1 70 Dr. Draper on Decomposition of Carbonic Acid by Leaves 



the decomposition in the prismatic spectrum. An account of 

 this was read by Dr. Kane to the British Association at its 

 meeting at Cork, and a memoir on the subject published in 

 the Phil. Mag. (Sept. IS^S). The result of these experiments 

 completely proved the power of yellow light in producing the 

 decomposition. The activity of the different rays seems to 

 follow very closely their illuminating power. Experiments 

 were also undertaken in Virginia, at my suggestion, by Dr. 

 Gardner who resided there, with a view of determining di- 

 rectly whether the greening of leaves was produced by the 

 same rays. As a matter of proof these experiments were not 

 however absolutely required, because in assuming, as I have 

 uniformly done, that the decomposition of carbonic acid and 

 the production of chlorophyll are the same phaenomenon, I 

 merely follow all botanical writers. These experiments proved 

 that the green colour is produced by yellow light, the other 

 rays following in the order of their illuminating power (Phil. 

 Mag., Jan. 1844<). This is the same result as before; I have 

 caused these experiments to be repeated in one of my labora- 

 tories in New York, and need scarcely add that they are ve- 

 rified. 



It is therefore with no small surprise that I read in Mr. 

 Hunt's recent treatise, entitled " Researches on Light," the 

 following passage in reference to the memoir published in the 

 Phil. Mag. (Sept. 1843), to prove that the yellow rays of light 

 and not the blue and violet chemical rays brought about the 

 decomposition in question. Mr. Hunt says, "Since this an- 

 nouncement, the very gloomy and uncertain state of the weather 

 has almost entirely prevented my testing the correctness, or 

 otherwise, of Dr. Draper's results. During a few faint gleams 

 of sunshine I have repeated the experiments in Draper's own 

 method, and I have found that bubbles of air have been libe- 

 rated in the tubes under the influence of the yellow and red 

 rays, but they have been carbonic acid. In the tubes which 

 were placed in the blue and violet rays alone, a perfect decom- 

 position had taken place, and the bubbles which were collected 

 were pure oxygen gas." — Researches, p. 198. 



Having recently, as the readers of this Journal know, ex- 

 pressed myself very plainly as to the character of Mr. Hunt's 

 book, I do not wish to make any allusion here, except in so 

 far as my experiments are called in question. It is true that 

 I cannot understand how an experiment, which in America 

 requires a most brilliant sun, and which involves some delicate 

 manipulation, is to be made in a very " gloomy and uncertain 

 state of the weather," and amid " a few faint gleams of sun- 

 shine." It would seem however that in England it can. 



