312 The Physiology of Plants BOOKIII 



quantity of carbon dioxide is made only two or three times 

 the normal amount. 



THE ENERGY RELATIONS. While the investigations just 

 discussed were occupying the attention of workers in the 

 domain of biological chemistry, others were being pursued 

 with equal earnestness arid perhaps greater or more com- 

 plete success in the realm of physics. The connexion 

 between light, chlorophyll, and the work of carbohydrate 

 formation was full of suggestiveness from this point of 

 view, and naturally led to the study of the sources of 

 energy for the work together with the methods of its 

 utilization. 



As we have seen, the optical properties of chlorophyll 

 had been investigated prior to 1860, and its absorption 

 spectrum had been mapped out by Brewster in 1834 and 

 by Draper some ten years later. A belief slowly grew up 

 that the photo-chemical process in the green leaf ought to 

 be considered a function of the total illumination it receives. 

 This was probably largely due to the fact that Draper's 

 results seemed to show that the maximum decomposition 

 of carbon dioxide was brought about by the yellow rays, 

 which are the most luminous a view which received very 

 strong support from Pfeffer, and was maintained by him 

 many years later. 



It was not till about 1868-70 that investigations were 

 seriously undertaken with a view to establish the exact 

 relations between the several factors. The first workers 

 in the field were Timiriazeff, Lommel, Miiller, and Pfeffer ,- 

 whose investigations proceeded separately but synchron- 

 ously for several years. In his first Russian paper, pub- 

 lished in 1868, Timiriazeff set out the problem which he 

 placed before himself : 



' To study the chemical as well as the physical condition 

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

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



