LONDON Royal Society, February 7. E. N. DA C. ANDBADE and P. J. HUTCHINGS: Mechanical behaviour of single crystals of mercury. In the mercury crystal the rhombohedral faces are glide planes, and the short diagonal is the glide direction. The crystal twins under strain on a plane through the long diagonals of two opposite faces acting as glide planes. In simple glide, twinning takes place when the twinning plane makes an angle of 45° with the axis of the wire. The rhombohedral face and the hexagonal basal plane are equally close-packed, but the former contains* a much more closely packed line than does the latter. Double and triple glide can take place. Hardening on one set of glide planes hardens the whole crystal. The critical shear stress at -43° C. is 9·3 gm. wt. per sq. mm. E. N. DA C. ANDBADE and J. C. MABTINDALE: Structure and physical properties of thin films of metal on solid surfaces. The films were prepared by cathodic sputtering under carefully controlled conditions, with a water-cooled anode. The films obtained were uniform, and appear to be amorphous with all types of microscopic examination. When they are maintained at a temperature of about 230° for silver, and somewhat higher for gold, the first stage of crystallisation takes place, which consists in the formation of birefringe-ment aggregates, of the order of 111 across, showing the spherulitic figure in polarised light. Prolonged heating at a somewhat higher temperature leads to rapid growth of the particles, which eventually become well-formed cubic crystals, all arranged with the (111) faces parallel to the supporting surface. The first aggregates are formed by the movement of the upper layers of the films, which are about 50 atoms thick, the further growth of the crystals being accompanied by the formation of areas from which the metal has retreated, leaving a thinner film. Crystallisation in such thinner films does not take place until a much higher temperature is reached than that required for the thicker films. M. BORN: On the theory of optical activity. This paper contains a detailed development of the theory of rotatory power given by the author in 1915. The molecule is considered as consisting of a set of isotropic oscillators coupled by Coulomb forces. The interaction is calculated by the perturbation method. The resultant formula is rather complicated but can be simplified very much for special cases. A molecule consisting of two equal pairs of oscillators perpendicular to one another and to their central line is worked out in detail; it gives the angle of rotation of the expected order of magnitude.