CHEMICAL COMPOSITION OF PLANTS. 
119 
within you which enables you to make this inquiry, that renders 
you thus precious ; — it is your soul that raises you above the inani- 
mate and brute creation. Though the body is sister to the worm 
and weed, the soul may aspire to the fellowship of angels. Oh, then, 
let me entreat you, sutfer not your chief thought to be given to the 
decoration of the perishable part, the mere temporary dwelhng-place 
of the immortal mind ! but seek to prepare this mind for admission 
into " the glorious company of the spirits of the just made perfect." 
LECTURE XX. 
PHYSIOLOGICAL VIEWS CHEMICAL COMPOSITION OF PLANTS PROXIMATE PRIN- 
CIPLES — CHEMICAL ANALYSIS OF THE SAP. 
We have, according to our method of arrangement, considered 
the anatomy of the vegetable in connexion with its physiology : that 
is, when treating upon each particular organ, we have remarked 
upon its uses in the life and growth of the whole plant. We have 
treated of the germination of the seed, the minute vessels which con- 
stitute the vegetable fabric, with the fluids which circulate through 
these vessels ; we have considered them as constituting, in various 
ways, three essential parts of woody plants, the bark, wood, and. pith. 
We have inquired into the manner in w^hich these separate parts 
are formed, and observed the great distinction in the growth of the 
stem.s of monocotyledonous and dicotyledonous plants. 
Yet, although we have attempted to show how plants grow, it is 
no easy thing to explain how they live. The great principle which 
operates in organic hfe, appears not to have been laid open to the 
eye of man. But by a careful observation of facts, we can learn all 
that it is important for us to know, in order to cultivate plants suc- 
cessfully; their habits, food, and the causes of their diseases and 
death. 
The physician who spends a long and laborious life in the study 
of the human frame, can give only the result of his observation. 
He finds a certain article efficacious in the relief of a particular dis- 
ease ; 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 corrects acidity in the stomach ; ask the reason of this, and he 
tells you that these are aikahes, substances which neutralize acids, 
and thus render them harmless ; inquire still further, why alkalies do 
thus affect acids, and the physician is as ignorant as yourselves. 
Before closing our view of the vegetable structure, we will, by the 
aid of chemistry, examine the elements which compose it. 
The growth of vegetables, and the increase of their weight, show 
that they imbibe some external substances, which are incorporated 
into their own substance. This constitutes nutrition, and distin- 
guishes living substances from dead matter. A stone does not re- 
ceive nourishment, although it may increase by an external accumu- 
lation of matter. "Vegetable substances, analyzed by a chemical 
process, have been found to contain carbon, oxygen, hydrogen, and 
sometimes nitrogen, sulphur, silex, 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. 
* Mirbel, " Elemens do Botanique.*' 
Recapitulation— A difference between the knowledge of facts, and of their causes— 
Substances which compose plants. 
