142 
PROXIMATE PRINCIPLES. 
tions , made by himself or others. He finds a certain article effi- 
cacious in the relief of a particular disease ; but he knows not 
why this should be so ; or if he is able to give some reasons, 
he is ultimately arrested in his speculations by a barrier which 
he cannot pass. Thus he knows that soda or pearl-ash correct 
acidity in the stomach ; ask the reason of this, and he tells you 
that these aie alkalies, substances which neutralize acids, and 
thus render them harmless ; inquire still farther, why alkalies 
do thus affect acids, and the physician is as ignorant as yourselves. 
Chemical Composition of Plants. 
Before closing our view of the vegetable structure, we shall, 
by the aid of chemistry, examine the elements which compose it. 
The growth of vegetables, and the increase of their weight, 
show that ihey imbibe some external substances, which are in- 
corporated into their own substance. This constitutes nutrition , 
and distinguishes living substances from dead matter. A stone 
does not receive nourishment, although it may increase by an 
external accumulation of matter. “ Vegetable substances, ana- 
lysed by a chemical process, have been found to contain carbon , 
oxygen , hydrogen , and sometimes nitrogen, sulphur, silex, (a 
flint-like substance,) the oxide of iron, soda, magnesia, and 
chalk These different substances are by the root, stems, and 
leaves of the plant, derived from the earth, air, and water. 
PROXIMATE PRINCIPLES. 
Vegetation produces chemical combinations which are distin- 
guished by the name of proximate principles. Although the 
proximate principles of plants are very numerous, but few of 
them are well known ; they are the result of the action of the 
vital forces of plants, and are, therefore, important subjects of 
investigation to those who pursue the study of physiological 
botany to any great extent. Carbon , oxygen, hydrogen, and 
nitrogen, are the constituent parts of the proximate principles of 
plants. These principles may be divided into two classes. 
I. Those principles which are composed of carbon, hydrogen 
and oxygen, without any nitrogen. 
II. Such as contain, besides the substances belonging to the 
other class, some nitrogen. There are few of this class. 
* Mirbel, “ Elemens de Botanique.” 
Chemical composition of plants — Examination of the elements which com- 
pose the vegetable structure — Proximate 'principles — Chemical relation — 
Chemical composition of the sap — Variety of combinations formed by a few 
elementary principles — Proximate principles divided into two classes — 
