82 LECTURE V. 



Nitrogen 41990.0. 



Oxygen 1116 



Carbon dioxide 431 



Total amount 5746 



The apparatus was exposed daily to the sunlight, and in the seventh day 

 the plants were withdrawn and the gas in the receiver analysed ; its com- 

 position was found to be, 



Nitrogen 4338 c.c. 



Oxygen ... 1408 



Carbon dioxide ... o 



5746 



Boussingault (1844) gave the direct proof that the leaves 

 take up the carbon dioxide which is present in such small 

 proportion in the air. He passed air through a large glass 

 receiver in which a branch of a Vine, bearing about twenty 

 leaves, was hermetically fixed, and he found that, when the 

 apparatus was exposed to light, the proportion of carbon 

 dioxide in the air which had passed through the receiver 

 was much smaller than that in the external air : in one case 

 the proportion found in the air which had passed through was 

 0.0002, whereas the proportion in the external air was 

 0.00045, and in another case the proportions were o.oooi and 

 0.0004. 



This sort of experiment has been repeated with various 

 plants by Vogel and Wittwer, by RauwenhofT, and by Coren- 

 winder, the results being in all cases of much the same kind. 



It is clear, therefore, that the leaves or, more generally 

 speaking, the parts of plants which contain chlorophyll, absorb 

 carbon dioxide from the air, under the influence of sunlight. 



There is no experimental evidence which would tend to 

 shew that this property is under any circumstances possessed 

 by plants or parts of plants which do not contain chlorophyll. 

 And yet it was thought for a considerable time that a portion 

 at least of the carbon dioxide which a plant requires, is 

 absorbed by the roots from the soil. Thus Liebig distinctly 

 asserted that roots absorb carbon dioxide, and Unger argued 

 that the increase of carbon in a plant within a given time 



