LECT. IX. ACTION OF LIGHT ON VEGETABLES. 215 



more quickly and completely changed into frogs under the 

 same condition. 



The colours of animals are brighter in proportion to the 

 intensity of the light to which they are submitted. It has 

 been asserted, that the quantity of carbonic acid exhaled by 

 the skin of an animal, is augmented by the action of the 

 solar rays. But not knowing which of the rays of the sun 

 produce these effects, we do not know whether these effects 

 are due to the chemical action of the rays, however proba- 

 ble this may otherwise appear. 



Action of Light on Vegetables. The action of light on 

 vegetables, although still obscure, is better known with re- 

 spect to its laws, and exercises a great influence upon the 

 life of these beings. It has been proved, that the respira- 

 tion of a plant, namely the decomposition of carbonic acid 

 effected by the green parts, the fixation of carbon, and the 

 exhalation of oxygen, take place only under the influence of 

 solar light : in darkness the plant, on the contrary, absorbs 

 oxygen and emits carbonic acid. In light, vegetables be- 

 come coloured, and their tissues harden ; whilst, in dark- 

 ness, they lose their colours, and their stems elongate and 

 become soft. A very vivid artificial light acts like that of 

 the sun, although in a much feebler degree. We possess 

 only a single fact capable of explaining this singular action 

 of the sun. It has been observed, in executing images 

 with the Daguerreotype, that the green parts of vegetables, 

 and, in general, all green bodies, are not represented, being 

 the contrary to what takes place with objects of other co- 

 lours. Now, since it is well established that, in the forma- 

 tion of images, by the well-known process of Daguerre, 

 these are owing to the influence of the chemical rays of solar 

 light, we are compelled to assume that the green parts are not 

 produced because they entirely absorb the rays. A very 



