liebig's organic chemistry. 
131 
by external influences, is disturbed, and, as a consequence 
of motion thus effected, produces structural form. To this 
the name of vital force, or vitality, is applied. The in- 
fluences, however, by which this force is disturbed, and the 
instruments by which it acts, are very different. Vegetables 
require for their nourishment matters reduced to their inor- 
ganic state as the result of decomposition ; but animals can 
only derive nourishment from highly organised particles. 
In the former the state of rest is disturbed by the influences 
of air, moisture, and light, while to the mere vegetative life 
there is superadded, in the latter, faculties of sensation and 
motion, acting through a nervous organism. These wide 
differences require a separate consideration. 
For the growth and reproduction of vegetable structure, 
the elements which constitute that structure must be present 
in the source of nourishment. To the soil upon which they 
rest is the most natural source to which to look for this sup- 
ply; and great importance has been attached to their com- 
position. To the matter recognized as the source of fertility 
the name humus has been applied, and the opinion advanc- 
ed that this substance is extracted by the roots of plants, 
and serves to the production of their tissues. This opinion 
the author considers as untenable. Humus does not appear 
capable of solution and consequent absorption by plants, un- 
less previously acted on by alkalies, and converted into the 
so-called humic acid; and even under the most favorable 
circumstances, the amount of alkali, even with the oxides 
of iron and manganese in the soil, is not by any means suf- 
ficient to render soluble in the form of humic acid the quan- 
tity of carbon which exists in the vegetable structure in any 
given space. This is shown by an estimation of the amount 
of carbon annually produced on one acre of woodland in 
comparison with the metallic oxides present, these latter 
being capable of conveying sufficient carbon to form only 
100 pounds of dry wood — but one-thirtieth of the actual 
amount produced. Carbon may also be conveyed away 
