Field's Chromatography

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rich, deep yellow, of considerable transparency; but the colour is acted upon by foul gas and exposure. Even in a book we have found it assume a dirty greyish cast, and a specimen which had been kept in a drawer, wrapped up in paper, became perfectly black in a few years. The presence of palladium interferes with the beauty of the original tint, but does not affect its stability. 63. _Thallium Yellow._ The new metal thallium yields in combination with chromic acid two yellow colours, a pale and an orange. They are not absolutely insoluble in water, and the sulphide of thallium being brown, would probably be damaged by impure air. But whatever their properties as pigments may be, their habitudes as such are not yet known. The present scarcity of the metal renders the colours produced from it mere scientific curiosities. 64. _Thwaites' Yellow._ Under this name chromate of cadmium was introduced some few years back. If well prepared, it is a fine soft powder of a very vivid light yellow colour. The compound is too soluble, however, to be of value, its washings even with cold water being continually tinged yellow. Hence it turns green after a time, and becomes otherwise discoloured. Like citron yellow and other chromates apt to assume a green cast, it should only be employed, if at all, when compounded for foliage tints, &c. This want of durability is to be regretted, for a good sample of cadmium chrome is marked by exceeding beauty, unsurpassed for clearness and purity by any other yellow. 65. _Turbith Mineral_, Or _Queen's Yellow_, is a subsulphate of mercury, of a beautiful lemon yellow colour, but so liable to change by the action of light or impure air, that it cannot be used safely, and hardly deserves attention as a pigment.


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