292 
them come together over a small area, exceeds the sum 
of the surface-tensions, the interfacial tension is negative. 
The result is an instantaneous puckering of the interface, 
as the commencement of diffusion and the well-known 
process of continued inter-diffusion follows. 
Consider next the mutual attraction between a solid 
and a liquid. Choose any particular area of the solid, 
and let a portion of the surface of the liquid be pre- 
liminarily shaped to fit it. Let now the liquid, kept for 
Fic. 6. 
the moment rigid, be allowed to come into contact over 
this area with the solid. The amount by which the work 
done per unit area of contact falls short of the surface- 
tension of the liquid is equal to the interfacial tension of 
the liquid. If the work done per unit area is exactly 
equal to the free-surface tension of the liquid, the inter- 
facial tension is zero. In this case the surface of the 
liquid when in equilibrium at the place of meeting of 
liquid and solid is at right angles to the surface of the 
solid. The angle between the free surfaces of liquid and 
solid is acute or obtuse according as the interfacial 
tension is positive or negative ; its cosine being equal to 
the interfacial tension divided by the free-surface tension. 
The greatest possible value the interfacial tension can 
have is clearly the free-surface tension, and it reaches 
this limiting value only in the, not purely static, case of a 
liquid resting on a solid of high thermal conductivity, 
kept at a temperature greatly above the boiling-point of 
6.. 
Fic. 8. 
the liquid ; as in the well-known phenomena to which 
attention has been called by Leidenfrost and Boutigny. 
There is no such limit to the absolute value of the inter- 
facial tension when negative, but its absolute value must 
be less than that of the free surface tension to admit of 
equilibrium at a line of separation between liquid and 
solid. If minus the interfacial tension is exactly equal to 
the free-surface tension, the angle between the free surfaces 
at the line of separation is exactly 180°. If minus the 
interfacial tension exceeds the free-surface tension, the 
NATURE 
| 
[Fuly 29, 
1886 
liquid runs all over the solid, as, for instance, water over 
a glass plate which has been very perfectly cleansed. If 
for a moment we leave the centre of the earth, and sup- 
pose ourselves anywhere else in or on the earth, we find 
the liquid running up, against gravity, in a thin film over 
the upper part of the containing vessel, and leaving the 
interface at an angle of 180° between the free surface of 
the liquid, and the surface of the film adhering to the 
solid above the bounding line of the free liquid surface. 
This is the case of water contained in a glass vessel, or in 
contact with a piece of glass of any shape, provided the 
surface of the glass be very perfectly cleansed. 
When two liquids which do not mingle, that is to say, 
two liquids of which the interfacial tension is positive, 
are placed in contact and left to themselves undis- 
turbed by gravity (in our favourite laboratory in the 
centre of the earth suppose), after performing vibra- 
tions subsiding in virtue of viscosity, the compound mass 
will come to rest, in a configuration consisting of two in- 
tersecting segments of spherical surfaces constituting the 
outer boundary of the two portions of liquid, and a third 
segment of spherical surface through their intersection 
constituting the interface between the two liquids. These 
three spherical surfaces meet at the same angles as three 
balancing forces in a plane whose magnitudes are re- 
spectively the surface tensions of the outer surfaces of the 
two liquids and the tension of their interface. Figs. 2 to 
5 illustrate these configurations in the case of bisulphide 
of carbon and water for several different proportions of 
the volumes of the two liquids. (In the figures the dark 
shading represents water in each case.) When the volume 
of each liquid is given, and the angles of meeting of the 
three surfaces are known, the problem of describing the 
three spherical surfaces is clearly determinate. It is an 
interesting enough geometrical problem. 
If we now for a moment leave our gravitationless 
laboratory, and, returning to the Theatre of the Royal 
Institution, bring our two masses of liquid into contact, 
as I now do in this glass bottle, we have the one liquid 
floating upon the other, and the form assumed by the 
floating liquid may be learned, for several different 
cases, from the phenomena exhibited in these bottles - 
—— 
gia 
