GROUP III  . Sulphides.—Some of the members of this group are liable to contain free sulphur, and some may give up this element to the metallic bases of other pigments . Thus cadmium yellow blackens emerald green, producing copper sulphide . Another pigment of this group, vermilion, is prone to a molecular change whereby the red form passes into the black variety . This change, frequent in water-colour drawings, is scarcely observable in works painted in oil . The sulphides comprise cadmium yellow (CdS), king's yellow (As2S2), realgar (As2S,), antimony, red (Sb2S3) and vermilion (HgS) . It is convenient to give places in the same group to the various kinds of ultramarine, blue, green, red, violet and native, for in all of them a part of the sulphur present occurs in the form of a sulphide . It may be stated that the sulphides of arsenic and antimony just named are dangerous and changeable pigments not suited for artistic painting .