ON CARBON. 53 



we find to be the case, for while the tasteless 

 leaves of the American aloe absorb only 0.3 of 

 their bulk of oxygen during the dark, the 

 leaves of some of the fir tribe which contain 

 volatile and resinous oils absorb ten times and 

 the leaves of the oak containing tannic acid, 

 fourteen times that quantity. 



This chemical action is shown also very 

 plainly in the leaves of the Cacalia ficoides 

 and others, which are sour in the morning 

 from the absorption of oxygen during the 

 night, tasteless at noon from the assimilation 

 of carbon during the sun's light, and bitter in 

 the evening, w^hen in addition to the carbon a 

 portion of the hydrogen of the water, a com- 

 ponent part of all bitter substances, has been 

 assimilated. 



Indeed the quantity of oxygen absorbed can 

 be estimated pretty nearly by the length of 

 time which the green leaves of plants require 

 to undergo an alteration in colour, by the in- 

 fluence of the atmosphere. Those which con- 

 tinue longest green will abstract less oxygen 

 from the air, than those which change colour 

 more rapidly. 



Thus the leaves of the beech and poplar 

 absorb eight or nine times their bulk of oxygen 

 in the same time that others, whose durability 

 as to colour is greater, absorb only two or three 

 times that quantity, and when these leaves are 

 dried, in the dark, in the vacuum of an air 

 pump, moistened with water and placed in a 

 vessel filled with oxygen, they are found to 

 change colour exactly in the proportion in 

 6 



