As pointed out in the paper by Andrade and Tsien, the situation as regards body-centred crystals is such that further information as to the glide element of crystals of this class is needed. In particular, there is considerable uncertainty as to the glide plane, different crystallographic planes being, apparently, operative with different metals. It is possible that with body-centred crystals the planes on which glide takes place depend upon the temperature with reference, say, to the melting point of the metal, since with certain hexagonal crystals such as magnesium, fresh planes have been found to come into operation at high temperatures. Molybdenum, which crystallizes in the body-centred system, has an extremely high melting point, about 2630° C., and so offers a wide range of temperature for experiment. In the following pages work on the glide elements in the temperature range 20 -1000° is described, and it is shown that while the glide plane at the lower temperatures is (112), at high temperature it is (110). The glide direction is in all cases [111], thus confirming the rule that the glide direction is the most closely packed direction. It is hoped later to extend the determinations to higher temperatures. 2—Preparation of the Crystals The crystals were prepared by the method devised by Professor Andrade for metals of high melting point, in which the wire is maintained at a high temperature by a current passing through it, and a local temperature gradient, which slowly travels down the wire, is obtained by a small subsidiary furnace surrounding the wire. For this method it is desirable not to use too thick a wire, and the metal used was 0∙25 mm. in diameter, supplied by the Tungsten Manufacturing Company. If such a wire be maintained by the passage of a current at a temperature some 1000° C. below its melting point, the individual crystallites grow, until the appearance shown in fig. 1, Plate 4 is obtained, where some of the crystals extend right across the wire. The crystal boundaries in this picture have been revealed by etching with dilute nitric acid.
The glide of single crystals of molybdenum