and the Function of Chlorophyll in Plants. 65 



turgescence of the cell is annihilated ; in short, the cell 

 exhibits all the phenomena of rapid and irreparable de- 

 struction. 



These phenomena are not direct effects of a high tempe- 

 rature produced in the cell by the radiation. By varying 

 the experiment by means of coloured screens, causing the 

 rays which delineate the image of the sun to pass through 

 coloured glasses or vessels containing coloured fluids, this 

 can be rendered highly probable, as I will show in my de- 

 tailed presentation of the subject. I will here merely mention 

 that the destruction of the contents of the cell in the way 

 above portrayed takes effect in all colours. It matters not 

 whether the image of the sun is produced as a warm red 

 image behind a solution of iodine in sulphide of carbon, or 

 as a green one behind a solution of chloride of copper, or 

 as a cold blue one behind ammonio-cupric sulphate ; the 

 result is always the same, provided only that the coloured 

 screens transmit a light of sufficient intensity. 



It is, however, easily perceptible, even without photo- 

 metric measurement, that blue light exerts a more powerful 

 action than red. Behind a solution of iodine in sulphide of 

 carbon so concentrated that, except the red up to the wave- 

 length 0*00061 millim., to the human eye it transmits no 

 portion of the spectrum, especially no blue, even in direct 

 sunlight, the phenomena described will not be seen to occur 

 even with long-continued action of the sun's image, al- 

 though here at least 80 per cent, of the total heat of the 

 white image is effective, and although this red image still 

 possesses a brightness unendurable even for a very short 

 time by any human eye, and, finally, although the first two 

 strong absorption-bands of the chlorophyll-spectrum fall in 

 the red of this image of the sun, and consequently this red 

 is absorbed in considerable quantity by the chlorophyll. 



On the other hand, a rapid and powerful action always takes 

 place behind even a dark solution of ammonio-cupric sulphate, 

 which absorbs the entire less-refrangible half of the solar 

 spectrum to about the wave-length - 00051 millim., and like- 

 wise behind a screen of deep-green glass, which is but very 

 slightly diathermanous. 



But, apart from the action being independent of the greater 

 or less diathermasy of the screen, the most direct proof can be 

 adduced that the destruction wrought in the cell does not 

 depend upon the thermal action of the radiation ; for it can be 

 shown that the occurrence of all the phenomena of destruction 

 of the cell and its contents in the light is exclusively condi- 

 tioned by the presence of oxygen in the surrounding atmosphere. 



Ann. db Mag. N. Hist. Ser. 5. Vol. v. 5 



