In a former edition of this work there appeared the following:--"Blue carmine is a blue oxide of molybdenum, of which little is known as a substance or as a pigment. It is said to be of a beautiful blue colour, and durable in a strong light, but is subject to be changed in hue by other substances, and blackened by foul air: we may conjecture, therefore, that it is not of much value in painting." In his estimate of this colour the author was certainly right. It is formed when a solution of bichloride of molybdenum is poured into a saturated, or nearly saturated, solution of molybdate of ammonia. A blue precipitate falls, which is a molybdate of molybdic oxide, hydrated, and abundantly soluble in water. When dried, it furnishes a dark blue powder, resembling powdered indigo, having a bitter, rough, metallic taste, and reddening litmus strongly. The solubility of this hydrated oxide is alone fatal to its employment as a pigment. It may, indeed, be rendered comparatively insoluble in water by ignition; but the anhydrous oxide so obtained is nearly black, and as a colour worthless. A more eligible preparation is the molybdate of baryta, produced by mixing solutions of molybdate of potash and acetate of baryta. A white, flocculent precipitate results, which rapidly condenses to a crystalline powder, and turns blue on ignition. It is, however, a costly compound, of little merit, and not likely to come into use. It is insoluble in water. 138. _Blue Ochre_, which has been improperly called Native Prussian Blue, is a native hydrated phosphate of iron of rare occurrence, found with iron pyrites in Cornwall, and also in North America. What Indian red is to the colour red, and Oxford ochre to yellow, this pigment is to the colour blue, being sober and subdued rather than brilliant. It has the body of other ochres, more transparency, and is of considerable depth. Both in water and oil it works well, dries readily, and does not suffer in tint with white lead, nor change when exposed to the action of strong light, damp, or impure air. As far as its powers extend, therefore, it is an eligible pigment, though not generally employed nor easily procured; it may, however, be artificially prepared. Answering to similar acid tests as ultramarine, it is distinguished therefrom by assuming an olive-brown hue on exposure to a red heat. 139. _Cobalt Prussian Blue._ Gmelin states that yellow prussiate of potash yields with a solution of oxalate of sesquioxide of cobalt a blue resembling Prussian blue--that, in fact, there can be obtained a Prussian blue with a base of cobalt instead of iron. In the moist state, the similarity is sufficiently great, but when washed and dried, the product is, with us, a dingy slate colour. Possibly, if such a blue could be produced, it might exceed in permanence the ferro- and ferri-cyanides of iron. Of course the compound would be much more expensive. _Copper Blues_