THE LEAVES. 77 



deprived of its air. The gas was therefore clearly evolved by 

 the leaves. Priestly recognized this gas to be oxygen, and 

 Ingenhouse showed that light was indispensable to its manifes- 

 tation, since it ceased to evolve itself from the leaves in dark- 

 ness. Such was the state of the question when Sennebier fully 

 demonstrated by experiment, that the oxygen evolved from the 

 leaves was the result of their decomposition of the carbonic 

 acid which was contained in them. 



This carbonic acid is chiefly abstracted from the atmosphere 

 by means of the stomata or pores of the leaves. Every one 

 must be aware that neither plants nor animals could live with- 

 out air, and if they both lived on the same air, the atmosphere 

 would soon become unfit for respiration. But the air taken 

 into the lungs of animals in the act of inspiration, imparts its 

 oxygen to the dark venous blood in the lungs, which combining 

 with, the carbon of the blood forms carbonic acid ; this gas is 

 expelled from the lungs in the act of expiration (ex, out, spiro 9 

 to breathe.) The blood thus oxygenated by breathing, loses 

 its dark color and is changed into that bright red arterial 

 stream which again circulates through the system for its nutri- 

 tion. Now the atmosphere would soon be thoroughly poisoned 

 by animals, but for the purifying influence exerted on it by 

 the vegetable creation. Carbonic acid, C02, which is com- 

 posed of one equivalent of carbon and two equivalents of 

 oxygen, is taken into the plant through the pores of its leaves, 

 and under the influence of solar light decomposed, the plant 

 fixing the carbon, which when thus assimilated, forms the chlo- 

 rophyl or green matter in them ; whilst the oxygen is set free 

 and escapes into the atmosphere, which gas is the food of 

 animals. 



This inspiration of the carbonic acid of the atmosphere, 



