It may be noted that Mr. Lowell objects to the carbon-dioxide theory of the formation of the snow-caps, that this gas at low pressures does not liquefy, but passes at once from the solid to the gaseous state, and that only water remains liquid sufficiently long to produce the 'blue colour' which plays so large a part in his argument for the mild climate essential for an inhabited planet. But this argument, as I have already shown, is valueless. For only very deep water can possibly show a blue colour by reflected light, while a dust-laden atmosphere—especially with a layer of very dense gas at the bottom of it, as would be the case with the newly evaporated carbon-dioxide from the diminishing snow-cap—would provide the very conditions likely to produce this blue tinge of colour.
It may be considered a support to this view that carbonic-acid gas becomes liquid at −140°F. and solid at −162°F., temperatures far higher than we should expect to prevail in the polar and north temperate regions of Mars during a considerable part of the year, but such as might be reached there during the summer solstice when the 'snows' so rapidly disappear, to be re-formed a few months later.
The Double Canals.
The curious phenomena of the 'double canals' are undoubtedly the most difficult to explain satis-
