746 
GENERAL CONDITIONS OF PLANT-LIFE. 
If these values are erected as ordinates upon the solar spectrum, taking its 
corresponding parts as abscissae, the result, as shown in Fig. 475, is that the curve of 
the different powers of light for causing evolution of gas corresponds in the main 
with the curve of subjective brightness of the same regions of the spectrum ; but 
does not coincide with the curve of heating power. 
Pfeffer's experiments had shown that the method first employed by me for 
determining the intensity of the action of light on water-plants, viz. counting the 
number of the bubbles of gas given off in a unit of time, gave nearly the same 
results as actual measurement of the gas, the result being in fact somewhat too 
great, and inexact in inverse proportion to the amount of gas given off. He then 
applied this method to determine the amount of oxygen given off from a small 
water-plant {Elodea canadensis) when exposed to a portion 1 3 mm. in breadth of a 
very intense solar spectrum 23 cm. long. In this experiment he had the advantage 
Fig. 475. — Graphic representation of the efficacy of rays of different refrangibility in causing the evolution of oxygen, com- 
pared with their brightness and heating power. The solar spectrum A—H serves as a base, on which lines to represent the 
three different effects are erected as ordinates ; the three curves are thus obtained which represent asbimilation, brightness, 
and heat. 
of being able to determine the amount of gas given off by the same plant in all 
the regions of the spectrum in successive very short spaces of time, and of thus 
avoiding various errors of observation which inevitably accompany eudiometric ob- 
servations, or at least are very difficult to get rid of. A number of observations 
conducted in this manner gave the following result as the mean capacities for 
decomposition possessed by the different regions of the solar spectrum, yellow 
being placed at 100: — 
Red .... 25-4 
Orange 
Yellow 
Green 
Blue 
Indigo 
Violet 
63-0 
lOO'O 
37'2 
221 
13-5 
7*1 
