304 



LECTURE XVIII. 



was placed in a second, wider cylinder, and the space between both filled with 

 one or other of the solutions previously mentioned, or with pure water. After 

 careful consideration and preparation, I employed as a measure of the decomposition 

 of carbon dioxide in the plant, the number of bubbles which escaped from the cut 

 surface of the stem in one minute ^. It was now possible to conduct the investigations 

 m such a manner that the plant could be observed alternately for one minute re- 

 spectively in white complete hght, in red-yellow, or in blue-violet light, one 

 immediately after the other, and the gas-bubbles counted. It turned out that in 

 the blue-violet light only very little carbon dioxide was decomposed, while (having 

 regard to accessory circumstances) the effect in red-yellow light was nearly as 



strong as in the full light which passed 

 through pure water. This result, as well 

 as the observations previously made by 

 Daubeny, Draper (1844), Cloe2 and Gra- 

 tiolet (1851), contradicted the prevailing 

 view of the physicists and chemists, that it 

 is the blue-violet part of the spectrum 

 I which almost alone brings about photo- 

 chemical effects. The decomposition of 

 carbon dioxide in the plant evidently de- 

 pends upon a photo-chemical effect; and 

 yet we here see that that portion of the 

 spectrum which is distinguished by phy- 

 sicists as the one chemically effective, is 

 relatively inactive, while the other half 

 of the spectrum is here the effective 

 one. I directly confirmed this apparent • 

 contradiction, again, by placing in the 

 upper part of the glass cylinder contain- 

 ing the plant a small apparatus which 

 enabled me, while observing the separa- 

 tion of oxygen, simultaneously to observe 

 the effect of the coloured light on pho- 

 tographic paper. When the light passed 

 through the blue-violet solution, the evolu- 

 tion of oxygen in the plant was extremely 

 small, while the photographic paper became deep brown; when, on the other 

 hand, the red-yellow solution was interposed, the plant evolved large quantities of 

 oxygen, while the photographic paper reacted but little and feebly. 



It may here simply be remarked that it was an inaccurate generalisation on 

 the part of physics and chemistry to designate the blue-violet portion of the spectrum 

 as the part chemically active, simply because the corresponding rays of light 

 cause silver salts to decompose and a mixture of chlorine and hydrogen to form 



Fig. 220.— The glass cylinder Ci contains the plant p 

 in water, in which the thermometer t is immersed. The 

 cylinder Ci is then placed in the larger cylinder Ca, and 

 the interspace y filled with the coloured liquid. /8 sup- 

 porting hooks. A disengages carbon-dioxide, which is 

 washed in water in the flask B. 



• With respect to the method of counting the bubbles employed by me, and much discussed 

 later, cf. Pfefifer, • Pflanzen-Physiologie^ p. 215. 



