and of Carbonates by the Light of the Sun. 173 



Saussure has already determined, that when plants are 

 forced to grow in an atmosphere of known volume, containing 

 carbonic acid gas, after the decomposition of the gas is com- 

 pleted, the total volume remains unchanged. As my experi- 

 ments were made with leaves immersed in water, 1 was de- 

 sirous of proving whether under these forced circumstances 

 the same result would still hold good. 



To a certain quantity of water, from which all air had been 

 expelled, confined in a jar over mercury, I passed 20 measures 

 of carbonic acid gas ; by a little agitation the water took up 

 15*50 measures of the acid. I now introduced into the jar 

 some leaves, taking the greatest care that no bubbles of air 

 should pass along with them. The jar was then placed in 

 the sunshine, and the decomposition completed. Corrected 

 for variation of temperature and pressure, the resulting vo- 

 lume of the gas in two experiments was 20, or precisely the 

 same as that of the carbonic acid. 



We may therefore infer that the volume of mixed gases 

 evolved is precisely equal to the volume of carbonic acid that 

 disappears. This leads us to some very remarkable conclu- 

 sions. 



When the leaves of plants under the influence of light de- 

 compose carbonic acid, they assimilate all the carbon, and a 

 certain proportion of oxygen disappears, at the same time 

 they emit a volume of nitrogen equal to that of the oxygen con- 

 sumed. 



This disappearance of oxygen and appearance of nitrogen 

 are thus connected with each other ; they are equivalent phe- 

 nomena. 



The emission of nitrogen is thus shown not to be a mere 

 accidental result, but to be profoundly connected with the 

 whole physiological action. 



I arrive also at this conclusion from experiments of another 

 kind. If the nitrogen that appears in company with oxygen 

 were obtained by diffusion from gas mechanically shut up in 

 the parenchyma of the leaf, it is plain, in the mode of opera- 

 tion which I have followed, in which leaves are immersed 

 under water, and no opportunity given them of restoring their 

 mechanically included air, if it were by any means withdrawn, 

 that the first portions of mixed gas evolved should be richest 

 in nitrogen, and that the per-centage amount should gradually 

 become less and less, as it was removed from the structure of 

 the leaf; this follows from the laws of the diffusion of gases. 

 But this is far from being the case. It very commonly hap- 

 pens that more nitrogen is evolved at the close of the process 

 than at its beginning. Thus, in one of the experiments I made, 

 in which it was found that there was 22*2 per cent, of nitrogen 



