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.