It would be most natural to conclude that the chromophore :CS is more powerful than :CO, and that :CSe is most powerful of all, as shown in our study of quinazoline compounds. It would equally follow that :S is a more powerful color-forming radical in a cyclic compound than that of :O; and :NH than that of :S; and :Se again most powerful of the whole series. Lesser and Weiss(41) in their research on selenoindigo stated that the selenium dyestuff, on account of its greater molecular weight than sulphur, shows a deeper blue. This hypothesis meets a difficulty in the case of coumorandione, thionaphthenequinone and isatin series, where the -NH- radical has an atomic weight of 15, and -S- 32, and showed the reversed order of color. This seems to be the case in the selenophene series also. Therefore this theory is not without exceptions. The diselenides present a very interesting study also. Methyl disulphide is colorless, but methyldiselenide(42) is a reddish yellow liquid. Methyl disulphide only becomes yellow when it is treated with chlorine, and in such cases (CH₃)₂S₂Cl₂ is formed(43), in yellow rhombic crystals. Ethyldisulphide is colorless: ethyl- disulphidedichloride is a faint yellow oil(44). But the corresponding ethyldiselenide is a red liquid(45). Phenyl disulphide is colorless, and phenyldisulphide dibromide is of mother-of-pearl appearance, and practically colorless(46), while phenyl diselenide forms pure yellow needles(47), and phenyldiselenide dibromide orange red ones. While phenyldisulphide is colorless, when an auxochrome group is added, such as NH₂, the compound is colored. This is the case with o-diaminodiphenyldisulphide(48) which is yellow both in solution and in crystalline form. In other words, an auxochrome in addition to the chromophore group transforms a colorless chromogen into a colored one. Therefore groups like -S.S- and -Se.Se- are chromophores in the same sense as -N:N-. This is in agreement with the chromophore ideas of Hugo Kaufmann. The -Se.Se- is a more powerful chromophore than -S.S-. This brings one directly to the inquiry as to why 2-phenylbenzo- selenazole, which contains a :Se radical, should be colorless; and that even 6-nitro-2-phenylselenazole, with the addition of a chromophore NO₂, should be only faintly colored. The benzothiazoles, their isomers and derivatives are mostly colorless, and similar causes are probably responsible in the case of the phenylbenzoselenazole, for its lack of color. But when this selenazole is combined with another chromophore, for example an azomethine grouping, the result is a more positively colored compound (in this case benzalaminoselenazole), the crystals being yellow. The corresponding thiazole derivative is light colored.