ix Leaves 169 



Connect the end of the exit tube with a glass cylinder 

 containing clear lime-water or baryta-water, and beyond 

 that through a perforated cork with a large vessel half 

 filled with water. It is evident that if we * allow water 

 to flow out from the base of the large vessel, and manage 

 the tubes rightly, there will be a passage of air from 

 above the seedlings, through the baryta-water cylinder, 

 into the final large vessel. As to the tube which enters 

 the vessel with the seedlings, connect this also with a 

 cylinder containing clear baryta-water, and beyond that 

 with an open tube packed with caustic potash. Now 

 set the apparatus agoing. As water flows from the 

 base of the terminal vessel, air passes since the whole 

 system is continuous through the caustic potash tube, 

 through the first baryta-water cylinder, through the vessel 

 with the seedlings, through the second baryta-water 

 cylinder, into the terminal vessel. Now the air which 

 enters is robbed of its carbonic acid gas by the caustic 

 potash, and the continued clearness of the first baryta- 

 water proves this. But the air which passes into the 

 second baryta-water clouds this (with a white precipitate 

 of insoluble carbonate of baryta), which proves that the 

 seedlings have been giving off carbonic acid gas. 



This experiment is not by any means the best that can 

 be made in order to prove that plants, like animals, give off 

 carbonic acid as they live, but it is one of the simplest. 

 For conclusiveness it will be necessary to use leaves instead 

 of seedlings, to estimate accurately the precise change of 

 gases, to test the difference between plants growing in dark- 

 ness and those in the light, and so on. But we have at 

 least indicated how the student may convince himself that 

 plants, like animals, breathe, for the liberation of carbonic 

 acid gas is associated with an absorption of oxygen and 

 with a slow combustion of the material of the plant, as in 



