He adduces evidence of an atmosphere, but of an exceedingly scanty one, since the greatest amount he can give to it is "not more than about four inches of barometric pressure as we reckon it"; and he assumes, as he has a fair right to do till disproved, that it consists of oxygen and nitrogen, with carbon-dioxide and water-vapour, in approximately the same proportions as with us.

This point he dwells upon repeatedly, stating, of these blue borders: "This excludes the possibility of their being formed by carbon-dioxide, and shows that of all the substances we know the material composing them must be water.

Then follows this remarkable passage: "Carbon-dioxide, because of its greater specific gravity, would also be in relatively greater amount so far as this cause is considered.

Whence as regards both water-vapour and carbon-dioxide we have reason to think them in relatively greater quantity than in our own air at corresponding barometric pressure."

I cannot understand this passage except as implying that 'water-vapour and carbon-dioxide' are among the heavier and not among the lighter gases of the atmospherethose which the planet 'parts with quickest.'

The first of these statements may be admitted as a fact which he is entitled to dwell upon, but the secondthe presence of large quantities of carbon-dioxide and aqueous vapour is a pure hypothesis unsupported by any item of scientific evidence, while in the case of aqueous vapour it is directly opposed to admitted results founded upon the molecular theory of gaseous elasticity.

He further supposes that water and carbon-dioxide issue from the interior into these fissures, and, in conjunction with sunlight, promote the growth of vegetation.

These heavy gases, mainly perhaps, as has been often suggested, carbon-dioxide, would, when in large quantity and of considerable depth, reflect a good deal of light, and, being almost inevitably dust-laden, might produce that blue tinge adjacent to the melting snow-caps which Mr. Lowell has erroneously assumed to be itself a proof of the presence of liquid water.

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.

For only very deep water can possibly show a blue colour by reflected light, while a dust-laden atmosphereespecially 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.