b-Naphthol, dissolved in hot concentrated sulphuric acid and heated for some time, yields the light brown, viscous b-naphtholsulphonic acid. A dilute solution of the latter, mixed with formaldehyde, remains clear; when heated on the water bath, however, it assumes a dark, reddish-yellow colour, and remains soluble in water and precipitates gelatine strongly. This condensation product, on adding excess of caustic soda, assumes a deep blue coloration, the alkaline solution giving no precipitate with gelatine; on adding acetic acid the solution turns brown, remains clear, and now precipitates gelatine. The concentrated b-naphtholsulphonic acid heated with formaldehyde on the water bath yields as condensation product a dark, reddish-yellow mass, soluble in water, which precipitates gelatine. A dilute solution, when allowed to act upon pelt, gave in a few days a light brown leather, the properties of which are very similar to those possessed by vegetable tanned leathers. The use of naphtholsulphonic and aminonaphtholsulphonic acids for the manufacture of synthetic tannins is protected by Ger. Pats., 293,640, 293,693, 293,042, and 303,640. [Footnote: _Cf._ Austr. Pat., 70,162.] It is a remarkable fact that non-condensed methane derivatives of naphthol, _e.g._, b-naphthol-a-methanesulphonic acid, dissolved in water and partly neutralised, are devoid of tanning character when allowed to act upon pelt. Neither does this substance precipitate gelatine, but it does give a deep blue coloration with ferric chloride. The condensation product of b-naphthol above referred to precipitates gelatine and aniline hydrochloride and gives a brown coloration with ferric chloride. Thionaphtholsulphonic acid, when acted upon by formaldehyde, yields a condensation product of the following constitution:-- HSO_4 ^ ^ SH SH ^ ^ HSO_4 | | | | | | | | |_____CH_2_____| | | v v v v This is a light yellow powder which, dissolved in water, yields an opaque solution; the latter only exhibits any tanning properties when it is not neutralised and even slightly acidified and then precipitates gelatine, aniline hydrochloride and barium chloride; dissolved in alkali, it forms a clear, yellow solution devoid of tannoid properties. Leather tanned with the acidified solution is very similar to those tanned with the phenolsulphonic acid condensation products; its colour, however, is more pronouncedly yellow.