Note 227 — Chapter XII

On the Colour of Steam under certain circumstances: by Professor Forbes; Philosophical Magazine, vol. xiv. p. 121, vol. xv. p. 25. In the first paper the following remarks occur:—“I cannot doubt that the colour of watery vapour under certain circumstances is the principal or only cause of the red colour observed in clouds. The very fact that that colour chiefly appears in the presence of clouds is a sufficient refutation of the only explanation of the phenomena of sunset and sunrise, having the least plausibility, given by optical writers. If the red light of the horizontal sky were simply complementary to the blue of a pure atmosphere, the sun ought to set red in the clearest weather, and then most of all; but experience shows that a lurid sunrise or sunset is always accompanied by clouds or diffused vapours, and in a great majority of cases occurs when the changing state of previously transparent and colourless vapour may be inferred from the succeeding rain. In like manner, terrestrial lights seen at a distance grow red and dim when the atmosphere is filled with vapour soon to be precipitated. Analogy applied to the preceding observations would certainly conduct to a solution of such appearances; for I have remarked that the existence of vapour of high tension is by no means essential to the production of colour, though of course a proportionally greater thickness of the medium must be employed to produce a similar effect when the elasticity is small.”