184 DISAPPEARANCE OF OXYGEN. 



true source is to be sought in some nitrogenized compound present in the leaf, which 

 is undergoing decomposition in a regulated way. 



814. Keeping this fact clearly before us, that the source of the nitrogen found thus 

 in company with the oxygen, given off under the influence of light, is some nitrogeni- 

 zed body existing in the leaf, the following experiments will show the simple and beau- 

 tiful law under which this phenomenon is conducted. 



815. 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 completed, the total volume remains unchanged. As my experiments were 

 made with leaves immersed in water, I was desirous of proving whether, under these 

 forced circumstances, the same result would still hold good. 



816. 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 volume of the gas in two experi- 

 ments was 20, or precisely the same as that of the carbonic acid. 



817. 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 re- 

 markable conclusions. 



818. When the leaves of plants, under the influence of light, decompose 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 consumed. 



819. This disappearance of oxygen and appearance of nitrogen are thus connected 

 with each other : they are equivalent phenomena. 



820. The emission of nitrogen is thus shown not to be a mere accidental result, 

 but to be" profoundly connected with the whole physiological action. 



821. I arrive also at this conclusion from experiments of another kind. If the nitro- 

 gen that appears in company with oxygen were obtained by diffusion from gas mechan- 

 ically shut up in the parenchyma of the leaf, it is plain, in the mode of operation 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 happens 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 in the total resulting 

 volume, the quantities that had been evolved in three successive periods of examination, 

 from the beginning to the termination of the experiment, were, 



1st period, 21-8 per cent, of nitrogen. 



2d " 18-8 



3d " 26-0 " " 



