It has often struck me that much more might be learned from the study of the secondary forms of crystalline minerals than we now know respecting the circumstances under which they were produced. Some years ago being chiefly acquainted with calcite as it occurs in Derbyshire, where the crystals are usually of the so-called dog-tooth shape, my attention was much attracted by the difference in the form of the crystals in Devonshire and Cornwall, where we so often meet with six-sided prisms. On reflecting on this difference I could not give any more satisfactory explanation than that the conditions under which they were formed must have been very different in some important particular. Various experiments, made both before and afterwards, sufficed to indicate at all events some of the possible causes of such a difference in the form of this mineral, but at the same time showed clearly that much special research would be required before we could draw such definite conclusions as could be desired.