94 Walter Stiles 
necessary to attach a definite weight to AD in order to keep it in 
position. 
The surface is thus the seat of special forces. How these come to 
be present can be understood from the following considerations: 
In the mass of the liquid we have an enormous number of mole¬ 
cules, each one of which is surrounded on all sides by numbers of 
other similar molecules uniformly distributed round it. The mutual 
attraction of the molecules therefore results in nothing more than 
keeping them pressed together, the pressure on any particular mole¬ 
cule being uniform in all directions. By the force of cohesion the 
liquid particles are kept together, and do not tend to separate as 
in the case of a gas. At the surface, of the liquid the state of affairs 
is different. A molecule at the very surface is subjected to the attrac¬ 
tive force of molecules in the interior of the liquid, but there is no 
similar balancing force at the exterior. This means that the surface 
layer is subjected to an inwardly directed pressure at right angles 
to the surface, a pressure which decreases rapidly in the direction 
away from the surface as the cohesive force of the molecules is more 
uniform in all directions. 
This inwardly directed pressure at the surface has two results. 
In the first place as all liquids are compressible to a small extent the 
surface layer of liquid will be compressed; in the second place as 
all the molecules at the surface are pressed towards the interior the 
surface will tend to contract to its smallest possible area. 
So far the surface has been spoken of as if there were nothing 
outside the liquid. As a matter of fact there must always be some 
substance in contact with the liquid. Now a force of attraction called 
adhesion exists between molecules of different substances, con¬ 
sequently the actual inwardly directed pressure at the surface is 
the result of the difference between the pressure due to cohesion 
of the liquid and that due to adhesion between the liquid and the 
external medium. Only when the cohesion and adhesion are equal the 
pressure will be zero. 
We thus see that the surface is in a state of tension by which 
it tends to reduce its area to the least possible. The surface tension 
of a liquid is defined as the force acting on unit length in the plane 
of the surface. From what has already been said it is clear that the 
surface tension depends not only on the nature of the liquid itself, 
but also on the medium with which it is in contact. The surface 
tension for instance of the surface water/air will not have the same 
value as the surface tension of the surface water/alcohol. When the 
