420 PLANTS AND THE ATMOSPHERE. 



comes to clear up all obscurities, there is need for the collected labors of severa 

 generations and the co-operation of many men of geniu%. It is not uninteresting 

 to study the history of these great discoveries, and I here undertake the recital of 

 the successive experiments which have determined he relations existing between 

 plants and the atmosphere ; I shall continue it aa far as the recent labors which 

 have recalled attention to the subject of which I treat. 



I. 

 Charles Bonnet, a physician of Geneva, towards the m'ddle of the 18th century, 

 was the first to enter, experimentally, upon the problem which occupies us. It 

 was the reading of a then celebrated woi'k " Le spectacle de la Nature" by 

 Pluche which decided his profession. He, at first, occupied himself with the 

 subject of spontaneous generation, a question already debated at that period and 

 of which, time has but served to inflame the discus^iion. He relinqiiished this 

 subject in order to treat of another, of which, perhaps, he did not foresee the pro- 

 lific nature ; he asked himself what is the function of leaves, and made two ex- 

 periments that have since remained classic. By ihe first, he proved that light 

 exercises upon the green parts of plants so lively an attraction that, being placed 

 in darkness, they direct and incline themselves towards the least openings which 

 bring daylight to them. The second demonstrated that, on being plunged into 

 water, plants give forth, under the influence of the sun, a great quantity of air ; 

 but at this point the discoveries of Bonnet were stopped ; he did not know what 

 that air was and co'uld not know it, since, at that period, the world was in utter 

 ignorance of the first principles of modern chemistry. 



Priestley, who was the rival, and, in some respects, the predecessor of Lavoi- 

 sier, was brought, by the very results of his discoveries, to study the action of 

 plants upon the atmosphere. He had just isolated the remarkable gas which 

 energetically maintains the combustion of candles and the respiration of animals, 

 and, for this reason, he had called it " vital air." He had, besides, discovered 

 that small animals shut up either in this or in atmospheric air, soon changed the 

 properties of these to such an extent that the animals ceased to live and that 

 candles were extinguished by the gases. But in reality, Priestley was not aware 

 of the true nature of oxygen, and, by a blind feeling of rivalry, refused all his 

 lifetime to adopt the theory of respiration just published by Lavoisier ; but he 

 knew, nevertheless, bow to deduce from his experiments a logical conclusion of 

 the greatest importance. Observing that these little animals vitiated the con. 

 fined air by their exhalations, he concluded that every individual of the animal 

 kingdom produced, in a continuous manner, the same efFfct upon the entire 

 atmosphere, and that they would infallibly die in it were there not in the play 

 of natural forces an inverse continuous action tending to restore the air to its 

 original purity, in proportion as it is vitiated by animal respiration. This coun- 

 terbalance, this regenerating action he sought and found in plants. Under an 

 air tight bell glass filled with air he placed an animal and a plant. The former 

 corrupted the air and died ; but at the end of a certain time Priestley discovered 

 that the latter had restored to the air its vital property or the purity necessary 

 for the support of life. It was on.e of the most Important facts in the world's 

 mechanism. From this moment it was known, without as yet entering into 



