Though unexceptionable as an oil-colour, both in solid painting and glazing, it does not work so well as some other blues in water; nor is it, unless carefully prepared, so well adapted for mixed tints, on account of a gritty quality, of which no grinding will entirely divest it, and which causes it to separate from other pigments. When extremely fine in texture, however, or when a considerable portion of gum, which renders it transparent, can be employed to give connexion or adhesion while flowing, it becomes no less valuable in water than in oil; but when its vivid azure is to be preserved, as in illuminated manuscripts and missals, little gum must be used. The fine greens, purples, and grays of the old masters, are often unquestionably compounds of ultramarine; and formerly it was the only blue known in fresco. Pure ultramarine varies in shade from light to dark, and in hue from pale warm azure to the deepest cold blue. Native ultramarine consists of silica, alumina, sulphur, and soda; its colouring matter seeming to be due to hyposulphite of soda and sulphide of sodium. In these respects, as well as in that of being decolourised by acids, the natural product resembles the artificial. As a precious material, the former has been subject to adulteration; and it has been dyed, damped, and oiled to enrich its appearance; attempts of fraud, however, which may be easily detected. In the preceding edition of this work the author adds--"and the genuine may be as easily distinguished from the spurious by dropping a few particles of the pigment into lemon-juice, or any other acid, which almost instantly destroys the colour of the true ultramarine totally, and without effervescence." With this statement, so far as it pretends to be a test for the two kinds, we are not inclined to agree. Genuine ultramarine is always decolourised by acids; but it depends on the mode and nicety of its preparation whether it is decolourised without effervescence: that this is the case the author himself admits in his article on artificial ultramarine. Moreover, the "violent effervescence" which he describes as ensuing on the latter being dropped into an acid, does not of necessity take place: in M. Guimet's finest variety, the brilliant ultramarine, acid produces little or no effervescence. Seeing, therefore, that both sorts are decolourised by acids, and that both may or may not effervesce therewith, the acid test must be considered fallacious. Experiments made with different samples of each, showed that native ultramarines offered greater resistance to acid than the artificial, taking longer to decolourise; and that the residues of the first were in general of a purer white than those of the last. It was also found that the brilliant ultramarine, above referred to, was less readily decolourised than other French or German kinds. * * * * * 137. _Blue Carmine._