81 
There appears to be almost as many varieties of 
extract as there are species of plants. The differ- 
ence of their properties probably in many cases 
depends upon their being combined with small 
quantities of other vegetable principles, or to their 
containing different saline, alkaline, acid, or earthy 
ingredients. Many dyeing substances seem to be 
of the nature of extractive 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 sub- 
stances when they are boiled in a solution of it. 
The combination is made stronger by the interven- 
tion of mordants, 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- 
nuts ; and by the evaporation of the solution to 
dryness. It appears as a yellow substance, pos- 
sessed of a highly astringent taste. It is difficult 
of combustion. It is very soluble, both in water 
and alcohol, but insoluble in ether. When a solu- 
tion of glue, or isinglass Q gelatine ), is mixed with 
an aqueous solution of tannin, the two substances, 
2 . e . the animal and vegetable matters, fall down in 
combination, and form an insoluble precipitate. 
G 
