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 



