514 
On tltp. Food of Plants. 
carbon, and consequent liberation of oxygen gas, is the result of 
something like digestion, it is much more likely, as Liebig sug- 
gests, that the evolution of carbonic acid, probably at all times, is 
due lo merely chemical actions constantly going on, and not to 
any vital process whatever. We know that this effect is constantly 
produced by moist vegetable matter,, especially when a little 
albuminous substance is present. Even wetted sawdust, placed 
in a vessel of oxygen, occasions the conversion of that substance 
into carbonic acid ; and it would be very odd if the green matter 
of the leaf (the chlorophylle, a body exceedingly prone to change) 
did not possess this property. Again, if we imagine, what is very 
likely to be true, that the acid principles of the plant are formed 
by the oxidation of such bodies as starch and sugar, we have 
another source of carbonic acidj which usually makes its appear- 
.ance in such reactions- — an abundant one in some particular cases, 
but perhaps commonly of less importance than the first mentioned. 
The permanent disappearance of a portion of oxygen is also ex- 
plained in this manner. Its office is to carry off hydrogen from 
the bodies in question. 
Once more, vegetables pump up from the soil and exhale from 
their leaves an enormous quantity of water. Now this water, for 
a reason presently to be described, is always largely impregnated 
with carbonic acid, which must thus be sent into the air with the 
aqueous vapour^ unless light be present to induce its decompo- 
sition. 
Now comes the important inquiry — does the quantity of oxygen 
emitted by a plant during the day, under ordinary circumstances, 
exceed that abstracted from the air by the same plant during the 
night? In other words, is the ultimate effect of a growing plant 
upon the air, after a succession of days and nights, the same as 
that produced by an animal, that is^ its vitiation ; or the reverse? 
Some of De Saussure's best experiments, among which may be 
placed the examples before quoted, besides those of others, in 
which the process was suffered to go on for more than one day, 
lead to a very decided affiirmative. In an artificial atmosphere, 
containing a very far larger proportion of carbonic acid than is 
ever found in the air (for plants exposed to light can bear this 
increase, up to a certain point, without injury, although they 
perish when the quantity becomes excessive), the oxygen has been 
seen day by day to increase nearly at the same rate as the car- 
bonic acid diminished, so long as the plant retained its healthy 
state, until the proportion contained in the experimenting jar 
showed a very notable excess over the normal quantity of the 
atmosphere. 
These experiments, and the conclusions deduced therefrom, 
have been called in question by some observers, and that recently, 
