VEGETABLE SUBSTANCES. 19 



the tissues, the fibres of flax, cotton, hemp, etc. are nearly pure lignin. To this substance 

 the coloring matter adheres when employed with a mordant, as the acetate of alumina- 

 It is in consequence of this affinity that colors are fixed, and continue unchanged for a 

 long period. 



Gum. Gum arabic is one of the most important gums. It is soluble in water ; is in 

 brittle transparent pieces, but never exhibits a tendency to crystallization. Gum may be 

 changed into sugar by means of sulphuric acid, and also by the vital action of the plant 

 which produces it. 



There is a great deal of interest in the question of the changes effected on starch in the 

 vegetable tissues. The origin of lignin is traced back to starch. It is known that the 

 grains of starch possess a structure quite different from those substances which are formed 

 by affinity. They have a structure analogous to certain animal organs, as the crystalline 

 lens of the eye ; and starch, when its changes are carefully noted, suffers a gradual 

 transition into lignin, the arrangement of the granules being such that they form fibrous 

 tubes in which considerable unaltered starch still remains adhering to the walls of the 

 fibre or tube*. This peculiar change may be better understood, when the composition of 

 starch is stated : it is composed of twelve atoms of carbon, ten of hydrogen, and ten of 

 oxygen. Starch also being formed of concentric layers, the outer one continually increases 

 in density by the absorption of water from the inner layers : this forms a space within it, 

 which may only constitute a cell ; but, in contiguous granules, they may form continuous 

 cells, or, in other words, a tube. 



Starch is regarded as the first stage in the organization of tissues. It may also perform 

 other offices : it may be changed into sugar or gum, acids, oils, coloring matter. Changes 

 of an analogous kind continually take place in the ripening of fruit. Let any one note 

 the changes in an apple, or a nut, from its earliest period up to the perfect fruit, and he 

 will witness numerous distinct products at different periods, which are changed or meta- 

 morphosed into each other. 



Dextrine. This is regarded as a kind of gum, which is formed from starch. It is 

 formeiby digesting starch in sulphuric acid. The proportions employed, are, one of sul- 

 phuric acid, fifteen of water and five parts of starch ; the mixture being heated to 200° 

 Fahr., and maintained at that temperature for some time. The starch is perfectly dissolved, 

 loses its peculiar character, and, instead of giving a deep blue violet color to iodine, im- 

 parts to it only a wine red. 



Extract, or Extractive matter. Extract is a watery solution of several substances, dried 

 down to the consistence of syrup. It is bitter and astringent usually, or sweetish bitter ; 

 but it can not be regarded as a proximate principle. It is an impure mixture of several 

 substances. Many medicinal substances are obtained in this state, but they can never be 

 regarded as chemical substances. The bitter principle of aloes, of hops, and of various 

 roots, are complex bodies. Their composition always depends upon the mode in which 

 they are prepared. 



* Kane's Chemistry, p. 652. 



