ASSIMILATION OK CARBON. 35 



vitiated by the respiration of animals or by combustion.* This un- 

 expected discovery immediately arrested the attention of vegetable 

 physiologists. Nevertheless, Priestley was not yet master, so to 

 speak, of the capital experiment which he had announced to the 

 world of science. He had not seized all the circumstances which 

 assure its success. Occasionally the leaves which were the subjects 

 of experiment did not cause the disengagement of any gas ; occa- 

 sionally, too, the air disengaged, far from being oxygen — far from 

 ameliorating the atmosphere, was found to be carbonic acid gas. It 

 was Ingenhousz who made out the influence of the solar light upon 

 the phe°nomenon in question. He proved, by a vast number of dis- 

 tinct experiments, that leaves exhale oxygen when they are exposed 

 to the light of the sun. He perceived, moreover, that in the dark 

 they vitiate the air, rendering it improper for respiration and com- 

 bustion. f 



But the origin of the oxygen disengaged from water by leaves 

 exposed to the light of the sun still remained to be discovered. It 

 was Sennebier who took this important step, by showing that it was 

 to the carbonic acid generally contained in water that leaves ex- 

 posed to the sun's light owed their faculty of evolving oxygen gas. 

 With this interesting fact, it was easy to render an account of all 

 the anomalies that had been successively announced : boiled water, 

 as Bonnet had observed, could not afford any air, and spring water 

 should usually give more than river water, as Ingenhousz had no- 

 ticed, for the simple reason that boiled water neither contains 

 carbonic acid gas nor any other kind of air ; and that well water 

 generally contains a larger quantity of carbonic acid in solution than 

 river water. 



In giving the grand features in the history of this brilliant discov- 

 ery of the°eighleenth century, it may be said that Bonnet was the 

 first who observed the phenomenon of the gaseous evolution effected 

 by the leaves of vegetables ;t that Priestley announced that the gas 

 disengaged was oxygen ; that Ingenhousz demonstrated the neces- 

 sity of the solar light to the production of the phenomenon ; finally, 

 that it was Sennebier, to whom was reserved the honor of showing 

 that the oxygen gas obtained under these circumstances is the pro- 

 duct of the decomposition of carbonic acid. 



It was, however, matter of supreme interest to study this decom- 

 position of carbonic acid in its last details. It was imperative, for 

 instance, to ascertain what relation existed between the volume of 

 the oxygen disengaged and the volume of the carbonic acid decom- 

 posed. This was admirably accomplished by M. Theodore de Saus- 

 sure in a long series of remarkable experiments, of which I shall here 

 endeavor briefly to state the main results. 



The conclusion which follows naturally from the discovery of 

 Sennebier, was that carbonic acid exercised a favorable influence on 

 vegetation by supplying plants with the carbon which enters into 



* Experiments and Observations, vol. ii. 



t Experiments on Vegetables. 



X Sur I'usage ries feuilles dan* /es planter, p. 31 



