298 THE POPULAR SCIENCE MONTHLY. 



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, ex- 

 posed young, feeble plants, from two to three inches long, to the differ- 

 ent rays of the spectrum, and observed that they regained a green col- 

 or with a maximum rapidity under the action of the yellow rays and 

 those nearest them. In one of his experiments, green color was pro- 

 duced, under the yellow rays, in three hours and a half, under 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 corre- 

 sponds neither with the maximum of heat, which is placed at the ex- 

 tremity of the red, nor with the maximum of chemical intensity, situ- 

 ated 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 University, 

 and the author of a very remarkable history of the intellectual devel- 

 opment 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 

 measuring the quantity of oxygen gas disengaged in each one of these 

 little vessels, he proved that the largest production of gas occurred in 

 the tubes exposed to the yellow and green light; the next, in the 

 orange and red rays. In 1848, Cloez and Gratiolet discovered the sin- 

 gular fact that the action of light on vegetation is more powerful when 

 it passes through roughened glass than when transmitted through 

 transparent glass. Julius Sachs, more lately, conceived the idea of 

 measuring the degree of intensity of light-action, upon aquatic plants, 

 by counting the number of gas-bubbles released by a cutting of a 

 branch exposed to the sun in water charged with carbonic acid. He 

 thus observed that the bubbles thrown off under the influence of orantre 

 light are very little less numerous than under white light, while the 

 branch put under blue light throws out about twenty times less. These 

 experiments are decisive. Neither the chemical nor the calorific rays 

 of the solar beam act on plants. The luminous rays only, and chiefly 

 the yellow and the orange, have that property. To these clearly-set- 

 tled results, Cailletet added a new fact, that green light acts on vege- 

 tation in the same way as darkness. He assigns this reason for the 

 feebleness of vegetation bathed in green light under the shade of large 

 trees. It is true, this discovery of Cailletet has been warmly ques- 

 tioned recently, but it has found defenders too, Bert among others 

 and we shall find soon that it harmonizes with the whole system of the 

 actions of light in the two kingdoms of life. 



A year ago, science had gone thus far, when a very distinguished 



