But a pressure of 2·1 lb. on the square inch is far less than that experienced by Coxwell and Glaisher in their great ascent; it is about one-half the pressure that is experienced on the top of the very highest terrestrial mountains. But the habitable regions of the Earth do not extend even so far upward as to the level of a pressure of 7·3 lb. on the square inch; that is, of half the terrestrial surface pressure. Plant life dies out before we reach that point, and though birds or men may occasionally attain greater heights, they cannot domicile there, and are, indeed, only able thus to ascend in virtue of nourishment which they have procured in more favoured regions. If we could suppose the conditions of the whole Earth changed to correspond with those prevailing at the summit of Mt. Everest, or even at the summit of Mont Blanc, it is clear that the life now present on this planet would be extinguished, and that speedily. Much more would this be the case if the atmosphere were diminished to one half the pressure on the summit of the highest earthly mountain. The tenuity of the atmosphere on Mars has another consequence. Here water freezes at 0° C. and boils at 100° C.; so that for one hundred degrees it remains in a liquid condition. On Mars, under the assumed conditions, water would boil at 53° C., and the range of temperature within which it would be liquid would be much curtailed. But it is only water in the liquid state that is useful for sustaining life. The above estimate of the density of the atmosphere of Mars is an outside limit, for it assumes that Mars has retained an atmosphere to the full proportion of its mass. But as the molecules of a gas are in continual motion, and in every direction, the lighter, most swiftly moving molecules must occasionally be moving directly outwards from the planet at the top of their speed, and in this case, if the speed of recession should exceed that which the gravity of the planet can control, the particle is lost to the planet for ever. A small planet therefore is subject to a continual drain upon its atmosphere, a drain of the lightest constituents. Hence it is, no doubt, that free hydrogen is not a constituent of the atmosphere of the Earth.