It may be objected that clouds are a form of moisture, not of air. They are, however, a perfect expression of aërial states and currents, and may sufficiently well stand for the element they move in. And I have put vegetation apparently somewhat out of its place, owing to its vast importance as a means of decoration, and its constant association with birds and men. § XVIII. 1. Abstract lines. I have not with lines named also shades and colors, for this evident reason, that there are no such things as abstract shadows, irrespective of the forms which exhibit them, and distinguished in their own nature from each other; and that the arrangement of shadows, in greater or less quantity, or in certain harmonical successions, is an affair of treatment, not of selection. And when we use abstract colors, we are in fact using a part of nature herself,--using a quality of her light, correspondent with that of the air, to carry sound; and the arrangement of color in harmonious masses is again a matter of treatment, not selection. Yet even in this separate art of coloring, as referred to architecture, it is very notable that the best tints are always those of natural stones. These can hardly be wrong; I think I never yet saw an offensive introduction of the natural colors of marble and precious stones, unless in small mosaics, and in one or two glaring instances of the resolute determination to produce something ugly at any cost. On the other hand, I have most assuredly never yet seen a painted building, ancient or modern, which seemed to me quite right.