ACTION OF LIGHT ON VEGETATION. 
745 
efficacious in this process as all the others put together. The most refrangible 
rays of the visible spectrum which act most energetically on silver chloride, &c., 
play a very subordinate part in the process of assimilation.' 
Draper placed glass tubes filled with water saturated with carbon dioxide in 
which he had placed green parts of plants, in the different coloured portions of a 
solar spectrum. Seven of these tubes were exposed simultaneously in the same 
spectrum. The following table gives the result of two experiments of this kind : — 
Part of the Spectrum. 
Dark-red 
Red-orange . 
Yellow-green . 
Green-blue 
Blue 
Indigo . 
Violet . 
Gas evolved. 
Experiment I. 
o 33 
20"00 
36-00 
O'lO 
O'O 
o-o 
O'O 
Experiment II. 
O'O 
24-75 
4375 
4*io 
I'OO 
O'O 
O'O 
Pfeffer experimented chiefly on leaves of the Cherry-Laurel and Oleander, which 
were placed in air containing carbon dioxide (shut off by mercury) in suitable glass 
vessels, and received the sunlight through coloured solutions (tested by the spectro- 
scope). The following was the result of sixty-four experiments : — If the amount 
of gas evolved in light which has passed through a stratum of water of standard 
thickness is represented by 100, the numbers here given are the corresponding 
quantities of carbon dioxide decomposed in light which has passed through equal 
thicknesses of the solutions named. 
^ _ . , . , Amount of carbon dioxide 
Colour 01 light. 1 
decomposed. 
Red, orange, yellow, green 
Green, blue, violet 
Red, orange-green, blue, violet 
Red, orange-blue, violet 
Red, orange 
Red-orange, yellow, green 
Quite dark 
Solution. 
Potassium bichromate 
Ammoniacal copper oxide 
Orcin 
Aniline-violet 
Aniline-red 
Chlorophyll 
Iodine solution 
88-6 
7-6 
53'9 
38-9 
32-1 
15-9 
(14*1 carbon di- 
oxide produced). 
From a comparison of these numbers Pfeffer deduced the following values for 
the decomposing power of the different regions of the spectrum, the action of white 
light being again placed at 100: — 
For Red-orange 
Yellow 
Green . 
Blue-violet 
32-1 
46-1 
15-0 
7-6 
100 
and from these is deduced the first statement of Pfeffer given above. 
