A peculiar combination between true tannage and pickling is to be found in the tawing process (tannage with potash, alum, and salt), whereby, firstly, the salt and the acid character of the alum produce a pickling effect, and secondly, the alum at the same time is hydrolysed, and its dissociation components partly adsorbed by the hide, thereby effecting true tannage. This double effect is still more pronounced in the synthetic tannins which contain colloidal bodies of pronounced tanning intensity on the one hand, inorganic and organic salts on the other, which then act as described above. Their real mode of action can only be explained with the aid of experimental data. The following chapters will deal with the different behaviour of the various groups of synthetic tannins. PART I SECTION I THE SYNTHESIS OF VEGETABLE TANNINS 1. TANNIN The first investigations of gall-tannin date from the year 1770, at which time, however, no exact differentiation between tannin and gallic acid was made. The first step in this direction was made when Scheele,[Footnote: Grell's _Chem. Ann._, 1787, 3, I.] in 1787, discovered gallic acid in fermented gall extract, and in the same year Kunzemuller [Footnote:_Ibid._, 1787,3,413.] separated gallic acid (or pyrogallol) as a crystalline body from oak galls. Dize [Footnote: _Jour. Chim. et Phys._, 1791, 399.] continued the investigations, which were brought to a conclusion with Deyeux' work [Footnote: _Ann. Chim._, 1793, 17, I.]; both recognised that the substance isolated was not a single substance, but was a mixture of gallic acid, a green colouring matter, a rosin (tannin?), and extraneous matter. Proust [Footnote: _Ibid._, 1799, 25, 225.] was the first to differentiate the crystalline gallic acid from the amorphous, astringent substance, which latter he named "Tannin." Amongst the numerous subsequent investigations of tannin must be especially noted the one by Berzelius [Footnote: Pogg,_Ann._, 1827, 10, 257.], who purified the potash salt and decomposed this with sulphuric acid. Pelouze [Footnote: Liebig's _Ann._, 1843, 47, 358.], later on, observed the formation of the crystalline gallic acid from tannin, when the latter is boiled with sulphuric acid; this had already been observed by J. Liebig.[Footnote: _Ibid._1843, 39, 100.] Both had noticed the absence of nitrogen. In addition to the methods of preparation of tannin then in vogue neutral solvents were mainly employed by subsequent investigators; Pelouze [Footnote: _Jour. Prakt. Chem._, 1834, 2, 301, and 328.] treated powdered galls with ether containing alcohol and water, and considered the upper layer to be a solution of gallic acid and impurities, the bottom layer to contain the pure tannin.