SURFACE-FILMS. 13 
2. Specific direction must be given to some chemical 
interactions, i.e., a preference to one out of many possible 
interactions, when one or both of the reacting substances are 
orientated, as also when specific intramolecular strains are 
set up by the specially directed intermolecular forces. 
It follows from. the above, and the reasoning is 
obviously equally applicable to liquid-solid and liquid-liquid 
interfaces, that all surfaces must tend to act as 
catalysts. It is presumably in this direction that we 
must look for the explanation of the action of certain 
enzymes studied by Bayliss, which work even when in coarse 
suspension (e.g., urease, lipase). In connection with this 
question it is perhaps worth pointing out that since chemical 
affinities are essentially intermolecular or interatomic 
attractions they must play an important part in determining 
the molecular strains at a surface, and may well do so in the 
highly specific way manifest in many enzyme actions, as also 
in adsorptions. And since spatial arrangements are of 
fundamental importance for the strains in a surface film, 
the same must sometimes be true of stereo-chemical 
structure. 
EXPANSILE SURFACE-FILMS. 
Thus far we have considered only the simple case of a 
free surface-stratum, a case in which the “‘ trans’’ surface 
intermolecular forces are relatively slight. Let us turn to a 
hquid surface-stratum in contact with an insoluble solid, a 
case in which the ‘‘ trans’’ forces may be considerable and 
may conceivably exceed those on the ‘‘cis’’ side of the 
surface. What does observation teach us concerning the 
conditions in the liquid under these circumstances? It is 
impossible to measure the tension in the liquid surface or to 
ascertain directly whether it is even contractile at all. But 
