PHYSICS, BIOLOGY, AND PSYCHOLOGY. 145 



elementary mechanical principles can we deduce the 

 behaviour of even the molecule of water in crystallisation ; 

 and similarly, from no elementary physical or chemical 

 principles can we deduce the behaviour of the organism. 

 It is owing to this empirical fact that the ordinary 

 working hypotheses of physics and chemistry are irre- 

 concilable with those of biology. 



The tacit assumption is often made that in mechanical 

 physics we reach a definition of the ultimate reality of 

 which the visible world consists. For many practical 

 purposes this definition, it is true, suffices. But even 

 in connection with heat, light, and electricity, the 

 definition is insufficient. In chemistry it breaks down 

 still more, and in biology the breakdown is complete. 

 Like pure mathematics, mechanical physics is only 

 an abstract science. We can use it for certain practical 

 purposes, but it tells us only a very little about reality, 

 and in only a very imperfect form. 



Let me illustrate my meaning by reference to the 

 kinetic theory of gases — a subject which has been specially 

 engaging me lately. For the kinetic theory of gases, 

 a gas is an assembly of molecules kept in motion by 

 heat, with the necessary consequence that each molecule, 

 whatever its mass may be, possesses on an average the 

 same amount of kinetic energy. Hence an equal number 

 of gas molecules will always produce the same bombard- 

 ment pressure at the same temperature, and from this 

 pressure we obtain an absolute scale of temperature. 

 In this way we can predict from the theory the three 

 well-known " gas-laws," called, after their discoverers, 

 Boyle's, Charles's and Avogadro's laws. These laws are 



embodied in the equation PV = RT, where P=^ pressure, 



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