170 PROCESS OF NUTRITION. CHAP. III. 



atmospheric air, and the water placed over mercury, 

 then the phenomenon occurs by which Hales was 

 deceived into the opinion that plants inhale air dur- 

 ing the day and give it out during the night. 

 Whilst the water is rapidly absorbed by the branch, 

 owing to the more copious perspiration of the leaves 

 during the day, the air is yet prevented from rushing 

 in by means of this very perspiration, although a 

 vacuum may be formed in the tube. The mercury 

 then ascends. But at night when the perspiration 

 and consequent absorption is but small, there is no 

 cause to counteract the intrusion of the atmospheric 

 air; the mercury again descends, and gives rise to 

 the apparently alternate inhalation and extrication 

 of air by day and by night, as understood by Hales. 

 But it is thus obvious, that the phenomenon is to 

 be ascribed to the nature of the apparatus. 

 On the The action and influence of oxygene are equally 

 conspicuous in the developement of the flower as in 

 the other parts of the plant. The flower-bud will 

 not expand if confined in an atmosphere of pure 

 nitrogene, and will fade much sooner than in an 

 atmosphere of common air. But in a confined at- 

 mosphere of common air, if placed in the shade, al- 

 though it does not alter the volume of its atmosphere, 

 at least in a perceptible degree, yet it replaces the 

 oxygene it absorbs by nearly an equal quantity of 

 nitrogene ; and in this respect the flower differs 

 from the other parts of vegetables, which when ve- 

 getating in the dark give out but little nitrogene, 



