PLANTS AND TilE ATMOSPHERE. 421 



the details, that plants and animak perform opposite fanctions, the latter render- 

 ing air unsuitable for the suppoi't of their life, the former repairing this evil. 

 The Royal Society of London, in 1*773, offered Priestley the Copley medal, and, 

 in presenting him with it, the president of this famous body thus characterized 

 Priestley's discovery : " Plants do not grow in vain ; each individual in the vege- 

 table kingdom, from the forest oak to the grass in the meadows, is useful to man- 

 kind. All plants preserve our atmosphere in a degree of purity necessary to 

 animal life. Even the forests of the most distant countries contribute towards 

 our preservation by feeding upon the exhalations from our bodies which have 

 become injurious to ourselves." Tlie glory of Priestley was, however, doomed to 

 be obscured. After such noble exertions, views so great and so general, after 

 these public rewards and eulogiums, Priestley, one day, took it into his head to 

 repeat his first experiments and obtained results diametrically opposite, that is to- 

 say, that plants, instead of purifying the air, seemed to him then to render it more 

 impure. Astonished at this inexplicable contradiction between the past and the 

 present, he multipled his tests by varying them and the only_ thing that he was 

 thereby enabled to aflSim was, that plants exhibit, alternately, the property of 

 purifying and that of vitiating the atmosphere. The law, therefore, which had 

 won for him the Copley medal was not a general one, and the consequences which 

 he had drawn from it were liable to dispute. Seeking refuge in America, 

 Priestley died in 1804, after a life agitated by religious discussions, having made 

 great discoveries in chemistry which he had not understood, and in vegetable 

 physiology, contradictory experiments that he was unable to reconcile. Priestley> 

 however, was noway mistaken ; plants do, in reality, perform alternately the two 

 functions which he had assigned to them, and the only thing that he had not 

 discovered was the condition which determines the occurrence, often, of the one, 

 the repairing function, and sometimes, of the other, the deleterious action ; a con-, 

 dition of which Bonnet had caught a glimpse and which Ingen-Housz was about 

 to make perfectly clear. Ingen-Houez was born at Breda, in I'ZSO ; he was a 

 physician, and went to England to study inoculation for small pox which was 

 then beginning to draw attention. It was in this voyage that he placed himself 

 upon the track of Priestley's labors and that be resolved to explain their contra- 

 dictions ; he found the cause of them in 1719, and here is how he sums up his own 

 discovery: "Hardly was I engaged in these researches before the most interesting 

 scene opened up before my eyes. I observed that plants have, not only "the 

 power of correcting the impurity of air in six or more days as the experiment of 

 Mr. Priestley seemed to indicate, but that they acquit themselves of this im- 

 portant duty in the most complete manner within a few hours ; that this wonder- 

 ful operation is not in any way due to vegetation, but to the influence of the sun's 

 light upon plants ; that it only commences some time after the sun rises above 

 the horizon and that it is entirely suspended during the darkness of night; that 

 plants shaded by high buildings or by other plants do not acquit themselves of 

 this duty, that is to say, do not ameliorate the atmosphere, but, on the contrary, 

 exhale noxious air and spread veritable poison in the atmosphere which surrounds 

 us ; that tlie production of good air begins to languish towards the close of the 

 day, and entirely censes with sunset; that all plants corrupt the surrounding air 



