It is a familiar fact, first pointed out by one of the present authors many years ago, that metal wires can be prepared which, when stretched, glide on a number of parallel faces. Such wires are usually spoken of as single crystals of the metal, and X-ray analysis has proved that the directions of the crystallographic axes of the metal are fixed throughout the wire. If, however, the wire consist of atoms arranged on an ideal crystal lattice, there seems no reason why there should be, among a set of crystallographically equivalent glide planes, certain more or less regularly spaced planes of weakness, along which glide takes place. The existence of these slip planes, periodically spaced, may be interpreted as evidence of a periodic secondary structure inherent in the crystal, for the preferential glide is not a cumulative process; that is, the resistance to glide along such planes does not become less as slip progresses, but greater. The phenomenon is, therefore, not due to certain chance planes, among a set of almost identical planes, starting as glide planes and then continuing as such, rather than their neighbours, because of progressive softening, but rather to certain preferred planes being disposed to glide. On the other hand, the phenomenon may be set down as due to a secondary structure not inherent in the crystal, but called into existence by strain, a dislocation of any one crystal plane producing a dislocation of a distant plane by some process of accumulation of small disturbances handed on from plane to plane. A third possibility is that the preferred slip planes would not be found in a lattice of perfectly pure metal, but are due to impurities, which may be either foreign metals or dissolved gases; these maybe supposed to segregate into particular planes and have a weakening effect. In this connection reference may be made to the electrical resistance of metals at low temperatures. Kapitza, in his extended investigations of the effect of a magnetic field on the resistance of metals, attributes the residual resistance of a metal to structural imperfections of the lattice, which he appears to associate with minute impurities. “It is known that in a metal which is not in a perfect crystalline state, and which contains even small traces of impurity, there exists a disturbance which increases the specific resistance.” The residual resistance is well known to decrease with increasing purity, as particularly exemplified by gold and platinum, while for mercury there is no residual resistance. If, therefore, the preferential glide on certain planes is due to impurities, pure mercury should not show it.
The mechanical behaviour of single crystals of mercury
