VEGETABLE SUBSTANCES. 131 



Woody fibre. When a piece of wood has been 

 boiled in water and in alkohol, until the soluble sub- 

 stances have been extracted from it, what remains 

 insoluble is the woody fibre, or lignin, which is the 

 basis of wood, and consists of long fibres, having 

 a considerable degree of transparency, without 

 taste, and unalterable by the air. It is insoluble in 

 water and alkohol. It is very inflammable ; and, 

 when distilled in a close vessel, yields an acid sub- 

 stance formerly thought to be a distinct acid called 

 the pyro-ligneous, but now known to be the acetic 

 acid with an empyreumatic oil. Pure acetic acid or 

 vinegar is now made from wood by distillation. 

 Wood consists of oxygen, carbon, and hydrogen ; 

 when burned, the carbon remains, constituting 

 charcoal. 



Colouring matter. The colours of vegetables 

 are owing to peculiar matters, which are extremely 

 numerous, and but little known. Many of them 

 are used as dyes and pigments. The extraction of 

 colouring matters from vegetables, and fixing them 

 on clo,ths, constitute the arts of dyeing and calico- 

 printing (which see). The colouring matters 

 sometimes are inherent in gums, sometimes in 

 resins, sometimes in fecula ; consequently they 

 require different chemical agents for their solution. 



Tannin. This principle is so called because it 

 is employed in the art of tanning leather. It is 

 also called the astringent principle. It is found 

 abundantly in the barks of several trees, particu- 

 larly the oak, and also in certain seeds. The gall- 

 nut and grape-seeds afford very pure tannin ; and 

 a substance called catechu, from India, consists 

 chiefly of it. Tannin is distinguished by its 

 forming a precipitate with glue, or isinglass. This 

 precipitate is insoluble in water, and is that which 



K g 



