402 Mr Kleeman, On the Nature of the Internal Work 
On the Nature of the Internal Work done during the Euapora- 
tion of a Liquid. By R. D. Kireman, D.Sc. (Adelaide), BA. | 
Emmanuel College. 
[Received 13 March 1914.] 
When a molecule passes from the liquid into the gaseous | 
state it absorbs energy in overcoming the attraction of the | 
molecules of the liquid, and in changing its internal energy. No 
energy is absorbed. or evolved which is due to a change in the 
kinetic energy of translation. This can be shown to follow from | 
the observed fact that the temperature indicated by a thermometer _ 
is independent of the nature of the material of the bulb, and 
consequently of the attraction it exerts upon the surrounding 
substance. The velocity of translation of a molecule in a substance 
when it passes through a point where the forces due to the 
surrounding molecules neutralize one another, is then independent — 
of the density of the substance*. The average distribution of the | 
molecules in the substance corresponds to each molecule bemg — 
situated at a point possessing the property mentioned. It follows — 
then at once that in separating the molecules of the substance by 
an infinite distance from one another energy can only be expended 
in the way described. Thus the internal heat of evaporation L ~ 
of a molecule when the saturated vapour behaves as a perfect gas 
may be written 
L=U + (Wg) ee @D), 
where U denotes the work done against molecular attraction, and 
u—U, the change in internal molecular energy, where wu, denotes 
the internal energy of a molecule in the gaseous state. Therefore 
if the liquid undergoes a small change in density the corresponding 
changes 1n the potential energy of attraction and internal molecular 
energy are dU and du. 
The quantity du has usually not been considered by most 
investigators of the properties of matter, or has been assumed — 
to be small in comparison with dU. Thus for example the equation — 
of state of van der Waals does not take it into account. It must — 
however occur in the equation of statet. On the other hand, other 
investigators have supposed that dw is of the same order of magni- 
tude as dU}. I have brought forward circumstantial evidence that 
du is small in comparison with dU§, but more direct evidence is — 
* Phil. Mag., July 1912, pp. 101—118. 
+ loc. cit., Sept. 1912, pp. 391—401. 
+ loc. cit., Jan. 1912, p. 111. 
§ Proc. Camb. Phil. Soc., xvi. Pt. 6, pp. 540—559 (1912). 
