-bonding in (CuC16)4-

conditions of breakdown in KC1 may be modified by ionic migration (Cooper, Higgin and Smith 1960).The [loo], [110] and [111] tracks were all observed in N a ~l by Cooper, Grossart and Wallace (1957).This is attributed to the difficulty in producing a truly spherically symmetrical field in a material that is not very anisotropic.It would be surprising if other members of the alkali halide family behaved qualitatively differently from KC1, KBr, and NaC1, and limited experiments suggest that NaF, and NaBr are respectively similar to KC1 and NaC1.The evidence of isotropic behaviour usually quoted consists of experiments performed using small numbers of unannealed NaCl specimens.I t is now evident that such experiments could not yield significant results.Neither this investigation nor the previous one (Cooper, Grossart and Wallace 1957) has provided evidence of marked temperature dependent transitions in the preferred track direction (Davisson 1959).Caspari's investigation (1955) indicates that these occur when breakdown is initiated by discharges in the ambient near to the anode, a condition which did not arise in the present or previous investigation.