422 PLANTS AND THE ATMOSPHERE. 



during the night ; that all the parts of plants are not employed in purifying the 

 air but only the leaves and green branches ; that acrid, fetid and even poisonous 

 plants acquit themselves of this office equally with those -which give forth the 

 sweetest odors and are the most wholesome, etc.*" 



Ingen-Housz attained also to the discovery of the force which determines the 

 respiration of plants ; this force, which had not even been guessed at, comes from 

 the sun; it is light. It diffuses itself in leaves, which absorb it, and accomplishes 

 the immense work of regenerating the atmosphere. Henceforward, the most 

 important as well as the most difficult step was taken ; but there still remained 

 fully as much to be performed. The sciences may be compared to the tub of 

 the daughters of Danaus ; each labors to fill it and none succeed, because every 

 discovery unveils a new horizon and removes farther away an end which is 

 never attained. According to Ingen-Housz, the question ought to be asked and 

 was indeed asked ; what is this change, determined in the atmosphere by animals> 

 and in what does the remedy which plants apply to it consist J It is the duty of 

 chemistry to reply, and, although he had not specially devoted himself to that 

 science, it was Lavoisier who gave the solution of this new problem. He found 

 it, the day on which he demonstrated that animals absorb oxygen, slowly consume 

 the organic substances on which they feed, and give forth, by expiration, a quan- 

 tity of carbonic acid containing all the carbon which they have consumed. 

 Vitiated or corrupted air, as Priestley and Ingen-Housz called it, was, therefore, 

 air deprived of oxygen and charged with carbonic acid, and, since plants purify 

 it, this fact seemed incontestably to show that they decompose carbonic acid, 

 retaining the carbon and restoring the oxygen to the atmosphere. 



Judging from the point to which chemistry had then attained it would seem 

 that everybody might have divined and made public this explanation. It was 

 not 60 however, and new experiments were still necessary in order to its discovery. 

 It was a Genevese who had commenced this long campaign and it was another 

 Genevese who had the honor of terminating it. His name was Sennebrier ; he 

 had been the friend of Charles Bonnet ; it was owing to his example that he had 

 embraced science, and, to his counsels that he studied the relations of plants to 

 the atmosphere. He found that plants placed in water which had been boiled do 

 not evolve any gas to the sun, but that they develope an abundant supply of 

 oxygen when this water has, beforehand, been charged with carbonic acid. He 

 thence concluded that this gas is necessary to the respiration of plants, that it is 

 decomposed by them, and thus he had the glory of giving a formula to the law 

 already prepared and discovered by his predecessors. The question might then 

 rightly be considered as solved ; but, during these researches, which had lasted 

 for more than half a century, many errors had crept in among the truths acquired, 

 and contradictory assiertions still left ample field of doubt upon various points of 

 detail. A review of all these phenomena was necessary ; this was undertaken by 

 Th, de Saussure, who, without adding any crowning fact to the pile of former 

 acquisitions, succeeded in giving to them an experimental confirmation that has 

 not since been contested. After these celebrated experiments there was a long 



• Experiences sur les v6g6taux, par T. Ingen-Housz, 1780, 



