Time produces in many cases a mellow and harmonious change in pictures, but occasionally alterations altogether unfavourable. To ensure the former and prevent the latter, the attention of the artist in the course of his colouring should be to the employment of such pigments and colours as are prone to adapt themselves, in changing, to the intended key of his colouring, and the right effect of his picture. Thus, if he design a cool effect, ultramarine has a tendency through time to predominate and aid the natural key of blue. He will, therefore, compromise the permanence of this effect, if in such case he employ a declining or changeable blue, or if he introduce such reds and yellows as have a tendency to warmth or foxiness, by which the colouring of many pictures has been destroyed. In a glowing or warm key, the case is in some measure reversed--not wholly so, for it is observable that those pictures have best preserved their colouring and harmony in which the blue has been most lasting, by the pigment counteracting the change of colour in the vehicle, and that suffusion of dusky yellow which time is wont to bestow upon pictures even of the best complexion. Unless introduced and guaranteed by houses of acknowledged reputation, newly discovered pigments are to be used with caution. Good colours have ever been prized with so true an estimation of their value, that to produce such, after so many ages of research is no ordinary accomplishment. But too many resplendent pigments, fruits of the fecundity of modern chemistry, have been found deficient. The yellow and orange chromates of lead, for instance, withstanding as they do the action of the sunbeam, become by time, foul air, and the influence of other pigments, inferior to the ochres. So the dazzling scarlet of iodine and mercury must yield the palm of excellence to the more sober vermilion, being a chameleon colour, subject to the most sudden and opposite changes. And the blues of cobalt, as always tending to greenness and obscurity, cannot rank beside ultramarine.