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 of tannin can be detected; and the yield (of leather, based on the pelt employed), which, from a practical standpoint, is so important, is so very low that it is hardly possible to speak of it as a tannin in the ordinary sense of the word. Formaldehyde must, therefore, be termed a pseudo-tannin. The tanning effect of formaldehyde is, according to Thuau, [Footnote: _Collegium_, 1909, 363, 211.] increased by those salts which bring about colloidal polymerisation of the formaldehyde, the resultant compounds being absorbed by the hide fibre. Fahrion considers this to be a true tannage, and is supported by Nierenstein [Footnote: _Ibid._, 1905, 157, 159.]:-- R.NH_2 R.NH-| +O.C.H. = CH_2 + H_2O R.NH_2 | R.NH-| (Hide.) H (Leather.)