I WAS glad to learn from his interesting letter in NATURE of Jan. 26, p. 130, that Mr. F. C. Guthrie has verified our observations on the thermoluminescence of natural blue rock salt, which were published in the Sitzungsberichte der Akademie der Wissenschaften in Wien (II. a), 132, 261, 1923, in collaboration with Miss M. Belar. Since then many samples of rock salt from various localities have passed through our hands, with the result that blue or violet pieces invariably show thermoluminescence, whilst colourless ones in general do not, only some very impure and opaque white pieces being an exception. It is scarcely to be doubted that the increased energy-content of the blue rock salt was acquired through absorption of some radiation, most likely of radioactive origin.