20 OF THE ASSIMILATION OF CARBON. 



volume of oxygen for every volume of carbonic acid 

 which has beeji decomposed. 



This remarkable property of plants has been 

 demonstrated in the most certain manner, and it 

 is in the power of every person to convince himself 

 of its existence. The leaves and other green parts 

 of a plant absorb carbonic acid, and emit an equal 

 volume of oxygen. They possess this property 

 quite independently of the plant ; for if, after being 

 separated from the stem, they are placed in water 

 containing carbonic acid, and exposed in that con- 

 dition to the sun's light, the carbonic acid is, after 

 a time, found to have disappeared entirely from 

 the water. If the experiment is conducted under 

 a glass receiver filled with water, the oxygen 

 emitted from the plant may be collected and exa- 

 mined. When no more oxygen gas is evolved, it 

 is a sign that all the dissolved carbonic acid is 

 decomposed ; but the operation recommences if a 

 new portion of it is added. 



Plants do not emit gas when placed in water 

 which either is free from carbonic acid, or contains 

 an alkali that protects it from assimilation. 



These observations were first made by Priestley 

 and Sennebier. The excellent experiments of De 

 Samsure have further shown, that plants increase 

 in weight during the decomposition of carbonic 

 acid and separation of oxygen. This increase in 

 weight is greater than can be accounted for by the 

 quantity of carbon assimilated ; a fact which con- 



