HISTORICAL AND INTRODUCTORY 9 



answer Scheele, who had insisted that plants, like animals, 

 vitiate the air. It was Ingen-Housz (142) who reconciled both 

 views and showed that purification goes on in light only, whilst 

 vitiation takes place in the darkness. Jean Senebier at Geneva 

 had also arrived at the same result. He also studied the con- 

 verse problem — the effect of air on the plant, and in 1782 

 argued (259) that the increased weight of the tree in Van 

 Helmont's experiment (p. 2) came from the fixed air, " Si 

 done I'air fixe, dissous dans I'eau de I'atmosphere, se combine 

 dans la parenchyme avec la lumiere et tous les autres elemens 

 de la plante ; si le phlogistique de cet air fixe est sOrement 

 precipite dans les organes de la plante, si ce precipite reste, 

 comme on le voit, puisque cet air fixe sort des plantes sous la 

 forme d'air dephlogistique, il est clair que I'air fixe, combine 

 dans la plante avec la lumiere, y laisse une matiere qui n'y 

 seroit pas, et mes experiences sur I'^tiolement suffisent pour le 

 demontrer." Later on Senebier translated his work into the 

 modern terms of Lavoisier's system. 



2. The Modern Period, 1800- 1860. 



{a) The Foundation of Plant Physiology. — We have seen 

 that Home in 1756 pushed his inquiries as far as the methods 

 in vogue would permit, and in consequence no marked advance 

 was made for forty years. A new method was wanted before 

 further progress could be made, or before the new idea intro- 

 duced by Senebier could be developed. Fortunately, this was 

 soon forthcoming. To Theodore de Saussure, in 1804 (244), 

 son of the well-known de Saussure of Geneva, is due the quan- 

 titative experimental method which more than anything else 

 has made modern agricultural chemistry possible : which formed 

 the basis of subsequent work by Boussingault, Liebig, Lawes 

 and Gilbert, and indeed still remains our safest method of in- 

 vestigation. Senebier tells us that the elder de Saussure was 

 well acquainted with his work, and it is therefore not surpris- 

 ing that the son attacked two problems that Senebier had also 

 studied — the effect of air on plants and the nature and origin 



