THEIR MODE OF ACTION. 393 



Such an influence and no other is exercised by the 

 vital principle over the chemical forces ; but in every 

 case where combination or decomposition takes 

 place, chemical affinity and cohesion are in action. 



The vital principle is only known to us through 

 the peculiar form of its instruments, that is, through 

 the organs in which it resides. Hence, whatever 

 kind of energy a substance may possess, if it is 

 amorphous and destitute of organs from which the 

 impulse, motion or change proceeds, it does not live. 

 Its energy depends in this case on a chemical action. 

 Light, heat, electricity, or other influences may in- 

 crease, diminish, or arrest this action, but they are 

 not its efficient cause. 



In the same way the vital principle governs the 

 chemical powers in the living body. All those sub- 

 stances to which we apply the general name of food, 

 and all the bodies formed from them in the organism, 

 are chemical compounds. The vital principle has, 

 therefore, no other resistance to overcome, in order 

 to convert these substances into component parts of 

 the organism, than the chemical powers by which 

 their constituents are held together. If the food pos- 

 sessed life, not merely the chemical forces, but this 

 vitality, would offer resistance to the vital force of 

 the organism it nourished. 



All substances adapted for assimilation are bodies 

 of a very complex constitution ; their atoms are 

 highly complex, and are held together only by a 

 weak chemical action. They are formed by the union 

 of two or more simple compounds ; and in propor- 

 tion as the number of their atoms augments, their 

 disposition to enter into new combinations is dimin- 

 ished ; that is, they lose the power of acting chem- 

 ically upon other bodies. 



Their complex nature, however, renders them 

 more liable to be changed, by the agency of external 

 causes, and thus to suff*er decomposition. Any ex- 

 ternal agency, in many cases even mechanical friction, 

 is sufficient to cause a disturbance in the equilibrium 



