CHAP. X. 
DIGESTION. 
313 
such effect at noon. The same phenomenon is said also to 
occur in other plants, as Cacalia ficoides, Sempervivum 
arboreum, &c. This stain in the litmus paper could not have 
arisen from the presence of carbonic acid, as that gas will not 
alter blue paper, but it must have been caused by the oxygen 
inhaled at night. " If," says De Candolle, " two plants are 
exposed, one to darkness and the other to the sun, in close 
vessels, and in an atmosphere containing a known quantity 
of carbonic acid, and are removed at the end of twelve hours, 
we shall find that the first has diminished neither the quantity 
of oxygen nor of carbonic acid ; and that in the second, on 
the contrary, the quantity of carbonic acid has diminished, 
while the quantity of free oxygen has increased in the same 
proportion. Or if we place two similar plants in closed vessels 
in the sun, the one in a vessel containing no carbonic acid, and 
the other in air which contains a known quantity of it, we shall 
find that the air in the first vessel has undergone no change, 
while that in the second will indicate an increase of oxygen 
proportioned to the quantity of carbonic acid which has dis- 
appeared ; and, if the experiment is conducted with sufficient 
care, we shall discover that the plant in question has gained a 
proportionable quantity of carbon. Therefore, the carbonic 
acid which has disappeared has given its oxygen to the air 
and its carbon to the plant, and this has been produced solely 
by the action of solar light." 
It is a very curious circumstance, however, that although 
the direct solar rays are requisite to produce a decomposition 
of carbonic acid in plants under experiment, yet that the 
most feeble diffused light of day, is sufficient to produce the 
result more or less in a natural state. Thus we find that 
plants growing in wells, in rooms partially darkened, in deep 
forests, on the north side of high walls, and on which not a 
single ray of sunlight ever fell, become green, and often 
perform all their functions, without much apparent incon- 
venience. Yet De Candolle found the purest daylight, the 
brightest lamp-light, insufficient to bring about the decompo- 
sition of carbonic acid. 
" It is not any kind of water in which oxygen will be evolved 
in the sunshine ; neither boiled water, nor distilled water, nor 
