1 64 Dr. Draper on the Decomposition of Carbonic Acid 



in the water; no glistening 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. 



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 direction, fell upon the 

 tubes. By bringing the trough nearer to the prism or moving 

 it further 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. 



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 evolving 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 in- 

 stance was this testing process of the power of each tube 

 for evolving gas omitted after the experiment in the spectrum 

 was over. 



Table of the Decomposition of Carbonic Acid by Light of dif- 

 ferent Colours. 



From this it appears, that the rays which cause the decom- 

 position of carbonic acid gas have the same place in the spec- 

 trum as the orange, the yellow, and the green, — the extreme 



