132 THE LIFE OF THE PLANT 



out of the water. We cover this bit of candle with a 

 glass bell (similar to the one in fig. 34) in such a way 

 that its edges dip into the water at the bottom of the 

 jar. The air in the jar will be thus enclosed and isolated 

 by a layer of water from the external atmosphere. The 

 bit of candle will go on burning under the glass bell for 

 a while and then die out. This means that the oxygen 

 necessary for the process of combustion has already been 

 exhausted from the air under the glass bell, and has been 

 replaced by carbonic acid, as the result of the com- 

 bustion. If at this moment we were to introduce 

 (through the neck of the bell) a burning splinter, it 

 would certainly die out in the same way as the candle. 

 But if we carefully introduce some leaves or a green 

 branch into the glass bell through the water, and place 

 the whole apparatus for a long time out in the light, we 

 shall find eventually that the splinter will continue 

 to burn under the bell which will mean that oxygen 

 has reappeared. Apparently the plant has transformed 

 into oxygen the carbonic acid formed by the burning 

 candle. We might have given another form to the 

 experiment : we might have put a mouse under the 

 bell instead of the candle and taken its death as a proof 

 of the fact that the air under the bell no longer con- 

 tained a sufficient quantity of oxygen for respiration. 

 By then introducing a green branch under it and 

 exposing it to the sun, we could have restored to the air 

 its power of supporting respiration by giving back to it 

 oxygen. 



Hitherto in all our experiments we have studied only 

 qualitatively the transformation of carbonic acid into 

 oxygen under the influence of the plant, or rather we 

 have estimated the relation between the disappearance of 

 carbonic acid and the appearance of oxygen only approxi- 

 mately. For the quantitative study of this phenomenon 

 science is provided with other methods incomparably 

 more precise, but the description of them would be 



