170 THE DECOMPOSITION OF CARBONIC-ACID GAS. [MEMOIR X. 



rays fell commenced giving off minute gas-bubbles, and 

 in about an hour and a half a quantity was collected 

 sufficient for accurate measurement. 



In Fig. 20, a a represents the trough, and R, O, Y, etc., 

 the tubes containing the leaves and carbonated water 

 placed so as to receive the spectrum, I b. 



ROY 



Fig. 20. 



The gas thus collected in each tube having been trans- 

 ferred 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 proc- 

 ess. 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. And 

 though I could never get the tubes to act precisely 

 alike, after a little practice I brought them sufficiently 

 near for my purpose. In no instance was this testing 

 process of the power of each tube for evolving gas omit- 

 ted after the experiment in the spectrum was over. 



From the following table it appears that the rays which 

 cause the decomposition of carbonic-acid gas are the or- 

 ange, the yellow, the green ; the extreme red, the blue, the 

 indigo, and the violet exerting no perceptible effect. We 



