320 LORY ON" THEIR RESPIRATION. [BOOK n. 



sun in the afternoon. It is clear that the mean temperature 

 during the experiment could be taken into account in the 

 first case only. 



The following are the results to which I have been led : 



1st. The volume of the gas surrounding the plant varies but 

 very little, even when the greater quantity of oxygen is con- 

 verted into carbonic acid. The irregularities observed, free from 

 the influence of all circumstances foreign to the phenomena of 

 respiration, are sometimes one way, sometimes another, and 

 they are so trifling that they cannot be taken into account 

 where precision is all but impossible. I placed the plants in 

 a closed flask, holding about a litre, and furnished with a bent 

 tube of inconsiderable volume, which I plunged in mercury. 

 On making the necessary corrections, T never found the level 

 vary more than two millimetres, even when the experiment 

 lasted nearly three days, until nearly the whole of the oxygen 

 was converted into carbonic acid. The means of these varia- 

 tions are altogether inappreciable. 



2nd. This first result is confirmed by analysis ; the nitrogen 

 being supposed constant, the sum of the oxygen and the 

 carbonic acid remains very nearly indeed, constant ; there 

 being, however, almost always a small diminution. Inde- 

 pendently of any error arising from the solubility of the 

 carbonic acid, this result may be naturally explained by two 

 circumstances ; there is always a little nitrogen given off, and 

 a little oxygen absorbed ; two causes which act together to 

 produce the difference in question, by keeping the total volume 

 very nearly constant. 



In a mixture of air and carbonic acid, the plant behaves in 

 the same way as in air alone; but as is natural, cateris 

 parihuSj a smaller quantity of oxygen is destroyed. 



3rd. In an atmosphere of pure hydrogen, the plants which 

 are not green, disengage a large proportion of carbonic acid 

 and a little nitrogen. Thus, as it might have been expected, 

 the disengagement of these gases does not correspond, 

 directly, with the absorption of oxygen ; they are only, as 

 in the respiration of animals, definite products of the reactions 

 accomplished in the tissues. 



4th. The elevation of temperature stimulates the respira- 



