116 



THE PHILOSOPHY OF BIOLOGY 





may contain about 54XIO 19 of them) that even such 

 a minute part of a body, or liquid, or gas as approxi- 

 mates to the infinitesimally small dimensions required 

 by the calculus, contains an enormous number of 

 molecules. 



Obviously we cannot investigate the individual 

 molecules. Even if experimental methods could be 

 so applied, such concepts as density, pressure, volume, 

 or temperature would have no meaning. Physics, 

 then, is based on collections of molecules, and the pro- 

 perties of a body are not those of a molecule of the same 

 body. Such concepts as temperature and pressure 



are statistical ones, and 

 are applied to the mean 

 properties of a large 

 number of molecules. 



We can best illus- 

 trate this by consider- 

 ing Maxwell's famous 

 fiction of the " sorting 

 demons." Let us im- 

 agine a mass of gas contained in a vessel the walls 

 of which do not conduct heat. Let there be a par- 

 tition in this vessel also of non-conducting material, 

 and let there be an aperture in this partition greater 

 in area than a molecule, but smaller than the mean 

 free path of a molecule. Now this mass of gas 

 has a certain temperature which is proportional 

 to the mean velocity of movement of the molecules. 

 The second law says that heat cannot pass from a 

 cold region in a system to a hot region without 

 work being done on the system from outside, nor can 

 an inequality of temperature be produced in a mass 

 of gas or liquid except under a similar condition. But 

 " conceive a being," says Maxwell, " whose faculties 



FIG. ii. 



