liebig's organic chemistry. 
183 
jected to the action of oxygen in the dark, then the chemi- 
cal affinity of the inorganic elements are brought into action, 
— leaves change their color, oils become resins, and various 
other changes take place, such as usually result from the 
chemical action of oxygen. The oxygen disappears during 
night, from entering into combination with portions of the 
organized structure, and the carbonic acid thrown off is that 
which has been absorbed by the unceasing action of the 
roots, and not being subjected to the proper influence, can 
not be assimilated by the plant. 
Experiments have been instituted to prove that carbonic 
acid is not sufficient for the nourishment and perfection of 
vegetable structure. But the only conclusion to be derived 
from them is, that other substances are also essential to this 
result, or that perfection of vegetable structure cannot be 
obtained without all the elements which are necessary to its 
composition, — grass cannot be formed without silex, — grain 
without phosphate of magnesia, &c. 
Although the carbon of the vegetable structure is derived 
from carbonic acid of the atmosphere, yet humus still acts 
an important part in the vegetable economy. This substance 
is not a constant and definite compound, but, on the con- 
trary, is matter in a state of transition. " Woody fibre in 
the state of decay is called humus" All woody fibre when 
exposed to the action of oxygen yields carbon, which under- 
goes a very slow combustion, and carbonic acid is produced. 
To effect this, moisture is necessary, and alkalies accelerate 
it, while acids produce a contrary effect. The carbonic acid 
in solution is absorbed by the roots, and this supply, though 
small, is most essential at that period in the year when the 
plants are destitute of those organs (leaves) which are capa- 
ble of drawing their nourishment from the atmosphere. — 
Also, the young plant, before it protrudes from the ground, 
is surrounded by it, and also derives its support partly from 
this source, before its organs have arrived at maturity. The 
effect of culture is to hasten the slow combustion of the 
