16 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
electrometer, by the changes which they occasion in the 
surface-tensions concerned. 
The existence of negative surface tensions is shown 
much more clearly however by some of the phenomena 
of capillarity. It is a familiar fact that when a capillary 
tube of insoluble solid is dipped vertically into a liquid 
that liquid either creeps up to a higher level inside the tube, 
e.g., water in a glass capillary, or ceases to rise before it 
reaches the level of the outside liquid, e.g., water 
in a greasy tube, or mercury in a clean glass capillary. 
In the former case the liquid is said to ‘‘ wet ’’ the solid and 
the lquid-air interface meets the solid-liquid imterface at 
6 
an acute angle; in the latter case the “‘ angle of contact ”’ 
is obtuse. Similarly with a plane solid plate the two 
‘‘angle of contact,’’ the magni- 
surfaces meet at a definite 
tude of which is characteristic for the particular solid and 
liquid and is either acute or obtuse. . | 
How is this difference in the equilibrium position of 
the same liquid with different solids or different liquids with 
the same solid to be accounted for? It is clear that ulti- 
mately it must depend upon the different magnitudes (and 
possibly different ranges) of the inter-molecular forces 
operative between the liquid stratum of thickness z and the 
solid within range across the surface and, since inter- 
molecular repulsions are only secondarily brought into 
operation as a consequence of precedent inter-molecular 
attractions, that it must depend primarily upon the 
different magnitudes of the attractions. It is clear also on 
general principles of energetics that in any material system 
the stronger inter-molecular attractions will tend to satisfy — 
themselves and therefore that the stronger the attractions 
between the solid and the liquid, the stronger would be the 
tendency for the solid and the liquid to acquire the maximum 
possible area of contact. We should therefore on a prin- 
ciple of minimal potential energy of inter-molecular 
