SENEBIER 19 



ceases entirely at sunset; that this office is not performed by the whole plant, but only 

 by the leaves and the green stalks; that even the most poisonous plants perform this 

 office in common with the mildest and most salutary; that the most part of leaves pour 

 out the greatest quantity of this dephlogisticated air from their under surface . . .; 

 that all plants contaminate the surrounding air by night; . . . that all flowers render 

 the surrounding air highly noxious, equally by night and by day; that roots and fruits 

 have the same deleterious quality at all times; . . . that the sun by itself has no power 

 to mend the air without the concurrence of plants." 



As revealed by this summary, Ingen-Hoiisz' main achievements were the 

 discovery of the importance of light for the "dephlogistication" of air by 

 plants, and the proof that the plants improve the air not merely by 

 absorbing its "mephitic" constituents (as suggested by Priestley), but 

 that they also actively produce "vital air" (oxj^gen). He proved this fact 

 by demonstrating the formation of oxygen bubbles by submerged leaves, 

 a technique which was to be widely used later in the study of photo- 

 sjmthesis. (Priestley had observed the oxygen bubble formation with his 

 "green matter" before; but, at that time, he did not yet realize that the 

 absorption of "bad air" by land plants and the liberation of "good 

 air" by "green matter" were merely two aspects of one and the same 

 phenomenon.) 



Ingen-Housz laid great emphasis, as shown by the title of his book, 

 on his discovery of the "poisoning" of the air by plants in the dark, and 

 he dwelt in detail on the gruesome dangers of keeping large trees in in- 

 habited rooms, or of spending nights in a closed space containing large 

 quantities of flowers, fruits or vegetables. This point became a subject 

 of violent controversy between Ingen-Housz and Senebier, the latter 

 denying any gas liberation by plants in darkness, and insisting that plants 

 produce no dangerous "effluvia" when they grow in the open air. The 

 fact of plant respiration was correctly observed by Ingen-Housz; but its 

 dangerous aspects were obviously over-stressed, perhaps for the sake of 

 philosophical satisfaction which he derived from the apposition of the 

 wholesome influence of plants during the day and their poisonous activity 

 during the night. The difference between inert gases (nitrogen and car- 

 bon dioxide) and active poisons did not become clear to the chemists 

 until much later. 



5. The Part of Carbon Dioxide : Senebier 



Ingen-Housz' haste in asserting his priority proved well founded. 

 Three years later, in 1782, Jean Senebier published, in his home town of 

 Geneva, three volumes of Memoires physico-chimiques sur Vinfluence de la 

 lumiere solaire pour modifier les etres des trois regnes de la nature et surtout 

 ceux du regne vegetal. After acknowledging in the preface the priorit}^ 

 and the importance of the work of Ingen-Housz, Senebier explained that 

 he started his experiments before the appearance of the latter 's book, 



