Dinaphthylmethanedisulphonic acid Ppte " Tanning Diphenylmethanedisulphonic acid " " " Dicresylmethanedisulphonic acid " " " Sodium dicresylmethanedisulphonate acid No ppte " Not tanning Dixylylmethanedisulphonic acid Ppte " Tanning Naphtholdisulphonic acid " Not tanning Not tanning Methylenedinaphthol " Tanning Tanning Hydroxyphenylmethanesulphonic " " " acid Not tanning Hydroxynaphthylmethanesulphonic Slight ppte " " acid Diaminonaphthylmethanedisulphonic Ppte Tanning Not tanning acid Dihydroxynaphthylmethanedisulphonic acid " " " Dichloronaphthylmethanedisulphonic acid " " Surface tanning Dinitronaphthylmethanedisulphonic acid " " " Dithionaphthylmethanedisulphonic acid " " Tanning Bromo _[Greek: b]_ naphthol [1] Slight ppte " " Rosolic acid_ [1] Ppte " " [Footnote 1: In alcoholic solution.] SECTION III TANNING EFFECTS OF MIXTURES AND NATURAL PRODUCTS 1. Mixture of Phenolsulphonic Acid and Formaldehyde The most important invention relatively to the search for new tanning materials was that of Weinschenk,[Footnote: Ger. Pat., 184,449.] who first showed that pelt may be converted into leather by the action upon it of mixtures of naphthols and formaldehyde. This process consists of two steps: the pelt is first immersed in a 0.25-0.50 per cent, formaldehyde solution, and secondly in an aqueous solution of -[Greek: a] or -[Greek: b] naphthol; this order may be reversed. If, on the other hand, a pasty mixture is made of formaldehyde and naphthol, and this is allowed to act upon the pelt, the latter is rapidly converted into leather, but the mixture must be administered very gradually or otherwise the insoluble methylenedinaphthol is formed outside the pelt and hinders any tanning effect. Leather obtained through the action of [Greek: a]-naphthol is, when freshly tanned, pure white and sufficiently soft and firm, but quickly assumes a brown colour on storing; if, however, [Greek: b]-naphthol is employed, a cream-coloured leather results, the colour of which turns only slightly more yellowish even when exposed to the direct rays of the sun.