from the point of view of chemical constitution have nothing in common
with the natural tannins. Not only are they of interest to the industry
from a practical point of view; they have also been examined very
closely from a chemical standpoint.

It is, however, necessary to differentiate with great exactitude between
the conception of _true tanning effect_ and _pickling effect_ when
considering the action of chemical substances on pelt (i.e., animal
hide, treated with lime, depilated, and the surplus flesh
removed). Whereas any _true tannage_ is characterised by the complete
penetration of the substance and its subsequent fixation by the pelt in
such a way that a thorough soaking and washing will not bring about a
reconversion (of the leather) to the pelt state; _pickling_, on the
other hand, is only characterised by the penetration of the substance in
the pelt and fixation to such an extent that a subsequent washing of the
pickled pelt will bring back the latter to a state closely approximating
that of a true pelt. Simple as such a differentiation appears, there
are still a number of cases occupying a position between the two
referred to, and which we may term _pseudo-tannage_. An example of the
latter is formaldehyde tannage; formaldehyde has for a long time been
employed in histological work for the purpose of hardening animal hide,
by which it is readily absorbed from solution whereby it hardens the
hide without, however, swelling it. A hide which has thus been treated
with formaldehyde absorbs the natural tannins with greater ease; this,
on the one hand, argues the probability of formaldehyde acting as a
pickling agent; on the other hand, it is also one of its characteristics
that it will either in neutral acid, [Footnote: R. Combret, Ger. Pat,
112, 183.] or, still better, in alkaline [Footnote: J. Pullman,
Ger. Pat, 111,408; Griffith, _Lea. Tr. Rev._, 1908.] solution, convert
pelt into leather. In a formaldehyde-tanned leather, however, no trace