LECTURE XV.] HISTORY OF CHEMISTRY. 327 



remarkable that the numerous experiments designed to ascer- 

 tain the maximum effect of the different parts of the spectrum 

 in the process of assimilation in plants have not led to uniform 

 results. Some find this maximum in the yellow, and others in 

 the red. The question is one of considerable importance. 



The fact recognised by Ingenhousz that the decomposition 

 of carbonic anhydride takes place in the green parts of plants, 

 soon led to the supposition that a connection existed between 

 the chlorophyll colouring matter and the chemical process of 

 assimilation; and Dumas, as early as 1844, stated the view 

 that the violet rays, which are the principal ones absorbed by 

 that colouring matter, must be the most active in effecting 

 assimilation. 176 Lommel, on the other hand, advocated the 

 opinion 177 that the rays lying between the lines B and C, of 

 Fraunhofer, might play the chief part in assimilation, because 

 they possess the greatest intensity, and also because they 

 correspond to a maximum of absorption by chlorophyll. 



Since the facts actually observed were not favourable 

 to either of these views, Pringsheim, in connection with his 

 investigations into the effect which light exercises upon the 

 processes of oxidation within the plant organism, advanced the 

 hypothesis and endeavoured to establish it that the chlorophyll 

 colouring matter is not the chemically active substance, but 

 that it merely serves as a screen in moderating the breathing in 

 the plant which would otherwise become excessive. 178 



Draper endeavoured to prove that, during its action, the 

 light must be absorbed (loc. at., Note 173). Bunsen and 

 Roscoe instituted quantitative experiments on this point, from 

 which it appears that, in the case of chlorine and hydrogen, 

 about one-third of the rays absorbed are used up in effecting 

 chemical work. But there are two kinds of cases which must 

 be distinguished : namely, those in which the light must supply 

 the energy necessary for the chemical process (which proceeds 



176 Essai de statique chimique ties etres organises, 24. 177 Pogg. 

 Ann. 143, 581. 178 Lichtwirkung und Chlorophyllfunction in der Pflanze. 

 1879. 



