In addition to these tanning materials the recovery of a substance possessing tanning properties from the so-called acid rosins has been made the subject of a patent; [Footnote: _Ger. Pat_., 36,019.] this rosin is formed when crude oil is treated with concentrated sulphuric acid in the oil refineries. The greasy substance is partly neutralised with alkali and is claimed to produce a very springy leather. The waste liquors obtained in the manufacture of cellulose, the so-called sulphite and sodium cellulose waste, have, however, been the subject of numerous investigations, and several hundred publications have appeared and a great number of patents [Footnote: "Literatur überiSulfitablauge" 1910-13. (Reprint from _WocheWochenblPapiePapierfabrikation_)] taken out, the first one being that of Mitscherlich [Footnote: _Jahresber. d. Chem_., 1893, 890; Ger. Pat., 72,161.] and Hönig [Footnote: _Chem. Centralbl_., 1902, ii. 174; Ger. Pat., 132,224.] The waste liquors contain large quantities of acids and lime, and in order to utilise the liquors for tanning purposes, the excessive sulphuric and sulphurous acids as well as the lime must be removed. The active tannin is no doubt the ligninsulphonic acid, and those cellulose extracts containing the largest amounts of free ligninsulphonic acid may also be considered the most efficient. According to the author,[Footnote: _Technikum_, 1912, 20, 156.] such sulphitecellulose extracts precipitate gelatine, aniline hydrochloride, ammoniacal zinc acetate, and basic coal-tar dyes, and give a greenish-black coloration with ferric chloride. These reactions indicate the presence of tanning matters in cellulose extracts. The official shake method of analysis gives the following results:--[Footnote: _Ibid_.] Tanning matters 23.0 per cent. Non-tannins 30.3 " Insoluble matters 0.7 " Water 46.0 " --------------- 100.0 per cent. Ash 4.3 " Sulphurous acid 0.6 " Many other substances have been used for tanning experiments, a number of them precipitating gelatine. Zacharias [Footnote: _Zeits. f. Ang. Chem_., 1907, 1645.] obtained leather by the action of many coal-tar dyes on pelt, similarly Herzog and Adler, by using Prussian blue, Neufuchsin, patent blue V, crystal violet, and colloidal gold. Most inorganic substances possess tanning properties when in the colloidal state, _e.g_., sulphur, halogens, chromium salts, iron salts, silver oxide, and the salts of mercury, copper, bismuth, zinc, lead, platinum, cesium, vanadium, and the rare earths (salts of cerium, lanthanum, didymium, neodymium, thorium, and zerconium). For practical purposes, however, only sulphur, chrome, and alum salts are used, the latter two being of the greatest importance. SECTION IV METHODS OF EXAMINING TANNING MATTERS