[GARVER] THE RANGE OF MOLECULAR ACTION 87 
general gas equation holds for the vapour of the given substance. Using 
the same symbols and meanings as in previous equations we have for 
R : 
the vapour-pressure © = 6 — T (gas equation) (F) 
mn 
where R is the gas constant, S2600000 which for a given vapour or gas 
must be divided by the molecular weight m of the substance, and T is 
the absolute temperature. If this value of © be substituted in the equa- 
: Enya ower FAN 
tion (D) and reduced we get a =p — T (liquid equation)  (G) 
2 € m 
where 2, m and T have the same values as in (F). 
Here p es T represents what in a former paper ! was described as 
m 
the “intrinsic pressure” of the liquid. But this equation throws an 
additional light on the subject. In the gas equation © represents 
the pressure per unit area exerted by the vapour. In the liquid equa- 
tion, Y is the tension in a film of a certain thickness. The ratio of this 
tension to the thickness of the film must be equal to the tension per unit 
area of cross-section of the film. If 2 €, twice the radius of action, i.e., 
the diameter of the sphere of action, may be regarded as the thickness 
of the film, then y/2 € is the tension per unit cross-section of the film 
in which the phenomena of surface-tension have their seat. But this 
value, y/2¢ = p Ae T is exactly the value of the intrinsic pressure 
m 
of the liquid. The intrinsic pressure of a liquid may be concisely de- 
fined as that pressure which the liquid would exert if it were a perfect 
gas having the same temperature and density as the liquid. This re- 
lation between the surface-tension and the gas equation would tend 
to show that the tension per unit area of cross-section of the film is nu- 
merically equal, but opposite in sign, to the pressure the liquid would 
exert if it were a perfect gas having the same temperature and density 
as the liquid in the film. 
In order to decide on the significance of € as applied to the general 
gas equation we may approach the question from another point of view. 
If in accordance with the generally accepted theory of the molecule, we 
assume that the average energy of translation of a molecule depends 
only on the absolute temperance and is independent of phase (liquid or 
vapour), then in consonance with the third Newtonian law, the molec- 
ular attraction observed in liquid films must be merely the reaction of 
what is observed in the free molecule when it is out of range of molec- 
ular attraction and exerts a pressure on the retaining walls. If this 

‘Jour. Phys. Chem. 14 7, 651, (1910). 
2 
