294- LIGHT. [BOOK n. 



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 decom- 

 position of carbonic acid in an obvious manner. 



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

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

 that in which nitrogen, hydrogen, or even oxygen, has been 

 dissolved, will produce the result. But if a small quantity of 

 carbonic acid is dissolved in the water, the green parts, stimu- 

 lated by the sun, disengage oxygen. Various ingenious means 

 have been contrived to prove this fact, and to show that the 

 quantity of oxygen given out is proportioned to the quantity 

 of carbonic acid decomposed. One of the prettiest experiments 

 is the following, by De Candolle : He placed in the same 

 cistern two inverted glasses, of which one (A), as well as the 

 cistern itself, was filled with distilled water, and had a plant 

 of Water Mint floating in it ; the other glass (B) was filled with 

 carbonic acid. The water of the cistern was protected from 

 the action of the atmosphere by a deep layer of oil. The 

 apparatus was exposed to the sun. The carbonic acid in the 

 glass B diminished daily, as was obvious from the water rising 

 in it ; and at the same time there rose to the top of the glass A 



