The hardening of plastic sulphur was investigated, and it was found that partial reversion to soluble sulphur prevents the securing in quasi-solid form of the whole of the amorphous sulphur present. It was discovered, however, that sulphur formed by precipitation in presence of concentrated acids does yield 100 per cent, of insoluble sulphur, and that only the impossibility of realising the requisite condition of very fine subdivision is therefore responsible for the smaller yields from melted sulphur which has reached the highest temperatures previous to being chilled.