CHAP, ir.] of Plants. Senebier. 495 



cause why plants at certain times vitiate the surrounding air, 

 a cause which neither Priestley nor Scheele had suspected. 

 He had discovered, he says, in the summer of 1779, that all 

 vegetables incessantly give out carbonic acid gas, but that the 

 green leaves and shoots only exhale oxygen in sun-light or clear 

 day-light. It appears therefore that Ingen-Houss not only dis- 

 covered the assimilation of carbon and the true respiration of 

 plants, but also kept the conditions and the meaning of the 

 two phenomena distinct from one another. Accordingly he 

 had a clear idea of the great distinction between the nutri- 

 tion of germinating plants and of older green ones, the in- 

 dependence of the one, the dependence of the other, on light ; 

 and that he considered the carbon dioxide of the atmosphere 

 to be the main if not the only source of the carbon in the 

 plant, is shown by his remark on a foolish assertion of Hassen- 

 fratz that the carbon is taken from the earth by the roots ; he 

 replied that it was scarcely conceivable that a large tree should 

 in that case find its food for hundreds of years in the same 

 spot. There was a certain boldness in these utterances of 

 Ingen-Houss, and a considerable confidence in his own con- 

 victions, for at that time the absolute amount of carbon dioxide 

 in the air had not been ascertained, and the small quantity of 

 it in proportion to the other constituents of air would certainly 

 have deterred some persons from seeing in it the supply of the 

 huge masses of carbon which plants accumulate in their 

 structures. 



Before Ingen-Houss in the work last mentioned explained 

 the results of his observations of 1779 in accordance with the 

 new chemical views, and laid the foundations of the doctrine 

 of nutrition in plants, JEAN SENEBIER \ of Geneva, made pro- 



1 Jean Senebier, bom at Geneva in 1742, was the son of a tradesman, and 

 after 1 765 pastor of the Evangelical Church. On his return from a visit to Paris 

 he published his ' Moral Tales,' and at the suggestion of his friend Bonnet 

 competed for a prize offered at Haarlem for an essay on the Art of Observation. 

 He was awarded the second place in this competition. In 1 769 he became 



