12 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
say: it must suffice to note that at a free surface as a 
matter of observed fact it invariably is accompanied by 
contractility, and it is difficult to resist the conclusion that 
the contractile state is essentially bound up with the fact 
that the molecular forces operating on the surface-stratum 
from the ‘* cis’’ side are larger than those operating on it 
* side of the surface. 
from the ‘‘ trans’ 
It will further be clear that if the surface molecules 
exert different attractions and repulsions along different 
radii they must necessarily, when exposed to the predomi- 
nantly one-sided intermolecular forces, suffer rotation until 
they become appropriately orientated. Since such orientation 
must make a considerable difference to the strains in the 
surface-layer, and therefore to the surface-tension, the same 
conclusion can be drawn also from the Gibbs-Thomson 
principle of minimal surface energy. It should be noted also 
that as it can rarely happen that the intermolecular forces 
operative between the molecules of a solvent and of a 
dissolved substance are exactly equal to those between the 
molecules of the solvent itself, any dissolved substance 
present in the surface-stratum must also be specifically 
spaced, and if its molecules have different fields of force 
along different radii must also be specifically orientated. 
The first suggestion of molecular orientation at a 
surface is due to Hardy of Cambridge, and the idea has 
been considerably developed by Irving Langmuir in U.S.A. 
with fascinating and important results. Let me point out 
two conclusions which follow from such orientation : 
1. The rate at which a chemical reaction takes place in 
a surface stratum would be modified, since the mathematical 
chances of two reacting molecules encountering each other, 
so to speak ‘* business end on’’ would be different at the 
surface and remote from it,—modified therefore in a way 
not taken into account by the Gibbs-Thomson principle. 
