Page:Popular Science Monthly Volume 2.djvu/312

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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 observed that they regained a green color with a maximum rapidity under the action of the yellow rays and those nearest them. In one of his experiments, green color was produced, 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 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 University, 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 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, Cloëz and Gratiolet discovered the singular 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 orange 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-settled results, Cailletet added a new fact, that green light acts on vegetation 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 questioned 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