PROXIMATE PRINCIPLES. 121 



The 3d order includes oils, was', resins, &c. 



Oils. These are fluid and combustible substances, which do not 

 unite with water. They are divided into Fixed and Volatile. »The 

 fixed oils are thick, and have little odour. 



The oil of sweet almonds, and olive oil, grow thick and opaque by 

 being exposed to the air. 



The Oil of Flaxseed, called hnseed oil, and some other oils, dry 

 without losing their transparency ; it is this quality which renders 

 linseed oil so valuable to painters. 



The Volatile oils are distinguished from the fixed oils by their aro- 

 matic odours, and their tendency to fly otf, from which circumstance 

 the term volatile is derived. Among these oils are those of the 

 orange, lavender, rose, jasmine, peppermint, and wintergreen. They 

 are sometimes greatly reduced by being mixed with alcohol, and are 

 then called essences. The volatile oils may be found in a great 

 variety of plants, particularly those of the Labiate family. 



The Aroma or aromatic property, consists chiefly of the odours 

 which are exhaled from plants, containing volatile oil ; to this oil is 

 owing the aromatic odour of the ginger plant, of the myrtle, rose, 

 and other sweet-scented plants. Aromatic plants are much more 

 common in hot, than cold countries ; most of aromatic spices are 

 found in the equatorial regions. 



Wa.r is found on the surface of the fruit of the bay-berry, (Myrica 

 cerifera.) Beeswax, though an animal production, is made by the 

 bees from the pollen of plants. 



Camphor has much analogy with the volatile oils; it is an extract 

 from the Ladrus camphora, or camphor-tree of Japan. 



Resin exudes from the pine, and some other trees ; it is diy, inso- 

 luble in water, but soluble in alcohol, and very inflammable. The 

 people in new countries often use, as a substitute for lamps, pine 

 knots, which abounding in resin, burn with a bright flame. The dif- 

 ference between resin and the volatile oils, appears to consist in the 

 action of oxygen upon the former ; for the oil in absorbing oxygen 

 from the air, passes into the resinous state. 



Resins mixed with volatile oils form balsams ; they are thick, 

 odorous, and inflammable substances, as, the balsam copaiva, and 

 the balsam of Tolu. 



These resins are sometimes mixed with gums, they are then called 

 gum-resins ; of this kind are gamboge, assafostida, guaiacum, aloes, 

 an extract from the Aloe pei-joliata. These gum-resins in flowing 

 from vegetables are sometimes white and liquid like milk, but they 

 usually iDecome brown and hard by exposure to the air. 

 Indian rubber,'^ or as it is sometimes called, gum elastic, is the 



firoduct of a South American tree, (Siphonia elastica,) an East 

 ndian plant, (the Urceola elastica,) and some other trees in the 

 equatorial regions ; by exposure to the air the gum hardens, be- 

 comes brown, and takes the appearance of leather ; it can neither 

 be dissolved by water nor alcohol. The juice of the milk-weed is 

 Baid to be similar to that of the plants from which the Indian rubber 

 is obtained.f 



* Caoutchouc. 



+ Mr. H. Eaton, (late professor of Chemistry at Transylvania University, Kent.) in- 

 formed me that he prepared a small quantity of the juice of the milk weed, (Ascle- 

 pias,) in such a manner that it could not be distinguished from the imported Indian 

 rubber, either in external appearance, or in its properties. 



What substances belong to the third order of the first class of proximate principles? 

 Describe the different vegetable oils— What causes the aroma of plants? — Wax — 

 Camphor— Resins— Indian rubber, 

 II 



