10 LITE IN NATUKE. 



thereby ; or if I compress an elastic body, my force 

 opposes elasticity, but the laws of elasticity are not 

 thereby altered. In truth, the forces of gravity 

 and elasticity thus receive scope to operate, and 

 display their laws. Just so it is in the living body. 

 The force of chemical affinity is opposed, and thereby 

 has scope to act ; its laws are not altered, but they 

 operate under new conditions. Owing to the oppo- 

 sition to chemical affinity, the living tissues ever 

 tend to decompose ; as a weight that has been lifted 

 tends to fall. 



But the living structures are not the only instances, 

 in nature, of bodies which tend to decompose. 

 There are several in the inorganic world : such 

 are the fulminating powders (iodide or chloride of 

 nitrogen, for example), which explode upon a touch. 

 There is a strong analogy between these and the 

 living tissues. In each case, there is a tendency to 

 undergo chemical decomposition ; in each case, this 

 decomposition produces an enormous amount of 

 force. Explosive powders may be compared to 

 steam that has been heated under pressure, and 

 which expands with violence when the pressure is 



