FUNCTION.] 



ACTION OF LIGHT. 



293 



placed in the sun, they part with oxygen. This fact has been 

 subsequently demonstrated by a great number of curious ex- 

 periments, to be found in the works of Ingenhouz, Saussure, 

 Senebier, and others. Saussure found that plants in cloudy 

 weather, or at night, inhaled the oxygen of the surrounding 

 atmosphere, but exhaled carbonic acid if they continued to 

 remain in obscurity. But, as soon as they were exposed to 

 the rays of the sun, they respired the oxygen they had pre- 

 viously inhaled, in about the same quantity as they received 

 it, and with great rapidity. Dr. Gilly found that grass leaves 

 exposed to the sun in a jar for four hours produced the 

 following effect : 



At the beginning of the experiment 



there were in the jar : 

 Of nitrogen .... 10-507 

 Of carbonic acid . . . 5-7 

 Of oxygen . . . .2793 



19-000 



At the close of the experiment there 



were : 



Of nitrogen .... 10507 

 Of carbonic acid . , . '37 

 Of oxygen .... 7'79 



18-667 



Heyne tells us that the leaves of Bryophyllum calycinum, 

 in India, are acid in the morning, tasteless at noon, and bitter 

 in the evening ; Link himself found that they readily stained 

 litmus paper red in the morning, but scarcely produced any 

 such effect at noon. The same phenomenon is said also to 

 occur in other plants, as Kleinia 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 



