is produced when bichromate of potash is added to a neutral salt of the protoxide of thallium, as an orange-yellow precipitate. The scarcity of the metal precludes their present introduction as pigments, but if the chromates of thallium were found to resist the action of light and air, and not to become green by deoxidation of the chromic acid, they might possibly prove fitted for the palette. It is a question whether their _very_ slight solubility in water would be a fatal objection; and, although they would be liable to suffer from a foul atmosphere, we are inclined to think the effects would not be so lasting as in the chromates of lead. Like lead sulphide, the sulphide of thallium ranges from brown to brownish-black, or grey-black; and, like it too, is subject to oxidation and consequent conversion into colourless sulphate. It is, however, much more readily oxidized than sulphate of lead; and hence the thallium chromates would doubtless soon regain their former hue on exposure to a strong light. Mr. Crookes, who discovered this new metal in 1861, believes that the deep orange shade observable in some specimens of sulphide of cadmium is due to the presence of thallium. He has frequently found it, he says, in the dark-coloured varieties, and considers the variations of colour in cadmium sulphide to be owing to traces of thallium. That thallium affects the colour is most probable, but it is not necessarily the cause of the orange hue. The tint of cadmium sulphide is a mere matter of manufacture, seeing that from the same sample of metal there can be obtained lemon-yellow, pale yellow, deep yellow, orange-yellow, and orange-red. With deference to the opinion of a chemist so distinguished, we hold that thallium rather impairs the beauty of cadmium sulphide than imparts to it an orange shade, the thallium being likewise in the form of sulphide, and therefore more or less black. On chromate of cadmium, made with bichromate of potash, thallium would naturally confer an orange hue. 173. _Uranium Orange_ is obtainable by wet and dry methods as a yellowish-red, or, when reduced to powder, an orange-yellow, uranate of baryta. It is an expensive preparation, superfluous as a pigment. 174. _Zinc Orange._ When hydrochloric acid and zinc are made to act on nitro-prusside of sodium, a corresponding zinc compound is formed of a deep orange colour, slightly soluble in water, and not permanent. * * * * * For a secondary colour, orange is well represented on the modern palette, and can point to some pigments as good and durable as any to be found among the primaries. Burnt Sienna, cadmium orange, Mars orange, neutral orange, and orange or burnt Roman ochre, are all strictly permanent. The so-called orange vermilions were, it will be remembered, classed among the reds. As semi-stable, must be ranked chrome orange; and as fugitive, Chinese orange, orange orpiment, and orange lead.