28 THE VITAL FUNCTIONS. 



substances are rendered more disposed to separate from the 

 fluid, and to become consolidated on the sides of the cells or 

 vessels, to which they are conducted from the leaves. This, 

 then, is the first modification in the qualities of the sap which 

 it undergoes in those organs. 



4. deration of the Sap. 



A CHEMICAL change much more considerable and im- 

 portant than the preceding is next effected on the sap by the 

 leaves, when they are subjected to the action of light. It 

 consists in the decomposition of the carbonic acid gas, which 

 is either brought to them by the sap itself, or obtained di- 

 rectly from the surrounding atmosphere. In either case its 

 oxygen is separated, and is disengaged in the form of gas; 

 while its carbon is retained, and composes an essential in- 

 gredient of the altered sap, which, as it now possesses one of 

 the principal elements of vegetable structures, may be con- 

 sidered as having made a near approach to its complete assi- 

 milation, using this term in the physiological sense already 

 pointed out. 



The remarkable discovery that oxygen gas is exhaled 

 from the leaves of plants during the day time, was made by 

 the great founder of pneumatic chemistry, Dr. Priestley: to 

 Sennebier we are indebted for the first observation that the 

 presence of carbonic acid is required for the disengagement 

 of oxygen in this process, and that the oxygen is derived 

 from the decomposition of the carbonic acid, and these lat- 

 ter facts have since been fully established by the researches 

 of Mr. Woodhouse, of Pennsylvania, Mr. Theodore de Saus- 

 sure, and Mr. Palmer. They are proved in a very satisfac- 

 tory 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 communication between 



