IN THE VEGETABLE KINGDOM. 179 



vegetable kingdom collectively, or at any rate the great majority of 

 vegetable substances, is nourished solely by carbonic acid, water, 

 and ammonia, and that, consequently, all organic bodies in the 

 vegetable kingdom are generated solely from these three inorganic 

 substances. 



Priestley and Sennebier first made the observation that the 

 leaves of plants, when exposed to solar light, absorb carbonic acid 

 and in its place exhale oxygen. The admirable experiments of 

 Saussure, and the later researches of Grischow, Boussingault, and 

 others, have elucidated many points connected with this subject. 

 We now know that it is not only direct solar light, but also ordinary 

 refracted light, that produces this phenomena, which depends not 

 upon the heating or the chemical rays of the spectrum, but mainly 

 upon its yellow and green rays (Draper), and that, moreover, the 

 green parts of the plant alone possess the faculty of exhaling 

 oxygen after absorbing carbonic acid. Plants only exhale oxygen 

 after the absorption of carbonic acid, which is probably taken up 

 through the roots from water or through the leaves from the air. 

 Boussingault has especially drawn attention to the extraordinary 

 rapidity with which the leaves abstract carbonic acid from the air. 

 The quantity of oxygen that is exhaled corresponds very nearly 

 with the amount of carbonic acid which has been absorbed. These 

 experiments were not, however, conducted with the exactness 

 necessary to warrant us in drawing definite conclusions ; for while 

 the volume of exhaled air was perfectly equal to that of the absorbed 

 air, there was always found in the exhaled air a small quantity 

 of nitrogen, the source of which could not be clearly ascertained. 

 The experiments appear, at all events, to prove that there is always 

 rather less oxygen developed than is contained in the carbonic 

 acid, and consequently, that the entire volume of the oxygen is 

 not returned to the air from the carbonic acid. Although a portion 

 of this gas may pass into those organs of plants which absorb 

 oxygen, although they are not green, certain chemical and other 

 grounds render it more probable that a large amount of the exhaled 

 oxygen may be derived from water, and that the carbonic acid 

 cannot therefore be decomposed at once into carbon and oxygen. 

 It would also appear, from the experiments of Saussure and others, 

 that the amount of exhaled oxygen does not depend upon the 

 mass so much as upon the extent of surface of the green parts of 

 plants. 



The undoubted fact that plants reverse this process during the 

 night, by developing carbonic acid after they have absorbed oxygen, 



N 2 



