LIGHT AND LIFE. 107 
diations which are called chemical or ultra-violet rays. The 
first affect the thermometer, the last occasion energetic 
reactions in chemical compounds. What is their influence 
upon vegetation ? Does solar light act by its colored rays, 
its heat-rays, or its chemical rays? 
The question has been subjected to many important ex- 
periments, and is, perhaps, not yet determined. Daubeny, 
in 1836, was the first to watch the respiration of plants in 
colored glasses, and he found that the volume of oxygen 
released is always less in the colored rays than in white 
light. The orange rays appeared to him most energetic ; 
the blue rays coming next. A few years later, Gardner, 
in Virginia, exposed young, feeble plants, from two to three 
inches long, to the different rays of the spectrum, and ob- 
served that they regained a green color with a maximum 
rapidity under the action of the yellow rays and those near- 
est them. In one of his experiments, green color was pro- 
duced, under the yellow rays, in three hours and a half; un- 
der orange rays, in four hours and a half; and under the blue, 
only after eighteen hours. Thus it is seen that the highest 
force of solar action corresponds neither with the maximum 
of heat, which is placed at the extremity of the red, nor 
with the maximum of chemical intensity, situated in the 
violet, at the other edge of the spectrum. Those radiations 
which are most active, from a chemical point of view, are 
the ones which have the least influence over the phenomena 
of vegetable life. 
Mr. Draper, at present a professor in the New York Uni- 
versity, and the author of a very remarkable history of the 
‘Intellectual Development of Europe,” undertook new and 
more accurate experiments about the same time. He placed 
blades of grass in tubes filled with water which was charged 
with carbonic gas, and exposed these tubes, near each other, 
to the different rays of the solar spectrum. Then, measur- 
ing the quantity of oxygen gas disengaged in each one of 
