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of their properties probably in many cases depends 

 upon their being combined with small quantities of 

 other vegetable principles, or to their containing differ- 

 ent saline, alkaline, acid, or earthy ingredients. Many 

 dyeing substances seem to be of the nature of extrac- 

 tive principle, such as the red colouring matter of 

 madder, and the yellow dye, procured from weld. 



Extract has a strong attraction for the fibres of 

 cotton or linen, and combines with these substances 

 when they are boiled in a solution of it. The com- 

 bination is made stronger by the intervention of mor- 

 dants, which are earthy or metallic combinations that 

 unite to the cloth, and enable the colouring matter to 

 adhere more strongly to its fibres. 



Extract, in its pure form, cannot be used as an 

 article of food, but it is probably nutritive when united 

 to starch, mucilage, or sugar. 



8. Tannin, or the tanning principle, may be pro- 

 cured by the action of a small quantity of cold water 

 on bruised grape-seeds, or pounded gall-nut ; and by 

 the evaporation of the solution to dryness. It appears 

 as a yellow substance, possessed of a highly astrin- 

 gent taste. It is difficult of combustion. It is very 

 soluble both in water and alcohol, but insoluble in 

 ether. When a solution of glue, or isinglass (gelatine} 

 is mixed with an aqueous solution of tannin, the two 

 substances, i. e. the animal and vegetable matters fall 

 down in combination, and form an insoluble precipi- 

 tate. 



When tannin is distilled in close vessels, the prin- 

 cipal products are charcoal, carbonic acid ? and inflanv 



