TANNIN 163 



of the collection of bark in the British Isles will be closely 

 connected with the utilization of waste wood in forest 

 problems. If it can be made profitable to bark the trees, 

 and dispose of the bark for tannin, the waste wood can be 

 distilled for the production of a much better quality charcoal, 

 and in practice, therefore, the two subjects are closely 

 connected. Calcareous soils probably produce more tannin 

 than others, and since, in the British Isles, it is only the 

 poorest land that can be left down to timber, this condition 

 does not often prevail. The proportion of tannin appears 

 to be greatest in bark removed about April or May. Charac- 

 teristics of the tannins are that they reduce Fehling solution, 

 are precipitated by basic lead acetate, give a blue-black 

 colour with ferric chloride, and are precipitated with many 

 bases. Phlobaphenes are the decomposed products of 

 the tannins proper, and are nearly always contained in 

 extracts of bark. They are red-coloured substances, and 

 though almost insoluble in water, they dissolve in solutions 

 of tannins. Whilst a great many of the common tannins 

 contain the glucose grouping, such is by no means invariably 

 the case. Gall nuts are very rich in gallo-tannic acid, and 

 may contain as much as 50 per cent. Ordinary tannin, 

 or gallo-tannic acid, is probably a compound containing 

 five molecules of di-gallic acid, with one molecule of glucose. 

 Catechin, whilst not properly tannin itself, is easily converted 

 into catechu tannin, a change which takes place readily 

 on heating to 120°, or slowly by merely boiling wdth 

 water. The common extracts from the acacia or mimosa 

 are usually mixtures of catechin and catechu tannin. The 

 catechin itself is used medicinally in India, or as a chewing 

 material. Tannin is abundant in the leaves, in all active 

 growing parts and in diseased parts, like galls. Any irritation 

 to the protoplasm appears to increase the amount of tannin. 

 Tannin is very common in all unripe fruits, but disappears 

 as the fruit becomes ripe. 



Rubber. — Rubber, or India rubber, is the material which 

 exudes as the result of an injury to many particular trees. 

 Rubber is generally derived by a process of coagulation from 



