These experiments prove that when pelt is treated with formaldehyde, phenolsulphonic acid, and vegetable tannins, the two former components effect, more or less, actual tannage; it is admittedly a matter of some difficulty to establish whether the effect is one of pickling or pseudo-tannage, or whether the tannage may be considered a true one. The final effect, however, is nearly always that of a true tannage, _i.e_., by varying the composition of the tanning solutions leather is obtained with properties identical with those tanned with true tannins of vegetable origin. The only difficulty encountered in these combinations is the property of formaldehyde, of precipitating the natural tannins, and it is hence essential, for practical purposes, to so arrange the combination that their value is not reduced by the property referred to. The fact that not only compounds already existing may convert pelt into leather, but that a similar effect is obtained _inside the pelt_, by their components, is indeed of theoretical interest. 3. Tanning Effects of Different Natural Substances In addition to the vegetable tannins, Nature has also provided other substances of vegetable origin, which, admittedly, do not effect tannage in their original state, but which may, by suitable treatment, acquire this property. The oldest information on this point is supplied by Resch, [Footnote: _Scherer's Jour_., 1801, 6, 495.] who carried out tanning experiments, using three parts of peat and one part of oak bark. By the action of nitric acid on substances of vegetable and animal origin, Hatchett, [Footnote: _Gehlen's Jour_., 1805, 1, 545.] Chevreul, [Footnote: _Ann. Chim_., 1810, 73, 36.] and Vogel [Footnote: _Jour. Chem. Phys_., 1812, 6, 101.] claim to have obtained tanning materials, whilst later, Buff [Footnote: _Ibid_., 1827, 51, 38.] obtained a material suitable for tanning purposes from indigo. By subsequent treatment with lime and soot, or tar, Ashmore [Footnote: _Dingier's Jour_., 1833, 48, 67.] claims to have converted pelt into leather. By treating peat with nitric acid, Jennings [Footnote: _Jahresber. d. Chem_., 1858, 666.] and Payne [Footnote: _ Chem. Centralbl_., 1908, ii. 554; Ger. Pat., 200, 539.] have produced artificial tanning materials. Skey [Footnote: _Chem. News_, 1866, 206; _Zeits. f. Chem_., 1866, 753.] obtained a dark brown extract, soluble in water and precipitating gelatine, by treating bituminous coal or lignite with nitric acid; by extracting coal with alkalies, Reinsch [Footnote: _Pharm. Centralh_., 1887, 141.] isolated a substance (pyrofuscine) which, when partly neutralised with carbon dioxide, was capable of converting pelt into leather.