320 THE NATURE AND TREATMENT OF 



the night, the soil emits excrementitioiis vapors, which are taken 

 into the animal system by the process of respiration. In the 

 act of rumination, vapor is also enclosed in the globules of 

 saliva, and thus reaches the stomach. Many plants which, 

 during the day, may be eaten with 'impunity by cattle, actually 

 become poisonous during the night ! This, I am aware, will 

 meet with some opposition, to meet which I quote from 

 Liebig: — 



" How powerful, indeed, must the resistance appear which 

 the vital force supplies to leaves charged with oil of turpentine 

 or tannic acid, when we consider the affinity of oxygen for 

 these compounds ! 



" 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 tem- 

 perature (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, pos- 

 sessed the power of separating oxygen from chemical combi- 

 nations, and of resisting its action, lose their power completely. 



" A precisely similar phenomenon is observed in animals. 



" The living animal body exhibits its peculiar manifestations 

 of vitality only at certain temperatures. When exposed to a 

 certain degree of cold, these vital phenomena 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 temperatures, in the same ratio in 

 which the tendency of the elements of the body to combine 

 wiiii llic oxygen of the air increases." 



