146 THE NEW PHYSIOLOGY. 



V = volume occupied by one gramme-molecule of gas, 

 T = absolute temperature, and R is a constant for 

 any gas. 



Now, it is evident that this equation can only hold 

 good if molecules are regarded as points with mass, but 

 without extension. Some mathematical physicists have 

 clung tenaciously to this idea and to the equation. 

 There we must leave them, because we are not dealing 

 with mathematical figments, but with reality in so far 

 as it is revealed to us in experience. As a matter of 

 fact, the equation PV=RT has only the appearance of 

 holding within certain limits of temperature and pressure. 

 If the temperature falls or the pressure increases suffi- 

 ciently, the value of PV becomes greater than RT, 

 because the volume of the molecules themselves begins 

 to count. Hence, if we call v the volume occupied by the 

 molecules, we must alter the equation to P (V— v) = RT. 



If the molecules were simply indifferent to one another, 

 so that they merely repelled one another on contact, 

 we should now have an equation expressing the behaviour 

 of a gaseous substance. But, as a matter of fact, even 

 the amended equation does not express the behaviour 

 of actual gases, for, with sufficient cooling, gases 

 condense to liquids. The molecules attract one another, 

 and with cooling their kinetic energy is reduced pro- 

 gressively so that on an average a constantly increasing 

 proportion of them must be within their mutual spheres 

 of attraction, like the bodies in a solar system, and 

 hence exercise no external pressure. We must, there- 

 fore, alter the significance of P, so that it means, not 

 external, but intermolecular pressure. We can, then, as 

 I have elsewhere endeavoured to show, not only extend 



