$4: LIFE OF A TREE. 
assist them in fulfilling their vastly important 
office—the decomposition of carbonic-acid gas ; 
and that when this is withdrawn, then this office 
ceases until the return of morning. De Can- 
dolle, the French botanist, tried to make plants 
decompose this gas, by exposing them to the 
rays of powerful lamps; but he could never 
succeed in causing them to do so. Hence it 
follows that sunlight is the agent which 
quickens this process into existence, and which, 
indeed, sustains it in activity when commenced. 
And if we come to ask what principle it is 
which exists in the light of the sun to cause it 
to effect these changes, we can only answer, 
that it appears to be the chemical or actinic 
rays, which are chiefly concerned in effecting 
it; 
At night, when these chemical rays no longer 
fall upon plants, as has been said before, the 
process stops. For a long time some talented 
philosophers actually thought that at night a 
precisely opposite decomposition took place: 
that is, that plants emitted carbonic-acid, and 
absorbed or drank in oxygen gas, instead of 
absorbing as they do in the day carbonic-acid, 
