SIR,
The paper by Reference Tobin and ItagakiTobin and Itagaki (1970) reminds me of a possibly useful experiment made many years ago in a cold room at the U.S. Snow, Ice and Permafrost Research Establishment. Rods 5 × 1 × 0.5 cm were cut from a Mendenhall Glacier single crystal with the long dimension approximately parallel to the c-axis. At – 10° C the rods were broken by slowly pressing over the edge of a bench. They usually broke in cleavage normal to the c-axis, yielding apparently perfectly flat mirror surfaces.