This supposition was adopted by Stiasny [Footnote: _Gerber_, 1905, 233.] and Kauschke [Footnote: _Collegium_, 1906, 362.] and the latter points out that these easily soluable substances exhibit tanning properties. Nierenstein [Footnote: _Ibid_., 1906 424.] was further able to show that by all processes of condensation between phenols (or hydroxybenzoic acids) and formaldehyde, compounds of the character of hydroxyaurine (or hydroxyaurinecarboxylic acid) were formed in addition to the insoluble hydroxydiphenylmethanes (or hydroxydiphenylmethanecarboxylic acids), the former possessing the characteristic tannophor group and hence precipitating gelatine, _i.e._, exerting tanning action. If the formation of leather is viewed in the light of Schiff's base, [Footnote: _Ibid_., 1905, 159.] one may consider the constitution of a hexahydroxyaurinecarboxylic acid leather as follows:-- _C_6H_2(OH)_3.COOH C{-C_6H_2(OH)_3.COOH |_}C_6H_2(OH)_2.COOH R-N In the preparation of these and similar condensation products, Nierenstein and Webster [Footnote: _Ber_., 1908, 41, 80.] observed a peculiar steric effect of the carboxyl group. Each 2.5 gm. of the phenol or the acid in question were dissolved in 30 c.c. of water, the solution brought to boil and 5 c.c. formaldehyde (20 per cent.) and 2.5 c.c. hydrochloric acid added drop by drop; the precipitate formed was filtered off after twenty-four hours, dried at 110° C. to constant weight, extracted (in a Gooch crucible) freely with water, and the residue again dried at 110° C. till constant. The following values were obtained:-- Total Insol. Aq. Sol. Aq. Oxy- Precipitate Diphenylmethane aurinecarboxylic in Grammes. Derivatives Acid.