THE LEAF 135 



not the way it is carried out. The main part of this 

 apparatus consists of two bent glass tubes (a) through 

 which the stream of aspirated air passes, and which 

 are meant to absorb the carbonic acid. For this purpose 

 one of them contains small pieces of caustic alkali. 

 Caustic alkali will become heavier in absorbing carbonic 

 acid ; and therefore what we have to do is to unfasten 

 the part of the apparatus marked a and to weigh it 

 before and after the experiment. The increase in 

 weight will indicate the quantity of carbonic acid 

 remaining in the air after it passes out of the globe. It 

 happened that under favourable conditions of illumina- 

 tion the air came out of the ball almost deprived of 

 carbonic acid. Consequently, in passing over the green 

 surface of the illuminated plant, the air has left behind 

 it almost all its carbonic acid ; in spite of the fact that 

 its particles are so scantily diffused in the atmosphere, 

 lost, so to speak, in the mass of its other components. 

 This result will become more comprehensible if we 

 recall the diffusion of carbonic acid into our artificial 

 cell.^ Then carbonic acid pressed into the cell only 

 because the cell contained none of this gas ; but in the 

 leaf also, since it is continually decomposed, it dis- 

 appears, without, so to speak, leaving any traces ; and 

 therefore, according to the laws of diffusion, it must 

 continually be replaced by fresh quantities from the 

 atmosphere. 



Boussingault's classical experiment was made more 

 than half a century ago, and has hardly ever been 

 repeated since. Lately a distinguished English chemist, 

 Horace Brown, undertook a whole series of similar ex- 

 periments of an improved form, by which he succeeded 

 in removing any remaining doubts upon the subject. 

 The insignificance of the total area of the minute 

 openings afforded by the stomata made it still seem 

 incomprehensible, however, that a plant should succeed 



1 See chapter ii. 



