1903.] The Cosmical Function of the Green Plant. 425 



Bunsen and Kirchhoff had just discovered that powerful means of 

 research spectrum analysis; Sir George Stokes had applied it to 

 the study of the colouring matter of blood; Desains and Tyndall 1 

 had worked out Melloni's thermoscopic methods ; Henri Sainte-Claire- 

 Deville had made his great discovery of the dissociation of carbon 

 dioxide; Bunsen, by simplifying the methods of gas analysis, had 

 quite recently put them at the disposal of the physiologist,, 

 and lastly, Boussingault had just published his classical researches on 

 the assimilation of carbon, showing that this process could be easily 

 studied on leaves or even on pieces of leaves detached from the living 

 plant. 



From the very outset, on the first page of my first Kussian paper, 

 which appeared in 1868, I formulated the problem in all its generality 

 in the following lines : " To study the chemical as well as the physical 

 conditions of this process, to follow the solar ray that effects it directly 

 or indirectly, up to the moment when we see it vanish on being trans- 

 formed into internal work, to find out the quantitative relation between 

 the energy absorbed and the work done here lies the brilliant though 

 perhaps arduous problem in attacking which modern physiologists ought 

 to unite all their forces." 



And I may now add that the very moderate results obtained after a 

 long series of years only confirm that, at all events, I did not overrate- 

 the difficulties of the problem. 



When I first set to work, the current idea was that the photo- 

 chemical process going on in the green leaf under the influence of 

 light ought to be considered a function of its luminosity. This belief 

 was chiefly based on J. W. Draper's classical experiments on the 

 spectrum. From a theoretical point of view it seemed to me highly 

 improbable that a chemical process so essentially endothermic, and 

 consequently depending on energy of radiation, should stand in a 

 direct relation to a purely physiological property of radiation, having 

 no existence outside the organ of sight. Light, taken in the narrow 

 acceptance of the word, does not exist for the vegetable world. But 

 facts brought forward by such an authority in this line of research as 

 Draper were not to be so easily dismissed on the single ground of their 

 improbability. However, a careful study of Melloni's classical memoir 

 on the shifting of the maximum of heat with the state of purity of 

 the spectrum, brought me on the track of a considerable experimental 

 flaw in Draper's researches. His spectrum was highly impure; in 

 fact it was obtained by means of a circular aperture f of an inch in 



diameter Wollaston's narrow slit not seeming to be in general use 



at the time. A sufficient explanation was thus obtained for the coin- 

 cidence of the chemical effect with the maximum luminosity in the 

 yellow and green rays, this part of the spectrum being practically 

 white, slightly tinged with these colours, and consequently acting by 



