Permeability 
95 
surface tension of a liquid is spoken of it is generally the surface 
tension of the liquid against air that is meant. 
The surface tension of a liquid, besides depending on the nature 
of the liquid and on the medium with which it is in contact, also 
depends on the temperature. 
It is outside the scope of this work to describe the different 
methods used for the measurement of surface tension. For these 
reference may be made to standard text-books of physics ( e.g . 
Poynting and Thomson, 1905; Winkelmann, 1908). The principles of 
the more ordinary methods consist respectively of: (1) the measure¬ 
ment of the height to which liquid will rise in a capillary tube; (2) the 
measurement of bubbles and drops (Quincke); (3) the measurement 
of the size of drops of the liquid as it issues from a narrow tube; 
(4) measuring the least pressure necessary to force bubbles of air 
from the orifice of a narrow tube dipping in the liquid (Jaeger); 
(5) the determination of the deformation produced in the cross- 
section of a stream of liquid issuing from an elliptical orifice; (6) 
forcing a stream of the liquid upwards through a small orifice and 
measuring the height to which it will rise; (7) the determination of 
the velocity with which waves travel over the surface of a liquid 
(Rayleigh); (8) observations of oscillations of a spherical drop of 
liquid (Lenard, 1887). These and other methods will be found de¬ 
scribed in physical text-books. 
The following table shows the surface tension of a number of 
liquids in contact with air, as determined by the capillary method, 
at 20 0 C. 
Table I 
Surface tension of certain liquids in contact with air 
at a temperature of 20° C. 
Substance 
Surface tension in 
dynes per cm. 
Water 
Acetic acid ... 
Ethyl alcohol 
Ether 
Chloroform... 
Olive oil 
7 2 ’53 
23-46 
22-03 
16-49 
25-88 
35*4 
