182 
liebig's organic chemistry. 
tinder the form of vegetable structure from the surface of 
any given soil, and yet the quantity of humus so far from 
decreasing, has increased in amount. This position is sus- 
tained by a series of arguments and calculations confirma- 
tory of this fact. Humus, moreover, is not necessary to 
some plants, which will live and flourish separate from any 
soil; aud there must have existed a period in the first for- 
mation of all soils, when no humus could have been present, 
this susbtance being the result of decay of previously exist- 
ing vegetable structure. Whence, then, is the source of this 
element ? The only other source is the atmosphere. An 
examination of the air results in the determination that, 
besides its main constituents, it contains a small but invari- 
able amount (1-1000 part of its bulk) of carbonic acid. 
Small as is this proportion to the whole amount, yet by 
<e calculation it can be shown that the atmosphere contains 
3306 billion pounds of carbon;" " more than adequate to all 
the purposes for which it is required." Carbonic acid, ex- 
isting in the atmosphere to this amount, is always in contact 
with the leaves of the plants, and, from its solubility, is 
carried down and conveyed to their roots by the rain. By 
experiment, it has been determined that when living vege- 
tables, surrounded by an atmosphere of carbonic acid, or 
immersed in a solution of this gas, are placed under the in- 
fluence of the sun's light, either direct or diffused, the car- 
bonic acid disappears, and is replaced by pure oxygen; but, 
in the absence of light, this effect ceases, and, on the con- 
trary, carbonic acid is given out. This latter fact has been 
adduced as disproving the decomposition of carbonic acid. 
To understand properly this difference of result, it is neces- 
sary to separate the vital action from a decomposition of a 
purely chemical nature. It is only when the static force of 
vitality is affected by the external influence of light, that 
this action is produced; but place the plant under other cir- 
cumstances, in which the action is merely chemical, and the 
result is very different. If a dead or living plant be sub- 
