64 SCIENTIFIC AGRICULTURE. 



EXTRACTIVE MATTER. 



Extractive matter (apotheme) exists in vegetables, and may 

 be obtained by steeping them in hot water, and then evapo- 

 rating the water, when a brown powder will remain, which is 

 but slightly soluble in water or alcohol, but soluble in alkalies. 

 Its nature is not well understood ; Dr. Kane, however, supposes 

 it may be identical with ulmic or hamic acid. It is not nutri- 

 cious. 



TANNIN. 



Tannin exists in the bark of most trees, but most abun- 

 dantly in the bark of the oak, horse chestnut and hemlock. It 

 is an astringent brownish powder, soluble in alcohol and water. 

 It has an astringent taste and is destitute of odor : it combines 

 with animal gelatine and forms an insoluble precipitate ; hence 

 by soaking the skins of animals in a solution containing tannin, 

 it is converted into leather, which is no longer subject to 

 putrefaction. It is composed of carbon, hydrogen and oxygen : 

 it is not nutricious : it precipitates most metallic solutions, and 

 is hence used in practical chemistry as a re-agent. 



COLORING MATTER. 



The matter which constitutes the basis of vegetable colors 

 is found in most plants. "The organic coloring principles, 

 [says Dr. Fownes,] with the exception of one red dye, cochi- 

 neal, are all of vegetable origin." The art of coloring is based 

 upon the affinity which exists between the coloring matter and 

 the fibres of the different fabrics to be colored. This is 

 stronger in woolen than in cotton and linen ; hence in dyeing 

 the two latter a third substance, called a, mordant, is used, 

 which strengthens their affinity: for this purpose, salts of 

 alumina, iron and tin are used. 



The coloring principle of vegetable blues is indigo: that of 

 madder red is alizarine: of madder yellow, xanthine: the 

 green color of plants depends upon a substance called chloro- 



