72 LIFE IN NATURE. 



has a constant tendency to return. The presence of 

 this same element, in the like solidified form, charac- 

 terizes also the explosive compounds ; gun-cotton, for 

 example, being formed by saturating the fibre with 

 nitrogen. It is hardly to be doubted that one great 

 element in the tendency of living structures to de- 

 compose, and to exert force, consists in the tendency 

 of the nitrogen to escape from the bondage in which 

 it is thus placed. And so a part of the activity of 

 the body would be due to the coercion, not of the 

 chemical but of the mechanical properties of its con- 

 stituents, by the union into which they have been 

 forced. 



For an example of the application of the idea of 

 Life, as a twofold operation of one force, to the details 

 of animal existence, we may refer to the development 

 of the caterpillar into the moth, which is generally 

 accompanied (as in the case of the silkworm) with a 

 special activity of secretion. The silk is produced 

 through a decomposing process ; it is less living than 

 the blood or tissues from which it is derived. Part of 

 the force that was embodied in these has been given 

 off; one portion of the creature's substance has 



