222 THE PHENOMENA OF MOTION 



This intensity of action or of resistance the plant 

 obtains by means of the sun's light ; the effect of which 

 in chemical actions may be, and is, compared to that 

 of a very high temperature (a moderate red heat). 



During the night an opposite process goes on in the 

 plant ; we see then that the constituents of the leaves 

 and green parts combine with the oxygen of the air, 

 a property which in daylight they did not possess. 



From these facts we can draw no other conclusion 

 but this ; that the intensity of the vital force diminishes 

 with the abstraction of light ; that with the approach 

 of night a state of equilibrium is established, and that 

 in complete darkness all those constituents of plants 

 which, during the day, possessed the power of separat- 

 ing oxygen from chemical combinations, and of resisting 

 its action, lose this power completely. 



A precisely similar phenomenon is observed in ani- 

 mals. 



The living animal body exhibits its peculiar manifes- 

 tations of vitality only at certain temperatures. When 

 exposed to a certain degree of cold, these vital phe- 

 nomena entirely cease. 



The abstraction of heat must, therefore, be viewed 

 as quite equivalent to a diminution of the vital energy ; 

 the resistance opposed by the vital force to external 

 causes of disturbance must diminish, in certain tempera- 

 tures, in the same ratio in which the tendency of the 

 elements of the body to combine with the oxygen of 

 the air increases. 



