DURING some experiments on the coloration of rock salt by the α-rays of polonium, we noticed that the surface exposed to the rays in air become dull. Examined under the microscope, it was found to be covered by minute birefringent crystals which could be identified as sodium nitrate. Similar crystals are also formed on exposing rock salt to the action of nitric acid fumes or to nitrogen dioxide prepared by heating lead nitrate. The crystals obtained when exposing rock salt to α-rays are evidently due to the formation of nitrogen dioxide by the rays in air. The action of the α-rays on the rock salt surface seems, however, also to intervene ; various samples of rock salt that show differences in the rate of colouring by the rays also show differences in the number of microscopic crystals formed, while no difference is found when nitrogen dioxide alone acts on the same samples. Sometimes the microscopic crystals show a marked orientation on the rock salt surface.
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WIENINGER, L., ADLER, N. Formation of Microscopic Crystals on Crystal Surfaces Exposed to α-Rays. Nature 163, 989 (1949). https://doi.org/10.1038/163989b0