50 VEGETABLE ACIDS TANNIN. 



Acids. Various acids are yielded by vegetables, chiefly from their fruit, but very 

 abundantly from the distillation of their -wood. Of the former are citric acid, from the 

 lemon Citrus, the acid juices of the apple (malic acid), pear, gooseberry, and other fruits 

 of our own climate, and the oxalic acid from the leaves of the sorrel, or Oxalis Acetosella. 

 All these acids appear to have distinct chemical characters, and to require distinct 

 names. 



Pyroligneous acid, or wood vinegar, is obtained from the distillation of almost all 

 kinds of wood, and is capable of perfect purification. It is colourless, abundant, and 

 cheap, and has therefore greatly lessened the demand for the coloured vinegar derived 

 from the fermentation of beer or wine, and more particularly in the preparation of such 

 pickles and other substances as would be deteriorated by immersion in coloured fluids. 

 The process is simple viz., the burning of billets of fast-growing wood, as poplar, in 

 closed iron tubes or kilns, and the separation of the empyreumatic oils, and other impure 

 substances, from the acid. This acid can be obtained in a highly-concentrated state, 

 a'nd is usually sold so that one part is equal in strength to eight of wine vinegar. It is 

 thus a convenient as well as necessary article for the use of persons on ship-board, or 

 for residents in new countries, where vinegar has not hitherto been made. 



Gallic acid is obtained from gall-nuts, and tannic acid from all sources supplying 

 tannin. 



Tannin. This is the chemical principle which is employed in the tanning of 

 leather, and produces its effect by acting upon the gelatine contained in the skin. It 

 is obtained from a great variety of sources, and not only from the oak bark, as is usually 

 supposed ; although it is probable that the excellence of good oak bark, and the ready 

 supply of it aiforded by our own country, will ever give it a preference in the estima- 

 tion of the manufacturer. Notwithstanding the supply of oak bark from our own forests, 

 so large a quantity as 1,200,000 cwts. of tanning materials were imported in 1849 ; but 

 it must be understood that the tanning principle forms but a small portion of the 

 barks and other materials thus imported. The following are the commercial substances 

 which contain tannin in quantity sufficiently large to render them efficient in the 

 tanning of leather : 



Oak bark, from various species of Quercus, but particularly the Quereus pedunculate* , 

 growing in England and the north of Europe. 



Cork-tree bark, from the Quereus Stiber, imported from Laruche and Rabat. 



Valonia, from another oak, the Quereus JEgilops, flourishing in the Morea, and the 

 south of Europe, and Asia. No less than 333,420 cwts. of this substance was imported 

 in 1849. 



Oak-galls, from the Qncrcus infcetoria of India and Turkey. 



Terra Japonica, Kutch, and Catechu, extracts from the Acacia Catechu, growing in the 

 East Indies. These substances contain a very large quantity of tannin. 



Sumach, in powder and in leaves, from Sicily and the south of Europe. It is the 

 product of the Rhus Coriaria. 



Besides the above principal sources may be mentioned Kino, the extract of the 

 Buchanania latifolia, of India; Divi-divi, of the Ccesalpinia coriaria, from South 

 America ; mimosa bark, and bark of the black wattle tree, Acacia mollisima ; hemlock 

 barf:, from the fir, Abies Canadensis, of the United States of America ; the bark of several 

 trees growing in New Zealand ; and the larch bark, Pinus larix, of Scotland. 



Opium. This highly important medicinal substance is procured from the Con- 

 tinent of India, and chiefly from the provinces of Behar, Benares, and other parts of 



