12 LIFE IN NATURE. 



explosion of gunpowder; or what the opening of 

 the valve that permits the steam to pass into the 

 cylinder, is to the motions of the steam-engine. 

 They do not cause the action, but permit it. The 

 cause of the muscular motion is the decomposition in 

 the muscle, as the cause of the motion of the piston 

 is the expansion of the steam ; it is the relaxing 

 of the tension. In the muscle, the chemical affinity 

 on the one hand, and a force which we will call, 

 provisionally, the vital force on the other, exist in 

 equilibrium ; the stimulus overthrows this equili- 

 brium, and thus calls forth the inherent tendency 

 to change of state. Magnets lose for a time their 

 magnetic property by being raised to a red heat; 

 if, therefore, to a magnet holding a weight suspended 

 heat enough were applied, it would permit the fall 

 of the weight. It is thus the stimulus "permits" 

 the function. 



So one of the most perplexing circumstances con- 

 nected with the phenomena of life becomes less 

 difficult to understand ; namely, that the most vari- 

 ous and even opposite agencies produce, and may be 

 used by us to produce, the same effects upon the 



