METABOLISM 49 



energy locked up in the ozone molecule in comparison 

 with the oxygen molecule. When we speak of this 

 energy as being locked up or " latent," we have 

 exhausted our knowledge of it. We know that it is 

 there, holding the atoms together by what we call 

 " chemical affinity," and that, when the atoms are 

 released from their bonds, this energy becomes 

 evident to our senses ; that is, it becomes kinetic, 

 and assumes various forms, such as heat, light, elec- 

 tricity, or motion. 



Two points of great significance must be noted in 

 the illustration just given ; first, the fact that the 

 more complex substance (O 3 ) has latent a great 

 deal more of the energy of combination than the 

 simpler one (O 2 ) ; and secondly, that the former 

 breaks down or gives up its latent energy more readily 

 than the latter. Few atomic combinations are as 

 simple as oxygen and ozone and at the same time 

 are so readily disrupted ; indeed, among the more 

 complex substances it is a general rule that the greater 

 the complexity of the molecule, the greater the 

 amount of its potential energy and the greater its 

 instability. 



The storing up of the potential energy of " chem- 

 ical affinity " is well illustrated by a so-called endo- 

 thermic compound such as acetylene, C^R^ '- 



Heat of combustion of C 2 = 2 X 96,980 cal. = 193,960 cal. 



Heat of combustion of H 2 = 2 X 34,960 cal. = 69,920 cal. 



Total 263,880 cal. 



Heat of combustion of C 2 H 2 (acetylene) . = 310,600 cal. 

 Difference . * 46,720 cal. 



