MEMOIR XL] THE FORCE INCLUDED IN PLANTS. 



The experiments of these earlier chemists had thus 

 established the important fact that from carbonic acid, 

 which is extensively diffused through the atmosphere 

 and in water, and even in the soil, through the influence 

 of sunlight, oxygen is obtained. The sunlight, then, is 

 the force which carries into effect the decomposition. 



There is thus a perpetual drain on the supply of car- 

 bonic acid, a perpetual tendency to its diminution, and 

 hence, for the order of nature to continue, there must be 

 an incessant supply. The source of that supply was very 

 strikingly indicated by some of Priestley's experiments. 

 " Having rendered a quantity of air thoroughly noxious 

 by mice breathing and dying in it, he divided it into 

 two receivers inverted in water, introducing a few green 

 leaves into one, and keeping the other receiver unaltered. 

 The former was placed in light, the latter in darkness. 

 After a certain time he found that the air in the former 

 had become respirable, for a mouse lived very well in it ; 

 but that in the latter was still noxious, for a mouse died 

 the moment it was put into it." 



To Priestley chiefly, though he was aided by other in- 

 vestigators, we must refer the honor of one of the great- 

 est discoveries of the last century. It was this, that the 

 two great kingdoms of nature, the animal and the vege- 

 table, stand at once in antagonism and alliance. What 

 is done by the one is undone by the other. Each is ab- 

 solutely essential to the existence of the other. There is 

 a never-ending cycle through which material atoms run. 

 Now they are in the atmosphere, then they are parts 

 of plants, then they are transferred to animals, and by 

 them they are conducted back to the atmosphere, to run 

 through the same cycle of changes again. The sunlight 

 supplies the force that carries them through these revo- 

 lutions. 



Previously to 1834 I had turned my attention to this 



