GG M. Pringsheim on the Action of Light 



The destruction does not take place in media free from 

 oxygen. 



If the experiments be made in a so-called microscopic gas- 

 chamber, through which various gases can be passed during 

 the experiment, the effects above described take place only in 

 atmospheric air and in media containing oxygen ; but they 

 constantly fail to appear, under otherwise like conditions, even 

 if the experiment be continued twice or three times as long, 

 in hydrogen or media containing no oxygen. But both when 

 atmospheric air passes through the gas-chamber, and when 

 oxygen is substituted for the air, the absorptions of the chlo- 

 rophyll remain unaltered, and the thermal actions of the sun's 

 image at least equal. If, however, the experiment be exces- 

 sively prolonged, in hydrogen, disturbances, it is true, will be 

 seen to occur ; but these, even in their first stages, are essen- 

 tially different from those described, and are easily shown to 

 be thermal effects. 



Even in a mixture of pure hydrogen and carhonic acid 

 cleared as much as possible of free oxygen, under these circum- 

 stances neither in any colour nor in white itself does any 

 photochemical action appear : the green cell remains therein 

 perfectly green and in every respect underanged. 



On the other hand, the abstraction of the carbonic acid has 

 not the slightest influence upon the occurrence of the action. 

 In air containing oxygen, from which by all possible means 

 the carbonic acid is abstracted before it enters the gas-chamber, 

 the decolorizing of chlorophyll, the destruction and death of 

 the cell take place as rapidly as in media containing carbonic 

 acid. 



The conclusions to be drawn from these experiments are 

 clear and simple. 



If we in the first place stop at the action of the light upon 

 the green colouring-matter, we find in the demonstrable de- 

 pendence of the phenomenon upon the presence of oxygen, 

 and in its independence of the abstraction of carbonic acid, 

 proof that the destruction of chlorophyll by light in the living 

 plant is an act of combustion influenced and promoted by the 

 light , and stands in no relation to the decomposition of carbonic 

 acid by the plant. 



By varying the experiment, e. g. shortening its duration by 

 stopping the action before complete destruction of the colouring- 

 matter in the chlorophyll-bodies, it can further be shown that 

 the cell and, in like manner, the individual chlorophyll-grain 

 are incapable of restoring the destroyed colouring- matter of the 

 partly decolorized chlorophyll-bodies, although with so brief 

 a duration of the action of the lisrht the cell behaves in all 



