LEWIS. — THERMAL PRESSURE. 151 



II. 



Thermal Pressure. 



When we 'coTisider again the laws which govern different phases of a 

 simple substance, as expressed in equations (1), (2), and (3), their great 

 simplicity and generality suggest that some equally simple physical expla- 

 nation is possible. As we have already seen, an increase in the external 

 pressure always produces an increase in the tendency of a substance to 

 escape from any phase. The pressure can influence this tendency only 

 by changing the conditions within the phase. Let us briefly analyze 

 these conditions. 



If in the interior of any homogeneous phase of infinite extent, not sub- 

 ject to gravity, we imagine a septum of infinitesimal thickness, i. e. a 

 mathematical plane, there is every reason to believe that upon each side 

 of this septum there would be a pressure exerted which would depend 

 upon the temperature, and would in general differ from the pressure 

 observed at the surface of the phase. This internal pressure will be 

 termed the thermal pressure of the phase. It would be expressed, in the 

 hypothetical terms of the kinetic theory, as equal to the number of 

 molecules passing from one side to the other in unit time through an 

 imaginary plane of unit area, multiplied by twice the average momentum 

 of each molecule in the direction perpendicular to the plane. 



An illustration of the meaning of thermal pressure is the quantity 



RT . . 



in the equation of van der Waals ; this quantity, if the theory of 



van der Waals is correct, represents the thermal pressure of a compressed 

 gas. Thermal pressure will denote .the pressure due to heat in distinction 

 from that due to attractive or repulsive forces. 



The actual pressure observed at the surface of the phase may be con- 

 sidered equal to the thermal pressure plus or minus the resultant of all 

 other repulsive or attractive forces acting in the phase. This resultant 

 will be called the attractive pressure and designated by a, — positive if 

 an attractive force, negative if repulsive. If the thermal pressure is 

 represented by yS we have the equation, 



/5 - a = P, (4) 



where P is the external pressure. This eciuation combined with equa- 

 tion (3) gives, 



