116 PHYSIOLOGICAL VIEWS. 



e. The 3d order includes oiU, wax, resins, &c. Oilff 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 odor. The oil of sweet al- 

 monds and olive-oil grow thick and opake by being exposed to the air. Tlie Oil 

 of Flaxseed, called linseed oil, and some other oils, dry without h)sing their trans- 

 parency; it is this quality which renders linseed oil so valuable to painters. Tlie 

 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 which are exlialed from plants containing volatile oil ; to 

 this oil is owing the aromatic odor of the ginger-plant, of the myrtle, rose, and 

 otlier sweet-scented plants. Aromatic plants are much more common in hot than 

 cold countries ; most of the aromatic spices are found in the equatorial regions. 

 Wax is found on the surface of the fruit of the bayberry (Myrica cerifera). 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 

 camphor a, 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. 

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

 which, abounding in resin, burn with a bright flame. The difterence 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 some- 

 times mixed with gums ; they are then called gum-resins. Of this kind are gam- 

 boge, asafoetida, guaiacum, and aloes, an extract from the Aloe perfoUata. 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 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 India-rubber is obtained. f The green principle (Chloro- 

 phyl). — It is to this principle that all the green parts exposed to liglit owe 

 theu' 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 : OpiMm, a narcotic principle extracted from the poppy ; 

 it is soluble in alcohol, slightly in water. Hematine 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, rye, 

 <fec. It is obtained by separatiftn from the starch. Flour owes much of its nour 

 ishing properties to gluten, which, in some respects, is analogous to animal prin- 

 ciples, being, hke them, subject to putrefaction. Jelly is the thickeneil juice ol 

 succuleiit fruits, as currants, quinces, and apples ; it is soluble in hot water, 



* Caoutcliouc. 



t Mr. H. 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. 



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

 the different vegetable oils — What causes the aroma of plants? — Wax — Camphor — Resins— India 

 rubber — Chlorophyl. — 141. What new element is foand in the second clasg of proximate principles t 

 What substances are found in this class ? 



