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- 

 appeai'ed ; 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 cai'bonic acid. 



" It is not any kind of water in which oxygen will be evolved 

 in the sunshine ; neither boiled water, nor distilled water, nor 



