Estimating the velocity of the molecules of the various atmospheric gases, according to the kinetic theory, Dr.Stoney finds that some of the smaller planets, and the moon, are gravitationally incapable of retaining all of these gases in the form of an atmosphere.

Among the atmospheric constituents that, according to this view, Mars would be unable permanently to retain is water vapor.

Indeed, he supposes that even the earth is slowly losing its water by evaporation into space, and on Mars, owing to the slight force of gravity there, this process would go on much more rapidly, so that, in this way, we have a means of accounting for the apparent drying up of that planet, while we may be led to anticipate that at some time in the remote future the earth also will begin to suffer from lack of water, and that eventually the chasms of the sea will yawn empty and desolate under a cloudless sky.
But it is not certain that the original supply of atmospheric elements was in every case proportional to the respective force of gravity of a planet.

The fact that Venus appears to have an atmosphere more extensive and denser than the earth's, although its force of gravity is a little less than that of our globe, indicates at once a variation as between these two planets in the amount of atmospheric material at their disposal.

This may be a detail depending upon differences in the mode, or in the stage, of their evolution.