166 Dr. Draper on the Decomposition of Carbonic Acid 



Comparing these experiments, made by the aid of absorp- 

 tive media, with those made by the prism, we are enabled to 

 come to a definite conclusion as to the character of the rays 

 which cause this decomposition. 



The true office of prismatic analysis is to determine the re- 

 frangibility of the rays which produce given actions; but in- 

 asmuch as rays of heat, rays of light, and tithonic rays are 

 found throughout the spectrum, in many cases the prism fails 

 to indicate to which of these imponderable agents phaenomena 

 are to be ascribed. The case before us furnishes a striking 

 example. Although the decomposition of carbonic acid is 

 most energetically brought about by rays whose index of re- 

 fraction corresponds to the yellow, yet that region of the spec- 

 trum is far from being devoid of heat and tithonicity. 



By considering however the prismatic analysis and the ab- 

 sorptive analysis together, the following facts appear : — 1st, the 

 place of maximum action in the spectrum corresponds to the 

 maximum of illumination; 2nd, at the place of the maximum 

 of heat (which in the prism here used is beyond the extreme 

 red) no decomposition whatever takes effect ; this appears 

 therefore to exclude calorific influence; 3rd, the point of 

 maximum action of the tithonic rays, which escape absorption 

 by the bichromate of potash, being towards the green, does 

 not correspond with the place of maximum decomposition, 

 which is the yellow ; this seems to exclude the tithonic rays ; 

 4th, the decomposition taking place almost as energetically 

 under the bichromate of potash as in the unobstructed beams 

 of the sun, and that salt absorbing all but a mere trace of the 

 tithonic rays, if the effect was due to them it ought to be re- 

 tarded to an extent corresponding to their loss by absorption, 

 which is far from being the case ; the retardation which is ob- 

 served appearing to be attributable rather to the loss of light 

 by reflexion from the faces of the trough, and the partial tur- 

 bidity (want oftranslucence) of its glasses and solutions. 



For these reasons I conclude that the decomposition of 

 carbonic acid by the leaves of plants is brought about by the 

 rays of light; and that the calorific and tithonic rays do not 

 participate in the phaenomenon. As was stated before, there- 

 fore, the rays of light are just as much entitled to the appella- 

 tion of chemical rays as those which have heretofore passed 

 under that name. 



I might observe in passing, that there is a degree of preci- 

 sion attached to results of the decomposition of carbonic acid 

 which is wholly wanting in most similar experiments. In the 

 stains on Daguerreotype plates, or on photographic papers, 

 though there is no difficulty in ascertaining the place of 



