Vegetable Substances. 283 



a portion of its hydregen, and combined with 

 oxygen. 



1 8. Camphor has been already noticed in the 

 lecture on Combustibles. 



1 9. Caoutchouc, or elastic gum, called also In- 

 dian rubber, is a curious but well known sub- 

 stance, extracted from two trees of South Ame- 

 rica, the haevea and jatropha, and probably from 

 some others of the same country. It extfdes in 

 the form of a milky juice, which concretes by 

 exposure to the air. Its concreting appears, 

 from some experiments, to be the effect of its 

 absorbing oxygen* Caoutchouc is composed of 

 carbon, hydrogen, nitrogen, and oxygen; but 

 the manner in which they are combined is not 

 known. 



20. Sandarachy a resinous substance obtained 

 from the juniper, is not a pure resin: for, when 

 dissolved in alcohol, an insoluble residuum re- 

 mains, which Dr. Thomson calls sandaracha. 



21. Grum resins differ in many respects from 

 resins. They are not so combustible, nor do they 

 melt so easily. Their specific gravity is greater. 

 They are only partially soluble in alcohol. They 

 are supposed to consist of gum, or an extractive 

 substance, and a body intermediate between oil 

 and resin. Galbanum, scammony, gum ammo- 

 niac, asafcetidn, belong to this class. 



22. The wood of plants, it is well known, 

 consists of longitudinal fibres, easily subdivided 



