222 THE REALITIES OF MODERN SCIENCE 



just possible the phenomenon of surface tension 

 vanishes. For temperatures below this value the 

 k.e. of the molecules is insufficient to render unavailable 

 the p.e. and tractation occurs. As the temperature 

 is decreased from the critical value the surface tension 

 increases, although at first very slowly. Now water 

 is ordinarily well below its critical temperature of 360 

 C., and in it therefore we find a very noticeable surface 

 tension. 



Because of this p.e. between molecules and also 

 because of the space occupied by molecules, the perfect 

 gas equation does not apply to an actual gas except 

 at temperatures well above the critical value. The 

 process of liquefaction is obviously not taken into 

 account by the perfect gas equation, for the molecules 

 of a perfect gas could be compressed and cooled in- 

 definitely without ever acting like a liquid, since they 

 have no p.e. It would be convenient if we had an 

 equation of state which was applicable not only to actual 

 gases but also to the same molecules in a liquid state. 

 One of the most successful attempts to develop such 

 an equation was that of Van der Waals. We may 

 consider his equation to be formed from the equation 

 PV=RT, by the introduction of two correction factors. 



The first of these is introduced to take into account 

 the fact that the volume V in the above equation is 

 too large by an amount dependent upon the actual 

 space occupied by the molecules. Let us represent 

 this amount by b and hence substitute V b for V. The 

 second correction factor takes into account the p.e. 

 of the molecules. Because this tends to decrease the 

 molecules tractate and the effect is the same as if the 



