308 Stonry— Of Atmospheres upon Planets and Satellites. 
having met with other molecules, either by reason of the nearly horizontal direction of their motion, or 
because of its low speed. The number of molecules that will thus fall back into the stratum will be a very 
inconsiderable proportion of the whole number passing through the stratum, so long as the temperature and 
density are at all like what they are at the surface of the Earth. In the lower strata of the atmosphere, 
therefore, the law by which the temperature and density decrease will not be appreciably affected by 
molecules thus falling back. But in those regions where the atmosphere is both cold and very 
attenuated, where accordingly the distance between the molecules is great and the speed with which they 
move feeble, the number of cases in which ascending molecules become descending without having 
encountered others will begin to be sensible. From this point upwards the density of the atmosphere will 
decrease by a much more rapid law, which will within a short space bring the atmosphere to an end.” 
It appears, then, that the atmosphere round any planet or satellite will, 
ceteris paribus, range to a greater height the less gravity upon that body is; 
and that if the potential of gravitation be sufficiently low, and the speed with 
which the molecules dart about sufficiently great, individual molecules will stream 
away from that body, and become independent wanderers throughout space. 
Thus, we shall presently see that, in the case of the Earth, a velocity of about 
eleven kilometres per second (nearly seven miles) would be enough to carry a 
molecule at the boundary of our atmosphere off into space, if the Earth were 
alone and at rest; and a somewhat less velocity of projection (about 10°5 km. 
per second) is sufficient, on account of the rotation of the Earth, and because 
westerly winds sometimes blow in the upper regions of the atmosphere. The 
modification introduced by these subsidiary causes will be examined in 
Chapter IV., and the amount of their effect will be determined. The behaviour 
of molecules is also slightly affected by the Moon, which is near enough sensibly 
to alter the orbits of molecules if shot up in some directions. 
Let us now consider what would happen if free hydrogen could remain in our 
atmosphere. Hydrogen is, in modern times, being supplied in small quantities to 
the Earth’s atmosphere by submarine* volcanoes and in other ways. Even if there 
were no tendency in hydrogen to leak away, it could not in the free state become a 
large constituent of our atmosphere, because, when it came to be a certain propor- 
tion of the atmosphere, it would, on the occasion of the first thunderstorm or on 
account of fires, enter into combination with the oxygen which is, in modern times, 
a large constituent of the atmosphere: but after each such explosion it would accu- 
mulate until it became a minor constituent like carbon dioxide were it not for the 
events described in this Paper; and in former times, before there was vegetation 
to evolve free oxygen, it might have been a large constituent but for those events. 
The free hydrogen which continues in modern times to be supplied in small 
quantities to the atmosphere is used up in some way. A little may be occluded, 
some may suffer surface condensation, and the rest is escaping. 
* The hydrogen evolved by terrestial volcanoes burns into water on reaching the air, and ceases 
to be free hydrogen. 
