THE “curious phenomenon” described by Mr. Frederic Case (NATURE, vol. xi. p. 249) is by no means an uncommon one. It is due to oxidation, and the conversion of a portion of the pyrites into soluble sulphate of iron. This decomposition is much aided by the presence of moisture; it is very doubtful whether it would occur at all in a dry atmosphere, and I suspect that the particular case in the Maidstone Museum, where the pyrites has thus crumbled, is near an outside wall, or otherwise exposed to humid influences. I have seen large heaps of pyrites thus decomposing at the foot of the troughs where coal-slack is washed before converting it into coke. The sulphate of iron used in the manufacture of Nordhausen sulphuric acid is commonly obtained by similar oxidation of pyrites, which is aided and economised in this case by previously roasting away a portion of the sulphur.