96 
Walter Stiles 
Table II 
Values of the surface tension of the same liquid against different 
substances at 20° C. (Data from Quincke) 
Surface tension in dynes per cm. against 
Liquid 
air 
water 
mercury 
Water ... 
80-97 
0 
41-77 
Mercury 
53-98 
41-77 
0 
Alcohol 
25-49 
— 
39-93 
Chloroform 
30-61 
29-52 
39-93 
Olive oil 
36-88 
20-56 
33-54 
The values given in this table are due to Quincke. His determinations 
are now generally regarded as somewhat high, and the figures given 
in Table I for surface tensions against air are lower in all cases than 
those found by Quincke. This does not affect the comparison of the 
surface tensions against different substances. 
With increase in temperature the surface tension diminishes. 
Eotvos (1886) has propounded the following relation between surface 
tension and temperature 
d (ot*) 
dd 
= — 2*1 
where(7 is the surface tension, v the molecular volume (i.e. molecular 
weight/density) and 9 the temperature whatever the value of 6 and 
whatever the substance. From this equation can be calculated the 
value at which the surface tension becomes zero; calculation shows 
this temperature differs very little from the critical temperature. 
(Poynting and Thomson, 1905.) 
Since the surface of a liquid is in a state of tension, it follows that 
when it contracts energy is released, and conversely, when a surface 
is increased work has to be done against the tension. Consequently 
the surface is the seat of energy. Referring to the example of the 
soap film in fig. 1, since the surface tension is the force exerted by 
unit length of the surface, and since there are two surfaces to the 
film, the weight required to keep the wire AD in equilibrium must 
be 2cr. AD. 
Now if the film is stretched so that BA and CD are increased in 
length by a quantity x, the work done in stretching the film is 
2 ( 7 . AD.x 
or 2 era) where at is the area by which the surface has been increased. 
Consequently the potential energy of a surface is the product of 
the surface tension and the area. This quantity is called the surface 
energy. 
