§ 16. To follow the subject farther would, however, lead us into doctrine of circular storms, and all kinds of pleasant, but infinite, difficulty, from which temptation I keep clear, believing that enough is now stated to enable the reader to understand what he is to look for in Turner's skies; and what kind of power, thought, and science are involved continually in the little white or purple dashes of cloud-spray, which, in such pictures as the San Benedetto, looking to Fusina, the Napoleon, or the Temeraire, guide the eye to the horizon more by their true perspective than by their aërial tone, and are buoyant, not so much by expression of lightness as of motion.[6] § 17. I say the "white or purple" cloud-spray. One word yet may be permitted me respecting the mystery of that color. What should we have thought--if we had lived in a country where there were no clouds, but only low mist or fog--of any stranger who had told us that, in his country, these mists rose into the air, and became purple, crimson, scarlet, and gold? I am aware of no sufficient explanation of these hues of the upper clouds, nor of their strange mingling of opacity with a power of absorbing light. All clouds are so opaque that, however delicate they may be, you never see one through another. Six feet depth of them, at a little distance, will wholly veil the darkest mountain edge; so that, whether for light or shade, they tell upon the sky as body color on canvas; they have always a perfect surface and bloom;--delicate as a rose-leaf, when required of them, but never poor or meagre in hue, like old-fashioned water-colors. And, if needed, in mass, they will bear themselves for solid force of hue against any rock. Facing p. 339, I have engraved a memorandum made of a clear sunset after rain, from the top of Milan cathedral. The greater part of the outline is granite--Monte Rosa--the rest cloud; but it and the granite were dark alike. Frequently, in effects of this kind, the cloud is darker of the two.[7] And this opacity is, nevertheless, obtained without destroying the gift they have of letting broken light through them, so that, between us and the sun, they may become golden fleeces, and float as fields of light. Now their distant colors depend on these two properties together; partly on the opacity, which enables them to reflect light strongly; partly on a spongelike power of gathering light into their bodies. § 18. Long ago it was noted by Aristotle, and again by Leonardo, that vaporous bodies looked russet, or even red, when warm light was seen through them, and blue when deep shade was seen through them. Both colors may, generally, be seen on any wreath of cottage smoke. Whereon, easy conclusion has sometimes been founded by modern reasoners. All red in sky is caused by light seen through vapor, and all blue by shade seen through vapor.