ATMOSl'UKUK 



FOOD OV PLANTS. 41 



The Absorption of ( arbonic Acid by Plants. — In 1771 

 Priestley, in Eiigluud, found th;it tlie leaves of plants im- 

 mersed in water, sometimes disengaged carbonic acid, 

 sometimes oxygen, and sometimes no gas at all. A few 

 years later Ingenbouss proved that the exhalation of car- 

 bonic acid takes place in the absence, and that of oxygen 

 in the presence, of solar light. Several years more elapsed 

 before Sennebier first demonstrated that the oxygen which 

 is exhaled by foliage in the sunlight comes from the car- 

 bonic acid contained in the water in whicli the plants are 

 immersed for the purpose of these experiments. It had 

 been already noticed, by Ingenhouss, that in spring water 

 plants evolve more oxygen than in river water. We now 

 know that the former contains more carbonic acid than the 

 latter. Where the water is by accident or purposely free 

 from carbonic acid, no gas is evolved by foliage in the 

 sunlight. 



The attention of scientific men was greatly attracted 

 by these interesting discoveries ; and shortly Percival, in 

 England, found that a |)lant of mint whose roots were 

 stationed in water, flourished better when the air bathing 

 its foliage was artificially enriched in carbonic acid than in 

 the ordinary atmosphere. 



In 1840 Boussingault furnished direct proof, of what 

 indeed was hardly to be doubted, viz.: the absorption of 

 the carbonic acid of the atmosphere by foliage. 



Into one of the oiiflces iu a tbree-necked glass globe he introduced 

 and fixed air-tight the brunch of a living vine bearing twenty leaves ; 

 with another opening he connected a tube through which a slow current 

 of air, containing, in one experiment, four-lOOOOths of carbonic acid, 

 could be passed into the globe. This air after streaming over the vine 

 leaves, at the rate of about 15 gallons per hour, escaped bj' the third 

 neck into an arrangement for collecting and weighing the carbonic acid 

 that remained in it. The experiment being set in process in the sun- 

 light, it was found that the enclosed foliage removed from the current 

 of air three-fourths of the carbonic acid it at first contained. 



Influence of the Relative Quantity of Carbonic Acid. — 



De Saussure investigated the influence of various propor- 



