22 LIFE IN NATURE. 



pounds in which the free manifestation of chemical 

 force has been impeded by other forces, a blow or 

 mechanical friction, or the contact of a substance 

 the particles of which are in a state of transforma- 

 tion, or any external cause whose activity is added 

 to the stronger attraction of the elementary particles 

 in another direction, may suffice to give the pre- 

 ponderance to the stronger attraction, and to alter 

 the form and structure of the compound." * 



And that a chemical change in the nervous tissue 

 does ensue from the action of the stimulus, is 

 proved by the fact that the same stimulus will not 

 reproduce the effect until after the lapse of a 

 certain interval. The necessity of time for the 

 renewal of the irritability is evidence of an altered 

 composition. 



And may we not, in this light, form a clear 

 and natural conception of the nervous force? A 

 galvanic current, we know, results from chemical 

 change in inorganic bodies. But when the nerves 

 of any part are stimulated a chemical change is set 



* Liebig. 



