It has been proposed to employ the beautiful golden-bronze crystals of meta-vanadic acid as a pigment. They possess, when finely ground, an intense colour, like that of a very rich golden ochre, but less earthy, and more brilliant. This pigment has remarkable covering power, and works admirably both as an oil and a water-colour. Although the material is somewhat costly, the price for which it could be prepared need not preclude its use.

But, unfortunately, this colour is not permanent. A few hours' exposure to sunshine of a water-colour wash of vanadium yellow suffices to change and deteriorate its hue in a marked degree.

Kings' Yellow: Orpiment - Jaune Royal - Konigsgelb.

The yellow arsenious sulphide (As2S3), though extremely beautiful in hue, cannot be relied on as a pigment. Even in oil or varnish its colour fades: Sir Joshua Reynolds' experimental canvas shows some pale brown patches which have once been kings' yellow, but which now have almost entirely disappeared. Strange to say, in one of his trials, a few quite visible crystals of orpiment are preserved. As it cannot be imagined that he used this pigment in this exceedingly coarse form, it would seem that a molecular aggregation of a part of the orpiment has taken place in the lapse of years. If this change has not occurred, then we may conclude that only the largest particles of the kings' yellow have escaped alteration. Under any circumstances the inadmissibility of kings' yellow to the palette of the artist is obvious: moreover, it cannot be safely mixed with any pigment containing lead or copper. It was known to the Egyptians.

Another compound of arsenic and sulphur (As2S2) has been employed as a pigment. It is of an orange-red hue and is known as realgar. Not only is it extremely poisonous, but it suffers change on exposure to light, and acts injuriously upon colours containing copper or lead. It was used by the Romans: I identified a fragment of it amongst the objects discovered at Silchester in the year 1896.