EXTRACT. 83 



rated, presents the tannin principle in a very pure 

 state. It is yellowish, very astringent, brittle, and 

 abundantly soluble either in alcohol or water. Its 

 combinations with iron are black, and by its union with 

 gelatine, it forms an insoluble compound. On account 

 of its astringency, it is of very great utility in medicine, 

 and the virtues of the Peruvian bark, Cinchona Officin- 

 alis, so celebrated as a febrifuge and tonic, are supposed 

 to reside chiefly in this principle. Its combinations 

 with iron form the basis of writing ink, and by its union 

 with gelatine, leather is formed, and hence we perceive 

 the utility of those barks which the tanner employs ; 

 they being in all cases strongly impregnated with the 

 tannin principle. In this country the bark of the 

 Hemlock, Pinus Canadensis, is preferred to every other, 

 but various species of Willow and Oak are scarcely less 

 valuable, in the manufacture of leather. 



EXTRACT.— That portion of the vegetable which 

 is soluble in water, but becomes insoluble when expos- 

 ed to the atmosphere, is denominated extract. It is 

 a very common product of vegetable analysis, and it 

 appears that its properties are not well defined, for the 

 extracts which are obtained from different plants, do 

 not appear to be precisely the same. Some of them 

 are employed in medicine, others in the arts. They 

 supply us with most of the colours used in dyeing, and 

 having a powerful affinity to the fibres of linen and cot- 

 ton, they communicate an almost indelible tinge to the 

 various textures with which they unite. The most 

 frequent of these colours are blue, red, yellow and 

 brown . 



BLUE. — Indigo presents a beautiful example. It is 

 obtained from the leaves of the Indigo plant, Indigofera 

 tintioria, which is cultivated extensively in India and 



