Vegetable Substances. 279 



and is a very important part of vegetables, if 

 we consider its uses in the arts. Besides the 

 tan contained in oak bark, there are at least 

 six other species, contained in 1st, Nut-galls; 

 2d, Catechu, which is obtained from a species 

 of mimosa; 3d, Dragon's blood ; 5th, Sumach; 

 5th, Fustic; 6th, Kino, an exudation from the 

 brown gum tree of Botany-Bay. They all unite 

 in the principle of astringency, and in striking a 

 black colour with salts of iron. Common writ- 

 ing ink is a composition of gallat of iron and 

 tannat of iron. 



Pare tan, from nut-galls, is a brittle substance 

 of a brown colour, and breaks with a vitreous 

 fracture. The most important property of tan 

 is its forming an insoluble compound with glue, 

 or gelatine as it is called by chemists. The skin 

 of animals consists almost entirely of gelatine, 

 more or less hard and compact, according to the 

 age and nature of the animal. With this the 

 tan unites in the process of making leather ; so 

 that the skin is entirely altered in its nature by 

 the process, and is formed into a third substance 

 by the combination of gelatine and tan. This 

 is a tedious process, the stouter hides requiring 

 from eleven to eighteen months, and sometimes 

 two years, before they are completely tanned. 



6. The bitter principle is common to many 

 plants, such as the quassia, the hop, &c. It has 

 not yet been chemically examined, but nitrat of 



