180 THE FORCE INCLUDED IN PLANTS. [MEMOIK XI. 



constituent, carbon, now ministering to the growth of the 

 plant. 



A stone trough standing in a garden received the 

 waste water from a pump. There had accumulated on 

 its sides a green slimy growth (conferva). From this 

 growth, on the west side of the trough, which was receiv- 

 ing the morning rays of the sun, bubbles of gas were con- 

 tinually forming ; and these, as they attained a sufficient 

 size, rose through the water and escaped into the air. 

 This effect on the west side diminished as the sun passed 

 towards the meridian, but at mid-day the north side of the 

 trough was in full activity. As evening came on, that in 

 its turn gave forth fewer bubbles, and was succeeded in 

 activity by the east side. At first it was thought that 

 these bubbles were nothing more than the gas which is 

 dissolved in all water, and analogous in composition to 

 atmospheric air, but closer examination showed that it 

 was "oxygen, nearly pure. During the night no gas what- 

 ever was disengaged. 



Priestley, Ingenhousz, Rumford, and other experiment- 

 ers of the last century investigated these facts carefully. 

 The conclusions to which they came may be thus sum- 

 marized : All ordinary natural waters contain carbonic 

 acid in solution ; leaves or other green parts of plants 

 placed in such water and kept in darkness exert no action 

 upon it, but in the sunshine they decompose the carbonic 

 acid, appropriating its carbon and setting its oxygen free, 

 as gas. Soon, however, the supply in the sample of water 

 is exhausted, and the action even in the sunlight ceases. 

 It is again resumed if more carbonic acid be artificially 

 dissolved in the water; and since the air expired from 

 the lungs in the act of breathing contains much of that 

 gas, it is sufficient, by the aid of a tube, or in any other 

 suitable manner, to conduct such expired air into the 

 water for the disengagement of oxygen to go on. 





