In papers previously read before the Society, the behaviour of sulphur when heated has been the subject of investigation. It has been shown that the transition from a pale-yellow mobile liquid to a deep-brown viscous one, which occurs as the temperature rises in the neighbourhood of 160°, is due to the production from the mobile sulphur (Sλ) of another distinct variety (Sμ). The proportion of the viscous variety (Sμ) is about 4 per cent, at the melting point (114·5°). At 160° it has become 11 per cent., at 170° 19 per cent., and at the boiling point 34 per cent.