Chromates of lead, says Prof. Barff, are produced by precipitating a lead salt with a salt of chromic acid, and the difference in tint is owing to the different quantity of the chromic acid which is present in the salt. The orange chrome is a basic chromate of lead, and basic chromate of lead contains more of the chromic acid than is present in the lemon chrome. The lightest chrome contains some sulphate of lead precipitated with the chromate. All these colours contain lead, and are therefore liable to the influence of sulphuretted hydrogen. Now, if a chromate of lead is bought hap-hazard anywhere, it may or may not be pure, but generally speaking, unless the chromates are obtained from makers who are careful in the preparation of their colours, they contain many other substances besides chromate of lead. For instance, they contain a quantity of the solution, in a dry state, from which they have been precipitated, and are by no means pure. Obviously, also, there is an inducement to men who sell cheap colours to adulterate them, to bring them down with whiting, and so forth. In that case the chrome loses its body; if it is brought down with lead sulphate it has more body; but when chrome yellows are prepared and mixed with good oils, and put on carefully, and those oils have time allowed them to become oxidised, perfectly dry in fact--not dry in the sense in which an artist considers a painting dry, but perfectly hard--then the action of sulphuretted hydrogen in the atmosphere will not be of such great moment as if the colours are impure, or if they are submitted to the influence of that deleterious gas before they have become perfectly hard. Another objection to the chromes, on the authority of Prof. Barff, is that they are soluble in alkali, and so are many other colours. If a painting is painted with chromes, and if that painting be washed with an alkaline soap, it is quite certain that some of the chromates will be dissolved. Consequently the minute and delicate touches, upon which the artist depended for some of the best effects of his picture, are removed with soap and water. No painting should ever be washed with soap and water at all; but there are certain colours which will withstand the action of soap, even if it is intensely alkaline, while others will not. Taking a precipitate of lemon chrome, if we act upon this with potash or soda, it dissolves up the yellow precipitate, and destroys it altogether.