The idea has been expressed that some substances, on a basis of their physical and chemical properties, may be classifiable simultaneously into two crystal classes.1 Such minerals as sylvite, sulfur, diamond, cuprite, and others have been stated to show conflicting properties when tested by various methods. Etch figure studies especially have not always led to conclusions that were in agreement with certain properties known to characterize some crystals. It is significant, however, that inconsistencies of this sort have arisen in some cases because of insufficient data or misinterpretation of the evidence at hand. In recent years, some attention has been given to the methods of crystal etching involving the use of optically active solvents, and, although published accounts of such investigations are scarce, the etch figure data thus far considered seem to have an important bearing on the problem of dual symmetry of crystals. Some of the earlier experimental . . .

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