THE ACTIVITIES OF THE ORGANISM 115 



that is, the motion of the molecules of the bodies to 

 which the heat is communicated. This molecular 

 motion is least in solids, greater in liquids, and greatest 

 in gases. If solids, liquids, and gases are in contact, 

 forming complex systems, the kinetic energy of their 

 molecules becomes distributed in definite ways, depend- 

 ing on the constants of the systems. After this re- 

 distribution the kinetic energy of these molecules is 

 unavailable for further energy transformations, so that 

 phenomena or change in the system ceases. There is 

 no longer effective physical diversity among the parts 

 of the system. 



We find that this conception of dissipation of 

 energy cannot be applied to the organism, at least not 

 with the generality in which it applies to physical 

 systems. Why ? Not because the conception is un- 

 sound, or because the physico-chemical reactions that 

 occur in material of the organism are of a different 

 order from those that occur in inorganic systems — they 

 are of the same order. The second law of energetics 

 is subject to limitations, and it is because it is applied 

 to organic happenings without regard to these limn- 

 tations that it does not describe the activities of the 

 organism as well as it describes those of inorganic 

 nature. 



What, then, are these limitations ? We note in 

 the first place that the laws of thermodynamics apply 

 to bodies of a certain range of size ; or at least the 

 possibility of mathematical investigation (on which, 

 of course, all depends) is limited to " differential ele- 

 ments " of mass, energy, and time. We cannot apply 

 mathematical analysis to bodies, or time-intervals of 

 " finite size," since the methods of the differential 

 and integral calculus would not strictly be applicable. 

 But molecules are so small (i cubic centimetre of a gas 



