Upon the more intimate union, or the blending and gradience of contrasts from one to another mutually, depend some of the most fascinating effects of colouring. The practical principle employed in producing them is important, and consists in the blending and gradating by _mixture_, while we avoid the _compounding_ of contrasting colours. That is, the colours must be kept distinct in the act of blending them, or otherwise they will run into dusky neutrality and defile each other. This is the case in blending and gradating from green to red, or from hue to hue--from blue to orange, or to and from coldness and warmth--from yellow to purple, or to and from advancing and retiring colours. It is the same in light and shade, or white and black, which _mix_ with clearness. Now, there are only two ways in which this distinctness in union of contrasts can be effected in practice: the one is by hatching or breaking them together in mixture, without compounding them uniformly; and the other is by glazing, in which the colours unite and penetrate mutually, without monotonous composition. The former process may be said to be the carrying out of the principle of separate colours to the utmost possible refinement, by using atoms of colour in juxtaposition, instead of in large spaces. And it is to be noted, in filling up minute interstices of this kind, that if the colour with which they are filled be wanted to show brightly, a rather positive point of it had better be put, with a little white left beside or round it in the interstice. This plan is preferable to laying a pale tint of the colour over the whole interstice. Yellow or orange, for instance, will hardly show, if pale, in small spaces; but they show brightly in free touches, however small, with white beside them. The latter mode is founded on the fact, that if a dark colour be laid first, and a little blue or white body-colour struck lightly over it, a more beautiful gray will be obtained than by mixing the colour and the blue or white. Similarly, if over a solid and perfectly dry touch of vermilion there be quickly washed a little very wet carmine, a much more brilliant red will be produced than by mixing the two colours. Transparency and opacity constitute another contrast of colouring, the former of which belongs to shade and blackness, the latter to light and whiteness. Even contrast has its contrast, for _gradations_ or _intermedia_ are opposed to contrasts or _extremes_; and, upon the right management of contrasts and gradations depend the harmony and melody, the tone, effect, and general expression of a picture. Thus, painting is an affair of judicious contrasting so far as regards colour, if even it be not such altogether.