126 VEGETABLE SUBSTANCES. 



Vegetable substances may be decomposed, or 

 separated into their elementary principles by various 

 means ; by heats, by acids, and by fermentation : 

 some of these processes occasion not only decom- 

 position, but also new combinations of the elements 

 that did not exist in the living bodies. 



The principal substances of which all vegetables 

 consist are. 



Mucilage^ or gum. — Various parts of vegetables 

 impart to water, particularly if boiled with them, 

 a certain viscous consistency : the substance so 

 dissolved is called mucilage. Some trees suffer 

 their mucilage to transude, either spontaneously 

 or by incisions made in them. AVhen it has 

 become concrete by drying in the air, it is called 

 gum. 



Gum is soluble in water, but not in oils or al- 

 kohol, the latter of which precipitates it from its 

 solution in water. It is insipid ; it does not un- 

 dergo any change by exposure to the air when dry. 

 The gums of different trees differ considerably in 

 their properties. Gum arable may be considered 

 as a very pure gum. Cherry-tree gum and gum 

 tragacanth do not dissolve in cold water ; but dis- 

 solve in boiling water, and, on cooling, assume the 

 state of a jelly. 



