VEGETABLE TANNAGE 29 



COOH, which possesses properties very similar to the 

 tannins, but does not precipitate gelatin and will not itself 

 make leather. Another of these substances is ellagic acid, 

 C^HgOg, a double lactone of a hexa-hydroxy-diphenyldi- 

 car boxy lie acid. This is deposited as an insoluble yellow 

 powder from infusions of many pyrogallol tans, by boiling 

 with dilute acids only, allowing them to stand for a few 

 days. In practice the deposit is found as mud at the bottom 

 of the tan pits, and also upon the leather, to which it strongly 

 adheres. It is technically known as " bloom." It is 

 insoluble in acids and cold alcohol, but soluble in alkalies. 

 It is a feeble dye-stuff. The pyrogallol tans yield very 

 different amounts of bloom. Other associated substances 

 are the sugars. In practice these sugars ferment to lactic, 

 acetic, and other acids which cause " sour " liquors. Such 

 liquors plump the hides and tend to give firm, thick leather. 

 These acids also probably cause increase of adsorption 

 of tannin by the hide and therefore assist in giving " good 

 weight." Solutions of pyrogallol tans all give a blue-black 

 colour with a dilute solution of ferric alum. If a solution 

 of sodium arsenate be added to an infusion of pyrogallol 

 tan diluted until no longer distinctly coloured, and the 

 mixture allowed to stand for about two hours, a green 

 colour develops at the surface of the liquid. The reaction 

 is due to gallic acid or a similar grouping, and is, in the 

 author's experience, the most satisfactory qualitative test 

 for the group. Another test is to mix equal volumes of a 

 0*4 per cent, infusion of tan and a 10 per cent, solution of 

 sodium bisulphite ; a few drops of 10 per cent, potassium 

 chromate aie added, and either a transient blood-red colour 

 or a more permanent deep purple is obtained. The former 

 ir is due to gallic acid. If a tannin infusion be largely 

 diluted with hard water and a little iodine solution added, 

 tin- pyrogallol tans yield either a purple-red or a dark 

 hliR- colour, the former being a reaction of gallic acid. 

 Pyrogallol tans yield no precipitate with bromine water. 

 They yield a yellow or brown colour when one drop of 

 infusion is added to concentrated sulphuric acid 



