that at the time they are melting a marginal blue band appears which accompanies them in their retreat, and this blue colour is said to prove conclusively that the liquid is not carbonic acid but water.  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.”

[Footnote 7:  In a paper written since the book appeared the density of air at the surface of Mars is said to be 1/12 of the earth’s.]

This is the only proof of the existence of water he adduces, and it is certainly a most extraordinary and futile one.  For it is perfectly well known that although water, in large masses and by transmitted light, is of a blue colour, yet shallow water by reflected light is not so; and in the case of the liquid produced by the snow-caps of Mars, which the whole conditions of the planet show must be shallow, and also be more or less turbid, it cannot possibly be the cause of the ‘deep blue’ tint said to result from the melting of the snow.

But there is a very weighty argument depending on the molecular theory of gases against the polar caps of Mars being composed of frozen water at all.  The mass and elastic force of the several gases is due to the greater or less rapidity of the vibratory motion of their molecules under identical conditions.  The speed of these molecular motions has been ascertained for all the chief gases, and it is found to be so great as in certain cases to enable them to overcome the force of gravity and escape from a planet’s surface into space.  Dr. G. Johnstone Stoney has specially investigated this subject, and he finds that the force of gravity on the earth is sufficient to retain all the gases composing its atmosphere, but not sufficient to retain hydrogen; and as a consequence, although this gas is produced in small quantities by volcanoes and by decomposing vegetation, yet no trace of it is found in our atmosphere.  The moon however, having only one-eightieth the mass of the earth, cannot retain any gas, hence its airless and waterless condition.

Water Vapour cannot exist on Mars.

Now, Dr. Stoney finds that in order to retain water vapour permanently a planet must have a mass at least a quarter that of the earth.  But the mass of Mars is only one-ninth that of the earth; therefore, unless there are some special conditions that prevent its loss, this gas cannot be present in the atmosphere.  Mr. Lowell does not refer to this argument against his view, neither does he claim the evidence of spectroscopy in his favour.  This was alleged more than thirty years ago to show the existence of water-vapour in the atmosphere of Mars, but of late years it has been doubted, and Mr. Lowell’s complete silence on the subject, while laying stress on such a very weak and inconclusive argument as that from the tinge of