BOOK I. PRIMARY PRINCIPLES OF PLANTS. 147 



CHAP. VII. 



Vegetable Chemistry, or jnimary Principles of Plants. 



635. As plants are not merely organised beings, but beings endowed with a species of 

 life, absorbing nourishment from the soil in which they grow, and assimilating it to their 

 own substance by means of the functions and operations of their different organs, it is 

 plain that no progress can be made in the explication of the phenomena of vegetable 

 life, and no distinct conception formed of the rationale of vegetation, without some 

 specific knowledge of the primary principles of vegetables, and of their mutual action 

 upon one another. The latter requisite presupposes a competent acquaintance with the 



** elements of chemistry ; and the former points out the necessity of a strict and scrupu- 

 lous analysis of the several compound ingredients constituting the fabric of the plant, 

 or contained within it. 



636. If the object of the experimenter is merely that of extracting such compound 

 ingredients as may be known to exist in the plant, the necessary apparatus is simple, 

 and the process easy. But if it is that of ascertaining the primary and radical principles 

 of which the compound ingredients are themselves composed, the apparatus is then com- 

 plicated, and the process extremely difficult, requiring much time and labor, and much 

 previous practice in analytical research. But whatever may be the object of analysis, or 

 particular view of the experimenter, the processes which he employs are either mechanical 

 or chemical. 



637. The mechanical processes are such as are effected by the agency of mechanical 

 powers, and are often indeed the operation of natural causes ; hence the origin of gums 

 and other spontaneous exudations. But the substances thus obtained do not always 

 flow sufficiently fast to satisfy the wants or necessities of man. And [men have conse- 

 quently contrived to accelerate the operations of nature by means of artificial aid in the 

 application of the wimble or axe, widening the passages which the extravasated fluid has 

 forced, or opening up new ones. But it more frequently happens that the process 

 employed is wholly artificial, and altogether effected without the operation of natural 

 causes. When the juices are enclosed in vesicles lodged in parts that are isolated, or 

 may easily be isolated, the vesicles may be opened by means of rasps or graters, and the 

 juices expressed by the hand, or by some other fit instrument. Thus the volatile oil may 

 be obtained that is lodged in the rind of the lemon. When the substance to be ex- 

 tracted lies more deeply concealed in the plant, or in parts which cannot be easily de- 

 tached from the rest, it may then become necessary to pound or bruise the whole, or a 

 great part of the plant, and to subject it, thus modified, to the action of the press. Thus 

 seeds are sometimes treated to express their essential oils. And if by the action of bruis- 

 ing or pressing heterogeneous ingredients have been mixed together, they may generally 

 be separated with considerable accuracy by means of decantation, when the substances 

 held in suspension have been precipitated. Thus the acid of lemons, oranges, goose- 

 berries, and other fruits, may be obtained in considerable purity, when the mucilage that 

 was mixed with them has subsided. 



638. The chemical processes are such as are effected by the agency of chemical powers, 

 and may be reduced to the following : distillation, combustion, the action of water, the 

 action of acids and alkalies, the action of oils and alcohols, and lastly fermentation. They 

 are much more intricate in their nature than the mechanical processes, as well as more 

 difficult in their application. 



639. Of the products of vegetable analysis, as obtained by the foregoing processes, 

 some consist of several heretogeneous substances, and are consequently compound, as 

 being capable of further decomposition ; and some consist of one individual substance 

 only, and are consequently simple, as being incapable of further decomposition. 



SECT. I. Compound Products. 



640. The compound products of analysis are very numerous in themselves, and much 

 diversified in their qualities. They are gum, sugar, starch, gluten, albumen, fibrina, 

 extract, tannin, coloring matter, bitter principle, narcotic principle, acids, oils, wax, 

 resins, gum resins, balsams, camphor, caoutchouc, cork, woody fibre, sap, proper juice, 

 charcoal, ashes, alkalies, earths, metallic oxides. 



641. Gum is an exudation that issues spontaneously from the surface of a variety of plants, in the state of a 

 clear, viscid, and tasteless fluid, that gradually hardens upon being exposed to the action of the atmosphere, 

 and condenses into a solid mass. It issues copiously from many fruit-trees, but especially from such as 

 produce stone-fruit, as the plum and cherry-tree. From plants or parts of plants containing it, but not dis- 

 charging it by spontaneous exudation, it may be obtained by the process of maceration in water. It has 

 been found by chemists to consist of several varieties, known by the names of gum arabic, gum tragacanth, 

 cherry-tree gum, and mucilage. Gum arabic, which is the most plentiful of all the gums, is the produce 

 of the mimosa nilotica, a native of the interior of Africa and of Arabia ; whence its name. When pure, it 

 is colorless and transparent, though sometimes it is tinged with yellow, varying in its specific gravity 

 from 1300 to 1490. (Davy's Agric, Chem., lect. iii.) It is insoluble in alcohol ; but is readily soluble in 



L 2 



