“…It is well known that, at low pressure, supercooling fre quently occurs the more easily the more impure the specimen [Westrum & McCullough (43)]. Gabrysh, Eyring & Vanhook (44) have found that the rates of the transformations in camphor at 2.8 kb and in phosphorus at 9.2 kb at room temperature were markedly affected by impurity, and they sug gest that the region of indifference may be vanishingly small for pure enough substances. Similar effects are of course well known in metallic alloys, and it seems possible that small amounts of impurity can harden molecular crystals as they can metallic crystals.…”