue fibres are partially converted into gelatin
by the action of strong acids and alkalies, as well as by heat. By
weak acids, they are swollen and gradually dissolved, and Reimer[C]
has found that the material may be reprecipitated by lime-water. It
forms an irregular fibrous mass, which has not the sticky feel of
gelatin, but is at once converted into that body by boiling. Rollet has
demonstrated that when hide and other forms of connective tissue are
soaked in lime- or baryta-water, the fibres become split up into finer
fibrils, and as the action proceeds, these again separate into still
finer, till the ultimate fibrils are as fine as can be distinguished
under a powerful microscope. At the same time, the alkaline solution
dissolves the substance which cemented the fibres together, and this
may be recovered by neutralising the solution with acetic acid, when it
comes down as a flocculent precipitate. This was considered by Rollet
to be an albuminoid substance; but Reimer has shown that it is much
more closely allied to the gelatigenous fibres, if indeed it is not
actually produced from them by the action of the alkaline solution.
Reimer used limed calf-skin for his experiments, and subjected it to
prolonged cleansing with distilled water, so that all soluble parts
must have been pretty thoroughly removed beforehand. He then digested
it in closed glasses with lime-water for 7-8 days, and precipitated the
clear solution with dilute acetic acid. He found that the same portion
of hide might be used again and again, without becoming exhausted,
which strongly supports the supposition that it is merely a product of
the partial decomposition of the hide fibre. The substance, which he
called "coriin," was purified by repeated solution in lime-water, and
reprecipitation by acetic acid. It was readily soluble by alkalies,
but insoluble in dilute acids, though in some cases it became so
swollen and finely divided as to appear almost as if dissolved. It was,
however, very soluble in common salt solution of about 10 per cent.,
though it was precipitated both by the addition of much water, and by
saturating the solution with salt. Reimer found that a 10 per cent.
salt solution was equally effective with lime-water in extracting it
from the hide, and that it was partially precipitated on the addition
of acid, and completely on saturating the acidified solution with
salt. Other salts of the alkalies and alkaline earths acted in a
similar manner, so that Reimer was at first deceived when experimenting
with baryta-water, because, being more concentrated than lime-water,
the coriin remained dissolved in the baryta salt formed on neutralising
with acid, and it was necessary to dilute before a precipitate could
be obtained. The slightly acid solution of coriin gave no precipitate
with potassium ferrocyanide, nor was it precipitated by boiling, being
thus distinguished from albuminoids. The neutral or alkaline solution
was not precipitated by iron or mercuric chloride, copper sulphate, nor
by neutral lead acetate; but was precipitated by basic lead acetate,
basic iron sulphate, and excess of tannin. Its elementary composition
is--carbon, 45·91: hydrogen, 6·57; nitrogen, 17·82; oxygen, 29·60; and
Reimer proposes the following equation as representing its relation to
hide fibre:--
Hide fibre. Water. Coriin.
C_{30}H_{46}N_{10}O_{12} + O + 2H_{2}O = C_{30}H_{50}N_{10}O_{15}.
[Footnote C: Dingler's Polyt. Journal, vol. 220, p. 167.]
_Hide Albumen._--The fresh hide, besides this coriin (which, very
possibly, is only evolved by the action of the lime), contains a
portion of actual albumen, viz. that of the blood serum and of the
lymph, which is not only contained in the abundant blood-vessels,
but saturates the fibrous connective tissue, of which it forms
the nourishment. This albumen is mostly removed by the liming and
working on the beam, which is preparatory to tanning. Probably for
sole-leather, the albumen itself would be rather advantageous if left
in the hide, as it combines with tannin, and would assist in giving
firmness and weight to the leather. It is, however, for reasons which
will be seen hereafter, absolutely necessary to get rid of any lime
which may be in combination with it. The blood also must be thoroughly
cleansed from the hide before tanning, as its colouring matter contains
iron, and, in combination with the tannin, would give a bad colour.
The reactions of blood and lymph albumen are very similar to those
of ordinary white of egg. It is precipitated by strong mineral acids,
especially nitric, and also by boiling. The precipitate produced by
strong hydrochloric acid redissolves by the aid of heat to a blue
or purple solution. Tribasic phosphoric, tartaric, acetic, and most
other organic acids, do not precipitate moderately dilute solutions
of albumen, but convert it into a sort of jelly, which, like gelatin,
does not coagulate, but liquefies on heating. It is precipitated by
neutral salts of the alkali metals. Blood-albumen slightly acidified
(with acetic acid) is precipitated by potassium ferrocyanide. It is
not precipitated by dilute infusions of oak bark, but is rendered
uncoagulable by heat, hence it cannot be employed to remove tannins
from their solutions.
_Elastic Fibres._--The elastic or yellow fibres of the hide are of
a very stable character. They are not completely dissolved even by
prolonged boiling, and acetic acid and hot solutions of caustic
alkalies scarcely attack them. Probably they do not combine with
tannin, and are very little changed in the tanning process.
_Hair, Epidermis, and Glands._--These are, as has been seen, all
derived from the epithelial layer, and hence, as might be inferred,
have much in common in their chemical constitution. They are all
classed by chemists under one name, "keratin," or horny tissue, and
their ultimate analysis shows that in elementary composition they
nearly agree. It is evident, however, that the horny tissues are rather
a class than a single compound.
The keratins are gradually loosened by prolonged soaking in water,
and, by continued boiling in a Papin's digester, are dissolved to an
extract which does not gelatinise on cooling. Keratin is dissolved by
caustic alkalies; the epidermis and the softer horny tissues are easily
attacked, while hair and horn require strong solutions and the aid of
heat to effect complete solution. The caustic alkaline earths act in
the same manner as dilute alkaline solutions; hence lime easily attacks
the epidermis, and loosens the hair, but does not readily destroy the
latter. Alkaline sulphides, on the other hand, seem to attack the
harder tissues with at least the same facility as the soft ones, the
hair being often completely disintegrated, while the epidermis is still
almost intact; hence their applicability to unhairing by destruction of
the hair. Keratins are dissolved by fuming hydrochloric acid, with the
production of a blue or violet coloration, like the albuminoids. They
also resemble albumen, in the fact that their solution in sulphuric
acid is precipitated by potassium ferrocyanide. By fusion with potash,
or prolonged boiling with dilute sulphuric acid, keratin is decomposed,
yielding leucin, tyrosin, ammonia, &c. The alkaline solution of keratin
(hair, horns, &c.) is precipitated by acids, and, mixed with oil and
baryta sulphate, is employed under Dr. Putz's patent as a filling
material for leather, for which purpose it acts in the same way as the
egg-yolks and meal used in kid-leather manufacture. Eitner attempted
to use it for the same purpose with bark-tanned leather, but without
much success. Putz has also proposed to precipitate the material after
working its solution into the pores of the leather.