130 VEGETABLE SUBSTANCES. 



fixed oils. They are also dissolved by alkaline 

 lys, and by the acids. Resins consist of oxygen, 

 carbon, and hydrogen ; and they are supposed to 

 be volatile oils saturated with oxygen. 



Bitumen is a substance having some analogy 

 with oils and resins, although differing in its consti- 

 tuents, and being also a mineral body. Pure bitu- 

 men is called naphtha, which is transparent, highly 

 inflammable, volatile, of a pungent odour ; it is found 

 in certain wells, and there are springs of it in several 

 parts of the world. When naphtha is exposed to 

 the air, it thickens, and becomes dark coloured; it is 

 then called petroleum, which is procured in the same 

 manner, and is used for burning in lamps. Maltha 

 or mineral pitch is a still farther thickened bitumen, 

 and when it has become solid it is called asphaltum. 

 Caoutchouc. This is the substance usually 

 known by the name of Indian rubber, and some- 

 times elastic gum. It was first brought from South 

 America. It exudes as a milky juice from a tree, 

 which thickens and hardens by exposure to the 

 air. The natives form it into bottles by covering 

 balls of clay with this juice ; the clay is afterwards 

 washed out after the caoutchouc is solid. When 

 caoutchouc is pure, it is white, the black colour 

 being owing to the smoke used in drying it. This 

 substance is extremely elastic. It is perfectly inso- 

 luble in water, but it may be softened by boiling, 

 so that its edges may be united together. It is not 

 soluble in alkohol : but it is soluble in ether ; and 

 when the ether is evaporated, the caoutchouc re- 

 mains unaltered in its properties : by this means 

 tubes and other instruments might be made of it, 

 but the method would be too expensive. It is so- 

 luble in some of the fixed and essential oils, as in 

 spermaceti and in oil of cajeput. 



