SENEBIER 21 



of his experiments, make these thirteen vohimes very tedious reading. 

 No wonder the historians were not too kind to Senebier. Harvey-Gibson, 

 in his Outlines of the History of Botany, denied him the recognition of the 

 one important step which he made beyond the confirmation of the dis- 

 coveries of Ingen-Hoiisz, and which was credited to him by Sachs and 

 Pfeffer — the realization of the part plaj'ed by "fixed air" (carbon dioxide) 

 in photosynthesis. 



The words "fixed air" do not occur at all in Ingen-Housz' first book 

 (1779), and in the second one (1789) they appear almost exclusively in 

 connection with the description of the products of plant respiration. He 

 found the effect of large ciuantities of "fixed air" to be deleterious to the 

 air-purifying activity of plants, and that of sniall quantities indefinite. 

 If chemical equations had been known at that time, Ingen-Housz would 

 have written the equation of photosynthesis in the following form: 



plants 

 (2.1) Air + light > something "phlogisticated" in the plants + 



dephlogisticated air 



He had no definite conception as to what kind of "air" is used for this 

 transformation. He even considered the possibilit.v of substituting, for 

 "common air" in (2.1), "inflammable air" (hydrogen), or water vapor. 

 Senebier, on the other hand, was aware even in his first work, pub- 

 lished in 1782, of the accelerating effect of "fixed air" on the production 

 of "pure air" by plants and wrote, "II paroit clairement, que Fair, fourni 

 par les feuilles exposees sous I'eau au soleil, est I'effet d'une combination 

 particuliere de I'air fixe, operee dans la feuille par le moyen du soleil," 

 and in another place, " Je n'admets pas que Fair commun de I'atmosphere 

 se tamise dans les feuilles des \'egetaux pour y deposer sa portion phlogis- 

 ticiue, et en sortir air dephlogistique apres cette depuration." This last 

 remark was directed against the hypothesis of Priestley, but apjilied 

 ('([ually well to the ideas of Ingen-Housz. As alternative to the picture 

 of a transformation of ordinary air into pure "dephlogisticated air," 

 Senebier suggested "que Fair fixe, dissous dans I'eau, est la nourriture que 

 les plantes tirent de I'air qui les baigne, et la source de I'air pur qu'elles 

 fournissent par I'elaboration qu'elles lui font subir." The transformation 

 of "fixed air" into "pure air" is the main subject of his second book 

 (1783), as shown by its title, and in his Experiences (1788) a special 

 chapter deals with the proof that "I'air rendu par les plantes exposees 

 au soleil, est le produit de I'elaboration de I'air fixe par le moyen de la 

 lumiere." Senebier shows convincingly, in polemics against Ingen-Housz, 

 who even in 1784 denied that fixed air is necessary for the "purification" 

 of air bj' plants, that no "dephlogisticated air" is formed from distilled 

 water, even if it is saturated with common air, provided the latter is free 

 from "fixed air." 



