116 
PHYSIOLOGICAL YIEWS. 
e. The 3d order includes oils, wax, resins, &c. Oils aro 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 odor. The oil of swdet al- 
monds and olive-oil grow thick and opake by being exposed to the air. The Oil 
of Flaxseed, called linseed oil, and some other oils, dry without losing their trans- 
parency ; it is this quahty which renders linseed oil so valuable to painters. The 
Volatile oils are distinguished from the fixed oils by their aromatic odors, 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 winter- 
green. 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, con- 
sists chiefly of the odors wliich are exhaled from plants containing volatile oil ; to 
this oil is owing the aromatic odor of the ginger-plant, of the myrtle, rose, and 
other sweet-scented plants. Aromatic plants are much more common in hot thau 
cold countries ; most of the aromatic spices are found in the equatorial regions. 
Wax is found on the surface of the fruit of the bay berry (Myrica c(rifera). Bees- 
wax, 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 
camphora, or camphor-tree of Japan. Resin exudes from the pine, and some other 
trees ; it is dry, insoluble in water, but soluble in alcohol, and very inflammable. 
Tlie people in new countries often use, as a substitute for lamps, pine-knots, 
which, abounding in resin, burn with a bright flame. The difference 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 l^olu. These resins are some- 
times mixed with gums ; they are then called gum-resins. Of this kind are gam- 
boge, asafcetida, guaiacum, and 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. India-ruhher^ 
or, as it is sometimes called, gum-elastic, is the product of a South American 
tree (Siphonia elastica), an East Indian plant (the Urceola elasHca), 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 India-rubber is obtained.f The green principle (Chloro' 
phyl). — It is to this pritjciple that all the green parts exposed to light owe 
their color ; chlorophyl undergoes changes in the different states of the plant, in 
autumn becoming brown or yellow. The change of color is attributed to the 
action of oxygen, into the formation of an acid. 
141. The second class of proximate principles consists of substances which, like 
the first class, are formed of carbon, hydrogen, and oxygen; but to these is added 
nitrogen. We here find : Opium, a narcotic principle extracted from the poppy ; 
it is soluble in alcohol, slightly in water. Hcmatine is the coloring principle from 
the Campeachy wood. Indigo is a coloring substance, obtained from several species 
of Indigofera, or indigo plant. Gluten is extracted from the cotyledons of the 
seeds of leguminous plants, as peas, beans; and from the albumen of wheat, rje, 
(fee. It is obtained by separation from the starch. Flour owes much of its nour 
ishing properties to gluten, which, in some respects, is analogous to animal prin- 
ciples, being, like them, subject to putrefaction. Jelly is the thickened juice of 
succulent fruits, as currants, quinces, and apples ; it is soluble in hot water, 
* Caoutchouc. 
t Mr. II. Eaton informed me that he prepared a small quantity of the juice of the milk-weed 
(Asclepias) in such a manner that it could not be distinguished from the imported india-rubber 
either in external appearance or in properties. 
AVhat substances belon;? to the third order of the first class of proximate principle j ?- -Describe 
the different vegetable oils— What causes the sroma of plants ?— Wax— Camphor -Re-iins— India 
ubber— Chlorophyl.— 141. What nev/ element is found in the second class of proximate princij)le» T 
What substances are found in this class ? 
