14 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
by reason of the fact that the solid is rigid, and therefore 
itself incapable of internal movement, it is possible to 
draw certain highly probable conclusions concerning the 
liquid surface from the movements seen when completely 
submerged masses are brought near together. 
Observation of fine solid particles suspended im liquids, 
and therefore in an environment where they are subject to 
numerous accidental approximations caused by currents and 
Brownian movements, shows two different types of 
behaviour. In some cases, e.g., kaolin in water containing 
lime salts, the particles run together; in other cases they 
remain obstinately separate, or at all events show no obvious 
tendency to run together, e.g., kaolin in pure water. I 
shall argue that in the former case the surface-stratum of 
the water must be actively contractile, but that in the latter 
it is probably, or at least possibly, actively expansile 
(neutrality or complete absence of surface-tension being 
excluded as impossible at a surface between two immissible 
substances). 
Assuming for the sake of argument that expansile 
strata really exist,” it is clear that whatever the strains set 
up in the two surface-strata of the hquid film separating 
two solid masses, whether they evoke a contractile or an 
expansile condition, those strains must begin to diminish as 
soon as the liquid is thinned below 2g so that some parts of 
it come within range of the solid on both sides and the 
molecular stresses become to that extent more equalised; 
consequently, either a contractility or an _ expansility 
would diminish. Diminution of the former would result in 
withdrawal of the liquid, since it would be no longer capable | 
of withstanding the superior pull of the surrounding thicker 
films, and spontaneous approximation of the solid masses 
would follow; diminution in an expansility would result in 
* Compare Clerk Maxwell, Encycl. Brii., 9th edn., Art. Capillarity, and 
Wo. Ostwald’s Handbook of Colloid Chemistry. English Translation. 1915. 
J. and A. Churchill. 
