and T12 = 0, as it should do.
Laplace does not treat systematically the question of interfacial
tension, but he gives incidentally in terms of his quantity H a relation
analogous to (47).
If 2T'12 > T1 + T2, T12 would be negative, so that the interface would
of itself tend to increase. In this case the fluids must mix.
Conversely, if two fluids mix, it would seem that T'12 must exceed the
mean of T1 and T2; otherwise work would have to be _expended_ to effect
a close alternate stratification of the two bodies, such as we may
suppose to constitute a first step in the process of mixture (Dupre,
_Theorie mecanique de la chaleur_, p. 372; Kelvin, _Popular Lectures_,
p. 53).