DECOMPOSITION IN THE PRISMATIC SPECTRUM. 



177 



779. The obvious inference which seems to arise from these facts is, that all the ox- 

 ygen collected is derived from the direct decomposition of carbonic acid. We shall 

 presently examine whether this is the correct inference. 



780. Having, by long boiling and subsequent cooling, obtained water free from dis- 

 solved air, I saturated it with carbonic acid gas. Some grass leaves, the surfaces of 

 which were carefully freed from any adhering 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 in- 

 serted 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 glisten- 

 ing air-film, 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. 



781. 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 horizontal di- 

 rection, fell upon the tubes. By bringing the trough nearer to the prism or moving it 

 farther off, the different coloured 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 after 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 



782. The gas, thus collected in each tube, having been transferred to another ves- 

 sel and its quantity determined, the little trough, with all its tubes, was freely exposed 

 to the sunshine. All the tubes now commenced actively evolving gas, which, when col- 

 lected and measured, served to show the capacity of each tube for carrying on the pro- 

 cess. 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 experi- 

 ment in the spectrum was over. 



TABLE OF THE DECOMPOSITION OF CARBONIC ACID BY LIGHT OF DIFFERENT COLOURS. 



783. From this, it appears that the rays which cause the decomposition 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 ef- 



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