Permeability 97 
It has been pointed out (Helm, 1887; Bayliss, 1915) that in 
general energy can be regarded as the product of two factors, an 
“intensity factor” of the nature of a force, and a “capacity factor” 
which is always a measure of size, such as mass, volume, area, length, 
etc. In the case of electrical energy for example, the intensity factor 
is difference of potential and the capacity factor strength of current. 
The capacity factors of two systems add together and the sum gives 
the capacity factor of the whole, the intensity factors cannot be 
added together. For instance in the case of heat, where the intensity 
factor is temperature, the sum of the temperatures does not give 
the temperature of the whole system. Differences in the intensity 
factor always tend to disappear. Thus two bodies in contact tend to 
come to the same temperature, or to the same potential. On the 
other hand there is as a rule no tendency for differences between 
capacity factors to disappear. 
In the case of surface energy, the intensity factor is obviously 
the surface tension, and the capacity factor the area of the surface. 
Now it is an important law of energetics that free energy always 
tends to reduce itself to a minimum, and hence the surface energy 
of a liquid will tend to diminish whenever this is possible. In 
most cases it is only the intensity factor which is capable of 
alteration to bring about reduction in the free energy, but in the 
case of surface energy both the intensity and capacity factors tend 
to diminish to the minimum possible value, that is the surface tension 
will reduce itself whenever possible and the surface will contract to 
the smallest possible. A reason for this has already been given in 
this chapter. 
The tendency for surface tension to diminish is probably a 
principle of great importance in regard to permeability. In the case 
of two pure and immiscible liquids in contact there is no possibility 
for the surface tension to be reduced as this is a function of the nature 
of the liquids and the temperature. The case of a liquid that contains 
more than one component requires more consideration. In this case if 
the surface tensions of two components in the pure state are different, 
the surface tensions of mixtures of the two are intermediate between 
those of the pure liquids. Thus in the case of mixtures of ethyl 
alcohol and water, if the surface tension of pure water against air 
is taken as unity, that of ethyl alcohol against air is only 0*302. 
The curves in fig. 2 show how the addition of various alcohols 
to water lowers the surface tension of the latter. So far how¬ 
ever it has not been possible to express the exact relationship 
