30 THE VITAL FUNCTIONS. 



that the presence of carbonic acid is required 

 for the disengagement of oxygen in this process, 

 and that the oxygen is derived from the decom- 

 position of the carbonic acid ; and these latter 

 facts have since been fidly established by the 

 researches of Mr. Woodhouse, of Pensylvania, 

 M. Theodore de Saussure, and Mr. Palmer. 

 They are proved in a very satisfactory manner 

 by the following experiment of De Candolle. 



Two glass jars were inverted over the same 

 water-bath ; the one filled with carbonic acid 

 gas, the other filled with water, containing a sprig 

 of mint ; the jars communicating below by means 

 of the water-bath, on the surface of which some 

 oil was poured, so as to intercept all communi- 

 cation between the water and the atmosphere. 

 The sprig of mint was exposed to the light of the 

 sun for twelve days consecutively : at the end 

 of each day the carbonic acid was seen to dimi- 

 nish in quantity, the water rising in the jar to 

 supply the place of what was lost, and at the 

 same time the plant exhaled a quantity of 

 oxygen exactly equal to that of the carbonic 

 acid which had disappeared. A similar sprig of 

 mint, placed in a jar of the same size, full of dis- 

 tilled water, but without having access to carbonic 

 acid, gave out no oxygen gas, and soon perished. 

 When, in another experiment, conducted by 

 means of the same apparatus as was used in the 

 first, oxygen gas was substituted in the first jar 



