31S	HISTORY OF CHEMISTRY         [i.ect. xv.
absorbed by that colouring matter, must be the most active
in effecting assimilation.^180 Lomrnel, on the other hand,
advocated the opinion,^181 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 moder-
ating the breathing in the plant which would otherwise
become excessive.^182
Draper endeavoured to prove that, during its action, the
light must be absorbed (Joe. cit, 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 with absorption of heat), as in the
case of the assimilation by the green parts of plants, and
those in which the chemical process takes place with the
evolution of heat, as in the case of chlorine and hydrogen.
The light appears, however, to do work in  both  cases,
although, in the latter case, it is merely preparatory work,
by which the obstacles to combination are overcome.    For
this effect a certain time  is required, and Bunsen and
Roscoe proposed to indicate this by the term photo-chemical
induction.
^180Essai de statique ciurnique dcs  tires  organises,   24.	^lrtl Fogg.
Ann. 143, 581.     383 Lichtwirkung und Chlorophyllfunction in der Pflanze!
1879,