The loose friable lumps into which the powder had aggregated were distinctly paler on the outside than in the interior, while the parts of the contents of the bottles which had been most exposed to light were paler than those which had been comparatively shaded. But a still more marked change had taken place in the samples to which the term 'pale' cadmium might be applied. These had generally become still paler, almost straw-coloured, especially where most exposed to light; but in some of the specimens orange specks were observed, resembling in hue what is usually called 'middle' cadmium. From the above observations it would seem that there is a tendency in differently tinted 'wet process' cadmium yellows to return to what we may call the normal or medium hue, but that the palest varieties are most subject to change. This change seems to arise in part from oxidation and hydration, for the bleached specimens gave indications of containing some white cadmium hydrate, when heated giving off a little water, and becoming brownish from the formation of the brown oxide of cadmium.

Such a bleaching of pale cadmium, if my explanation be correct, is in a measure explicable if we recollect that this variety occurs in a very fine state of division, and on this account is more liable to chemical change. In water-colour painting, where there is no effective protection through the presence of a hydrofuge medium, this fading of 'wet process' pale cadmium is notorious. In oils this cadmium, like the others, is generally thought to be permanent. My faith in the inalterability of cadmium pigments, even in oil and allied media, has, however, been somewhat shaken during recent years. Cadmium orange has almost perished where used in Leighton's lunette 'Arts of Peace,' in the Victoria and Albert Museum, a work executed in spirit-fresco. I regard the passage of the pale and of the deep cadmium yellow when in powder into the normal or middle variety as dependent chiefly, if not entirely, upon molecular changes. Moreover, the pale cadmiums are rarely found free from admixture, and their alterability may be in part owing to the foreign ingredients they contain. More recent researches by G. Buchner and N. von Klobukoff confirm the conclusions drawn from my early experiments.

There can be no doubt that cadmium sulphide exists in two if not in three molecular states, differing not only in colour but in crystalline form and in specific gravity. Thus pale cadmium has the specific gravity 3.9 to 4.5, while the red modification is denser - 4.5 to 4.8. And when the pale variety, dry and in powder, is rubbed strongly with a piece of agate, its colour deepens and reddens in a very decisive manner. The same change occurs when a water-colour wash of cadmium yellow is exposed for a year or so to sunlight in a perfectly dry atmosphere. This phenomenon is clearly analogous with that shown when the yellow mercuric iodide is altered into the scarlet form by pressure. It is perhaps safer to employ an ivory palette knife rather than one of steel in manipulating the cadmium pigments.

Aurora yellow is a bright and beautiful pigment consisting essentially of cadmium sulphide. It has more opacity than most of the other varieties of cadmium and possesses a pure yellow hue. Its stability is greater than that of many other varieties of this pigment. Daffodil yellow is the name given to another variety of cadmium sulphide, prepared at a red heat and containing a small quantity of magnesia. Neutral orange is a mixture of cadmium yellow with Venetian red.

Cadmium yellows are sometimes adulterated with Indian yellow, baryta and strontia chromates, and chromates of lead. Indian yellow shows its presence by blackening and giving off tarry fumes when the pigment, in the state of dry powder, is strongly heated in a test-tube. The chromates may be detected by the green colour produced when the sample is warmed with alcohol and dilute sulphuric acid. The lead chromates or chrome yellows, and the orange and red basic chromates of the same metal will blacken when the substance in which they are present is moistened with weak ammonium sulphide. Free sulphur in pale cadmium yellows comes off as a vapour when the sample is heated, but it may be better detected by the solvent action upon it of carbon bisulphide. Baryta-white may be detected by its insolubility in hot strong hydrochloric acid, in which cadmium sulphide dissolves.

Cadmium red and cadmium orange are slightly translucent when compared with the paler and yellower varieties of this pigment, and possess very full and glowing hues. They work well as oil and water colours. Mixed with zinc-white or flake-white, deep and middle cadmiums yield several beautiful colours, some of which closely resemble the different varieties of true Naples yellow, and are now employed very largely in lieu of the latter pigment. Pure cadmium yellow, when heated moderately, becomes orange-red or red, but regains its pristine hue on cooling. If, however, the heat be considerably raised in the presence of air, some of the sulphur in the compound burns, and the residual mass presents a dull brown colour. 'Manganese oil' accelerates the drying of the cadmium colours, which is sometimes inconveniently slow.