SURFACE-FILMS 15 
inability of the thinned film to resist the superior side- 
thrust of the surrounding thicker films, so that quid would 
be pushed in between the solids and cause their spontaneous 
separation. I must not conceal the probability, however, 
that the above reasoning does not convey a complete picture 
of the tension changes in a thinning liquid film. Such a 
picture is impossible without greater knowledge of the 
relative ranges and changes of inter-molecular attractions 
and repulsions with distance than is available. ‘The one I 
have sketched is submitted as a working hypothesis which 
contains much relative truth and demands the recognition 
of expansile surface strata or ‘ negative tensions.’ 
Phenomena showing contractile liquid films are of 
daily occurrence in laboratory’ work. Spontaneous 
approximation of suspended solids is seen, for example, in 
the flocculation of many suspensions, in certain colloid 
precipitations, in the formation of rouleaux by red blood 
corpuscles, and in the agglutination or ‘‘ clumping ”’ of 
bacteria first described by Gruber and Durham and utilised 
for the diagnosis of typhoid fever in the well-known Widal 
test. Spontaneous dispersal of clumped particles of insoluble 
solids (presumably by negative surface tensions) may be 
seen on adding appropriate electrolytes to the water in which 
they are suspended, e.g., on adding NaOHto kaolin clumped 
by CaCl,. The forcible imbibition of water by many insoluble 
colloids may also be instanced as a phenomenon demanding 
a belief in the real existence of negative surface tensions. 
Probably the process of dissolving solids which yield colloid 
solutions is, in its earlier stages at least, essentially of the 
same nature also. As regards the electric charges shown 
by Hardy to be acquired or lost in the dispersals and 
agelutinations of suspended solids, it is assumed that, 
although they often play an essential part, they are only 
indirectly responsible for the movements and that for the 
most part they operate as they appear to do in the capillary 
