PROXIMATE PRINCIPLES. 
121 
The 3d order includes oils, wax, 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 linseed 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 off, 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. 
Wax 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 Laurus camphor a, or camphor-tree of Japan. 
Resin exudes from the pine, and so^me other trees ; it is dry, 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 5 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, assafoetida, guaiacum, aloes, 
an extract from the Aloe perfoliata. These gum-resins in flowing 
from vegetables are sometimes white and liquid like milk, but they 
usually become brown and hard by exposure to the air. 
Indian rubber ,* or as it is sometimes called, gum elastic, is the 
product of a South American tree, (Sxphonia elastica ,) an East 
Indian 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 
said to be similar to that of the plants from which the Indian rubber 
is obtained.!' 
* 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 7 
Describe the different vegetable oils—What causes the aroma of plants?—Wax- 
Camphor—Resins—Indian rubber. 
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