356 POISONS, CONTAGIONS, MIASMS. 



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 increase, 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 

 substances 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 over- 

 come, in order to convert these substances into 

 component parts of the organism, than the chemical 

 powers by which their constituents are held toge- 

 ther. If the food possessed life, not merely the 

 chemical forces, but this vitality, would offer resist- 

 ance 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 simpler compounds ; and in 

 proportion as the number of their atoms augments, 

 their disposition to enter into new combination is 

 diminished ; that is, they lose the power of acting 

 chemically upon other bodies. 



Their complex nature, however, renders them 

 more liable to be changed, by the agency of exter- 

 nal causes, and thus to suffer decomposition. Any 

 external agency, in many cases even mechanical 



