The different species of mica have very nearly the same forms and **interfacial** angles, and they not infrequently occur intergrown together in parallel position.

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).

(48) ° and in general the functions 0, or 4), must be regarded as capable of assuming different forms. Under these circumstances there is no limitation upon the values of the **interfacial** tensions for three fluids, which we may denote by T12, T23, T31.

P. 463) deduced relative to the **interfacial** tensions of three bodies.

(52) According to (52), the **interfacial** tension between any two bodies is proportional to the square of the difference of their densities.

The densities a 1, 'a' 2, Q3 being in descending order of magnitude, we may write T 31 = (01 - 02+a, - o"3)'T0 =T 12+ T 23 + 2 (e 1 - (72) (02-0.3)Tc; so that T31 necessarily exceeds the sum of the other two **interfacial** tensions.

The problem is to make the sum of the **interfacial** tensions a minimum, each tension being proportional to the square of the difference of densities of the two contiguous liquids in question.

Although bismuth is readily obtained in fine crystals by artificial means, yet natural crystals are rare and usually indistinct: they belong to the rhombohedral system and a cube-like rhombohedron with **interfacial** angles of 92° 20' is the predominating form.

It crystallizes in rhombohedra belonging to the hexagonal system, having **interfacial** angles of 87° 40'.

If T12 denote the **interfacial** tension, the energy corresponding to unit of area of the interface b Q FIG.