VEGETABLE ALKALIES. 133 



18 immediately produced. By macerating a piece of raw hide in a 

 solution of tannin, the same combination takes place even into the 

 very interior of the tissue ; the whole of the tannin quits the solu- 

 tion by degrees to combine with the gelatine of the skin. 



It is not in the bark only that tannin is encountered, it has been 

 found in different organs of plants. Sir Humphrey Davy has stated 

 these quantities of tannin as constituents of 100 parts of the follow- 

 ing substances : 



Nutgalls ... . . 27.4 



Oak bark 6.3 



Chestnut bark .... 4.3 



Elm bark 2.7 



Willow bark 2.2 



Inner white bark of an aged oak . 15.0 

 The same of young oaks , . 16.0 



The same of the Indian chestnut . 15.2 

 The inner colored bark of the oak . 4.0 

 Sicilian sumac . . 16.2 



Malaga sumac . . . 10.4 



Souchong tea . . . 10.0 



Green tea .... 8.5 



Bombay catechu . . . . 54.3 

 Bengal catechu . . . .48.1 



Gallic acid. This acid is found united with tannin in the greater 

 number of barks, or along with the astringent principles of plants. 

 Gallic acid appears to be the product of a kind of fermentation un- 

 dergone by tannin, as the process by which it is prepared seems to 

 indicate, and which consists essentially in exposing for about a 

 month a quantity of nutgalls reduced to powder and kept constantly 

 moistened. The solution of gallic acid does not precipitate gela- 

 tine. 



I have added in a table the composition of the principal vegetable 

 acids. I shall speak of the composition of fat acids when I come to 

 treat of fatty substances. 



The different vegetable acids do not vary essentially in composi- 

 tion, save in a single instance. With one exception they consist of 

 definite proportions of carbon, hydrogen, and oxygen. The excep- 

 tion alluded to is the hydrocyanic acid, which contains no oxygen, 

 but a large quantity, nearly 52 per cent., of azote. 



OF THE VEGETABLE ALKALIES. 



The alkaline bases which are formed in the course of vegetation, 

 always contain a certain proportion of azote. Their general prop- 

 erties are those of alkalies ; their watery or alcoholic solutions re- 

 store the blue color of the reddened tincture of turnsole, and they 

 constitute salts by combining with acids. In their manner of be- 

 having they bear a certain analogy to ammonia. Like ammonia, 

 the organic alkalies combine with the hydrates of the oxacids, and 

 when they are deprived of their water of crystallization, they fix the 

 hydracids without losing weight. 



12 



