108 NATURE AND LIFE. 



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 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, con- 

 ceived 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 b'ght 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 deci- 

 sive. Neither the chemical nor the calorific rays of the so- 

 lar 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 harmo- 

 nizes with the whole system of the actions of light in the 

 two kingdoms of life. 1 



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

 distinguished botanist, Prillieux, published the result of a 

 course of experiments made with an entirely different pur- 

 pose, and taking up the study of the action of light from 

 a new point of view. Resting on the twofold consideration 

 that the distinctly-colored rays are not equally luminous, 



1 Bert ascertained that green light stops the motions of sensitive 

 plants. 



