under the Influence of Yelloia Light, 171 



Neither do I understand how it is possible that the violet 

 ray or any other ray can cause leaves to evolve pure oxygen. 

 It has happened to me in certain investigations in vegetable 

 physiology to make more than 200 analyses of gas from vege- 

 table leaves. They have never emitted pure oxygen, but a 

 mixture of nitrogen and oxygen, and very often as much as 

 fifty per cent, of the former gas. This result is the same as 

 has been obtained over and over again for more than fifty 

 years. M. Saussure, Dr. Daubeny, and all other competent 

 writers are in agreement upon this point. 



Neither can I understand how it is possible that an analysis 

 can be made of the gas emitted by the violet ray. With a 

 brilliant sun and proper precautions taken to exclude extra- 

 neous light from the tubes, I have never been able with the 

 violet light to cause leaves to evolve a solitary huhble of any 

 Jcind of gas 'whatsoever. 



I am therefore obliged to infer that there must be some 

 error in Mr. Hunt's method of experimenting, or a want of 

 care in his manipulation. His results on the action of light 

 upon plants are undoubtedly altogether erroneous. 



The readers of the Philosophical Magazine who have pe- 

 rused my memoirs, will, I hope, have seen abundant reason to 

 understand that no facts are ever there reported without a 

 thorough evidence of their correctness. In the limited space 

 to which a periodical writer is restricted, much collateral evi- 

 dence must be laid aside, or communications would become 

 inordinately long. So, in my memoir on the decomposition 

 of carbonic acid, I might have added many other facts. 

 Among the rest I might have given analyses of the gas emitted 

 by the yellow ray. I will conclude this paper by furnishing 

 evidence on that point. 



Five tubes, each three-eighths of an inch in diameter and 

 six inches long, were inverted in a small trough of water con- 

 taining carbonic acid, with which the tubes were also filled. 

 Some blades of grass, nearly of the same size and volume, were 

 placed in each tube. This grass had been kept for two days in 

 the dark, in a bottle filled with carbonated water. During this 

 time the film of air which envelopes all new leaves was re- 

 moved, the grass became perfectly free from all adhering 

 gaseous matter, and when in the carbonated water exhibited 

 a dark green aspect. 



I have previously found that leaves thus soaked emit, under 

 the influence of the light, a larger amount of nitrogen than 

 usual; this comes from the incipient decay of some of their 

 nitrogenized constituents. When, under these circumstances, 



