FROM a recent note by Iredale and Gibson1, it seems that the authors have not been able to isolate the absorption spectrum of HgS, observed by me2, from that of sulphur vapour in the range of temperatures they have worked, and they are of opinion that the absorption continua ascribed by me to HgS probably belong to sulphur vapour, because these lie in the neighbourhood of the absorption maxima of sulphur vapour at 4000 A. and 2670 A. when raised to the temperature of 400° C.