Failures in the process of burning carmines, and preparing the purple of gold, frequently afford good marrones. Compounds more or less of that hue are likewise furnished by copper, mercury, &c. Some ochres incline to marrone when calcined: indeed we have remarked in many instances that the action of fire anticipates the effects of long continued time; and that several of the primary and secondary colours may, by different degrees of burning, be converted into their analogous secondary, tertiary, or semi-neutral colours. The one marrone or brown-marrone pigment at present employed, brown madder, is permanent. FOOTNOTES: [B] This was written previous to the opening of the new rooms of the Royal Academy at Burlington House. In these, among other improvements, the subject of wall space has been considered. CHAPTER XIX. ON THE SEMI-NEUTRAL, GRAY. Of the tribe of semi-neutral colours, GRAY is third and last, being nearest in relation to black. In its common acceptation, and that in which we here use it, gray, as was observed in the third chapter, denotes a class of cool cinereous colours faint of hue; whence we have blue grays, olive grays, green grays, purple grays, and grays of all hues, in which blue predominates; but no yellow or red grays, the prevalence of such hues carrying the compounds into the classes of brown and marrone, of which gray is the natural opposite. In this sense the _semi-neutral_ Gray is distinguished from the _neutral_ Grey, which springs in an infinite series from the mixture of the neutral black and white. Between gray and grey, however, there is no intermediate, since where colour ends in the one, neutrality commences in the other, and vice versâ. Hence the natural alliance of the semi-neutral gray--definable as a cool coloured grey--with black or shade; an alliance which is strengthened by the latent predominance of blue in the synthesis of black, so that in the tints resulting from the mixture of black and white, so much of that hue is developed as to give apparent colour to the tints. This explains why the tints of black and dark pigments are colder than their originals, so much so as in some instances to answer the purposes of positive colours. It accounts in some measure for the natural blueness of the sky, yet not wholly, for this is in part dependent, by contrast, upon the warm colour of sunshine to which it is opposed; for, if by any accident the light of nature should be rendered red, the colour of the sky would not appear purple, in consequence, but green. Again, if the sun shone green, the sky would not be green, but red inclined to purple; and so would it be with all colours, not according to the laws of composition, but of contrast; since, if it were otherwise, the golden rays of the sun would render a blue sky green.