THE LEAF 133 



out of place here on account of the technical details 

 involved. I will only say that in those methods we 

 make use of the property of carbonic acid, already 

 familiar to us, of being absorbed by caustic alkali. 

 A definite amount of carbonic acid is supplied to a 

 plant, or to a single leaf, placed in a graduated glass 

 tube closed at its upper end (as in fig. 37), which is 

 then exposed to the light. The experiment being over, 

 we determine, by means of an alkali, the quantity of 

 carbonic acid remaining in the tube. Knowing how 

 much gas was introduced, and how much is left, we can 

 tell how much has disappeared. 



This method has helped to solve many interest- 

 ing problems ; for instance, that of determining the 

 percentage of carbonic acid in the air most favourable 

 for the plant. The experiment proves it to be some- 

 thing like eight per cent. ; a greater percentage is 

 evidently noxious to the plant. 



Another question dealing with the same point 

 deserves our attention. We have proved experimentally 

 that plants decompose carbonic acid supplied to them 

 in our apparatus ; but it is questionable whether we 

 are entitled to infer from these experiments that a 

 plant in its natural condition is also able to decom- 

 pose the carbonic acid of the air. You remember that 

 we supplied in our experiments comparatively large 

 quantities of carbonic acid to the plant, as a rule in 

 the proportion of several parts in a hundred, whereas 

 the atmosphere contains only a few ten-thousandth 

 parts of it. It may seem unlikely that a plant should 

 be able to discover and assimilate the particles of car- 

 bonic acid so sparingly diffused in the air. In order 

 to settle this question, the renowned French chemist, 

 Boussingault, made the experiment which we will 

 now demonstrate. We take a large glass globe 

 with three apertures (fig. 39). A vine branch with 

 leaves is introduced through the lower opening into 



