230 THE REALITIES OF MODERN SCIENCE 



compressed and has an excess of energy, for the mole- 

 cules have not yet converted their p. e. into k.e. The 

 contraction has consumed external energy instead of 

 available internal energy. From c to d this molecular 

 p.e. is released. When it is converted into k.e., the 

 molecules may travel beyond their positions of stable 

 equilibrium until their relative separations are too 

 small, as represented by the point e. As the molecules 

 return to a separation corresponding to stable equi- 

 librium the pressure increases, as represented by the 

 line ef. The molecules thus arrive at the same con- 

 dition as if the successive states had been as repre- 

 sented by the line bf, and there had been no over- 

 shooting of the mark. 



We cannot carry a substance through this hypotheti- 

 cal isothermal, for we cannot carry it as far as either the 

 maximum or the minimum. Van der Waals's equation, 

 however, indicates such a form for the isothermal of a 

 pure substance. Thomson's hypothesis appears con- 

 ceivable and is supported to some extent by the 

 experimental entry into these regions of instability. 

 The effects which scientists observe in pressure and 

 temperature are, however, average effects of very large 

 numbers of molecules. If we could only observe the 

 interactions of individual molecules we should need no 

 hypothesis. But if we could observe and direct the 

 actions of individual molecules we could also make 

 possible far more efficient utilization of molecular 

 energy and even be able to alter the processes of life 

 itself. It is the intuition of scientists, however, that 

 it is impossible to control the action of molecules except 

 in so far as their average effects are concerned. 



