10 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
contaminated surface near the rod, with its lower tension, 
is hopelessly outclassed by the clean water surface with its 
higher tension, and is therefore rapidly pulled over the 
whole surface. Observe that no such rapid pulling outwards 
of a coloured soap solution occurs when it is introduced 
down the inside of a glass tube directly into the depths of 
the water—it occurs only in the surface layer. 
Every liquid has its free surface layer down to a very 
small depth from the surface in a similar contractile 
- condition, and the force which it exerts, 1.e., the surface- 
tension, can be measured in many ingenious ways. Its 
magnitude has been found to differ for different liquids, and 
it is always lower in the warm liquid than in the cold. ‘The 
addition of a soluble substance in some cases increases it, 
e.g., sodium chloride in water; in other cases, as you have 
seen, diminishes it, e.g., soap or proteins in water. 
If we ask how the free surface-stratum of liquid comes 
to possess this contractility, we must concern ourselves at 
once with molecules and intermolecular forces of which our 
present knowledge is very inadequate. Arguing purely 
a priori on the basis of that knowledge it would be difficult 
to deduce the conclusion that the free surface stratum must 
necessarily be in a contractile state. Even when fortified by 
the knowledge that it invariably is in such a state it is at 
present impossible to arrive at any clear explanation of the 
molecular arrangements which give rise to it.. It will 
nevertheless be profitable for other reasons to proceed as if 
we had such an explanation in view. 
Of molecules it is safe to assume that they must often 
have other than a spherical shape. Of intermolecular forces - 
it is certain that (1) they consist of attractions and 
repulsions, (2) they are of enormous magnitude when two ~ 
molecules are very close together, but become negligible 
when the molecules are more than a certain minute distance 
