Whereas the evaluation of vegetable tanning matters necessitates determinations of their practical applicability in addition to qualitative and quantitative analyses, the latter two determinations are of practically no value when dealing with synthetic tannins. The way in which tanning matters obtained by chemical means exert their action, in addition to the intensity with which they convert pelt into leather, is the only criterion of their quality for practical (tanning) purposes; both may be demonstrated by experimental tests. When dealing with the natural tanning materials it is desirable to know their contents of actual tanning matter, from which their special qualities as tanning agents may be deduced. Where the vegetable tanning materials have already been converted into extracts, it is essential to establish the identity of the original material used by the qualitative reactions of the extract in addition to the quantitative estimation of actual tannin contents. It is frequently necessary to examine whether the extract in question has been actually prepared from the material giving the extract its name, or whether the extract has suffered the addition of other extracts of tanning materials of but low quality. Such determinations may be undertaken by microscopical observations and by means of qualitative and quantitative reactions; for this purpose many colour reactions and precipitation methods are available in addition to the determination of the molybdenum figure (Lauffmann),[Footnote: Collegium, 1913, 10.] the alcohol and ethyl acetate figures and microscopical examination (Grasser).[Footnote: Ibid., 1911, 349.] Of other adulterants tending to reduce the quality of extracts may be mentioned sugars, mineral salts, and coal-tar dyes; [Footnote: Grasser, _Collegium_, 1910, 379.] for the determination of these, the special literature should be consulted. [Footnote: Grasser, "Handbuch f. gerbereichem. Laboratorien" (Leipzig, 1914); Procter-Paessler, "Gerbereichem. Untersuchungen" (Berlin, 1901).] Two methods are devised for the purpose of quantitatively determining the tannin contents, both of which employ hide powder, and which are known as the "shake method" and the "filter bell method" respectively: the former is adopted as the official method of the "International Association of Leather Trades' Chemists" (I.A.L.T.C.). [Footnote: And also by the Society of Leather Trades' Chemists.-_Transl._]