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XIII. The Variation of Molecular Surface-Energy ivith Temperature. 
By William Ramsay, F.R.S., and John Shields, D.Sc., Ph.D. 
Received March 14,—Read March 23, 1893. 
[Plates 24, 25.] 
Introduction. 
1. The well-known relation of volume-energy to temperature, expressed by the 
equation 
p)v = RT, 
and the simple relations between p, v, and T when the substance examined is in the 
state of gas, together with the constancy in the value of R, if the weights of unit 
volumes of gases taken are proportional to their molecular weights, have led us to 
make experiments on the surface-tension of liquids in the hope of discovering 
relations as direct and simple. We were guided to this research by experiments 
by E5tv6s, which shall be discussed later.* We have not been disappointed; it 
appears that a similar formula expresses with moderate accuracy the variation of 
surface-energy with temperature; it may be stated in similar terms, using the 
symbols y for surface-tension, i.e., the force exerted along a linear centimetre of 
the liquid ; 5 for a surface over which are distributed a number of molecules the same 
for all liquids ; k for a constant analogous to R in the gaseous equation ; and t for 
temperature measured in Centigrade degrees downwards, the critical temperature 
being taken at zero. An equation completely analogous to the gaseous equation, 
pv = RT, should therefore be ys = kt. 
That this is reasonable follows from the following consideration ;—In the gas 
equation, pv = RT, if v be kept constant, p decreases with ftdl of T, until where 
P = 0, T = — 273°, or absolute zero; the origin of the scale of absolute temperature 
is also the origin of the scale of pressure. 
Correspondingly, in the liquid surface equation, if s be kept constant, y decreases 
with T, until where y = 0, a condition of affairs which takes place at the critical 
*OsTWALD (‘Lelirbuch der Allgemeinen Chemie,’ p. 1130) has also pointed out the bearings of the 
experiments of Eotvos in this direction, and has indicated the possibility of determining the molecular 
weights of liquids by investigation of this “ colligative ” property. 
20.10.93. 
