DRAPER'S EXPERIMENTS. 311 



off during exposure to different rays of the solar spectrum. From 

 his results it appears that tk the rays which cause the decomposi- 

 tion of carbonic acid gas have the same place in the spectrum as 

 the orange, the yellow, and the green ; the extreme red, the blue, 

 the indigo, and the violet exerting no perceptible effect." 1 



Draper lays great stress upon the interesting fact previously 

 noticed by Daubeiry, that the chemical rays appear to have no 

 effect upon the work of assimilation. He does not, however, 

 offer any explanation of the curious fact that the chemical activ- 

 ity of the plant is dependent upon other rays than the chemical 

 for its excitation. 



827. The principal results obtained with submerged water 

 plants b} r Cloe'z and Gratiolet, 2 who exposed Potamogeton and 



1 A Treatise on the Forces which produce the Organization of Plants, 1844, 

 p. 177. The method of experimenting is detailed l>y Draper as follows : 

 " Having, by long boiling and subservient cooling, obtained water free from 

 dissolved air, I saturated it with carbonic acid gas. Some grass leaves, the 

 surfaces of which were carefully freed from any adherent bubbles or films 

 of air by having been kept beneath carbonated water for three or four days, 

 were provided. Seven glass tubes, each half an inch in diameter and six 

 inches long, were filled with carbonated water, and into the upper part of each 

 the same number of blades of grass were placed, care being taken to have all 

 as near as could be alike. The tubes were inserted side by side in a small 

 pneumatic trough of porcelain. It is to be particularly remarked that the 

 blades were of a pure green aspect, as seen in the water; no glistening air- 

 iilm, such as is always on freshly gathered leaves, nor any air bubbles, were 

 attached to them. Great care was taken to secure this perfect freedom from 

 air at the outset of the experiments. 



"The little trough was now placed in such a position that a solar spectrum, 

 kept motionless by a heliostat and dispersed by a flint-glass prism in a hori- 

 zontal direction, fell upon the tubes. By bringing the trough nearer to the 

 prism or moving it farther off, the different colored spaces could be made to 

 fall at pleasure on the inverted tubes. The beam of light was about three 

 fourths of an inch in diameter. In a few minutes alter the commencement 

 of the experiment the tubes on which the orange, yellow, and green light fell 

 commenced giving off minute gas bubbles ; and in about an hour and a half 

 a quantity was collected sufficient for accurate measurement. 



"The gas thus collected in each tube having been transferred to another 

 vessel and its quantity determined, the little trough, with all its tubes, was 

 freely exposed to the sunshine. All the tubes now commenced actively evolv- 

 ing gas, which, when collected and measured, served to show the capacity of 

 each tube for carrying on the process. If the leaves in one were more sluggish, 

 or exposed a smaller surface than the others, the quantity of gas evolved in 

 that tube was correspondingly less. As may be readily supposed, I never 

 could get tubes so arranged as to act precisely alike ; but after a little practice 

 I brought them sufficiently near to equality. And in no instance was this 

 testing-process of the power of each tube for evolving gas omitted after the 

 experiment in the spectrum was over." 



2 Annales de Chimie et de Physique, ser. 3, tome xxxii., 1851. p. 67. 



