en of will appear
more or less thick, until the tanning is completed; it has been
generally supposed, that the tan in the tanpits had no other effect
upon the leather than that of hardening and bracing the fibres of the
skin, which has been relaxed by the preliminary of tanning. Mr. S----,
however, examined the operation more closely, and discovered that there
existed in the tan a principle which was soluble in water, by which the
tanning was brought about. That this principle afterwards became fixed
in the leather in consequence of a particular combination between the
said principle and the skin; and this combination produced a substance
that was not soluble in water; all this has been demonstrated by Mr.
S----, in the most evident manner. It is well known that if leather,
which has not been tanned, is boiled in water, it is in a short time
almost entirely dissolved therein. This solution, by being
concentrated, produces a jelly, or size, which, by farther evaporation,
and being dried in the air, becomes what is called glue. Mr. S----
having, in the course of his experiments, examined the effects of a
solution of tan upon a solution of glue, observed that they were hardly
mixed together before a white felamentous precipitate took place, owing
to a combination of the glue with the tanning principle contained in
the solution of tan. This precipitate is insoluble in water, either hot
or cold, and acquires colour by being exposed to the light. The
foregoing experiment furnishes a true explanation of the process of
tanning; for it will easily be conceived that the solution of tan acts
upon the hides (from which glue is produced) in the same manner as it
acts upon glue; this is what really happens in common tanpits, and Mr.
S----'s new method, in which the solution of tan gradually penetrates
the hides, and as it penetrates combines with it, producing a gradual
change of colour that is very observable, till at last the colour of
the hide is changed throughout, and it acquires a compact texture and
marbled appearance, like that of a nutmeg: by this it plainly appears,
that a precipitation also takes place in the action of tanning,
although the hide is not dissolved, but merely swelled so as to enable
the solution to penetrate it more easily. The property which animal
jelly, or glue, possesses, of being precipitated by a solution of the
tanning principle, furnishes a means of discovering what substances may
be useful in tanning