6 SOIL CONDITIONS AND PLANT GROWTH 



unfortunately failed to confirm his earlier results because he over- 

 looked a vital factor, the necessity of light. He was therefore unable 

 to 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 converse problem — the effect of 

 air on the plant, and in 1782 argued (262) 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'atmosph^re, se com- 

 bine dans la parenchyme avec la lumi^re et tous les autres Siemens de 

 la plante; si le phlogistique de cet air fixe est sfirement pr6cipit6 

 dans les organes de la plante, si ce pr6cipite reste, comme on le voit, 

 puisque cet air fixe sort des plantes sous la forme d'air d^phlogistiqqe, 

 il est clair que I'air fixe, combine dans la plante avec la lumi^re, 

 y laisse une matiere qui n'y seroit pas, et mes exp6riences sur 

 r^tiolement suffisent pour le demontrer." Later on Senebier trans- 

 lated his work into the modern terms of Lavoisier's system. 



2. The Modern Period, 1 800-1 860. 



We have seen that Home in 1756 pushed his inquiries as far as the 

 methods in vogue would permit, and in consequence no marked ad- 

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

 further progress could be made, or before the new idea introduced by 

 Senebier could be developed. Fortunately this was soon forthcom- 

 ing. To Theodore de Saussure, in 1 804 (243), son of the well-known 

 de Saussure of Geneva, is due the quantitative statistical method which 

 more than anything else has made modern agricultural chemistry pos- 

 sible : which formed the basis of subsequent work by Boussingault, 

 Liebig, Lawes and Gilbert, and indeed still remains our safest method 

 of investigation. Senebier tells us that the elder de Saussure was 

 well acquainted with his work, and it is therefore not surprising that 

 the son attacked two problems that Senebier had also studied — the 

 effect of air on plants and the nature and origin of salts in plants. 

 De Saussure grew plants in air or in known mixtures of air and carbon 

 dioxide, and measured the gas changes by eudiometric analysis and the 

 changes in the plant by " carbonisation ". He was thus able to 

 demonstrate the central fact of plant respiration — the absorption of 

 oxygen and the evolution of carbon dioxide, and further to show the de- 

 composition pf earbon dioxide and evolution of oxygen in light. Car- 



