Stonsy— Of Atmospheres upon Planets and Satellites. 307 
Of this series of communications, though known to many, only imperfect printed 
accounts have appeared; and it is the object of the present communication to 
present the subject in a more complete form. The opportunity will be taken of 
substituting better numerical results for those originally given, by basing them on 
the fact which has recently come to our knowledge, that not only hydrogen, but 
helium also, with a density twice that of hydrogen, can escape from the Earth. 
The most notable change that this makes is, that what was before probable is 
now certain—that water cannot in any of its forms be present upon Mars. 
Crarter I.—Of the Fundamental Facts. 
In order to see why neither hydrogen nor helium remains in the Earth’s 
atmosphere, and why there is neither air nor water on the Moon, it is neces- 
sary to understand the conditions which determine the limit of an atmosphere. 
These were investigated under the kinetic theory of gas by the present writer 
in a memoir communicated to the Royal Society in May, 1867: see his Paper 
“On the Physical Constitution of the Sun and Stars” in the Proceedings of the 
Royal Society, No. 105, 1868, from page 13 of which it will be convenient to 
make the following extract* :— 
‘23. Let us consider what it is that puts a limit to the atmosphere. Let us first suppose that it 
consists of but one gas, and let us conceive a layer of this gas between two horizontal surfaces of indefinite 
extent, so close that the interval between them is small compared with the mean distance to which mole- 
cules dart between their collisions, but yet thick enough to have, at any moment, several molecules within it. 
Molecules are constantly flying in all directions across this thin stratum. Some of them come within the 
sphere of one another’s influence while within the layer, and therefore pass out of it with altered direction 
and speed. Let us call these the molecules emitted by the layer. If the same density and pressure 
prevail above and belowythe layer, the molecules which strike down into it will, on account of gravity, 
arrive with somewhat more speed on the average than those which rise into it. Hence those molecules 
which suffer collision within the stratum will not scatter equally in all directions, but will have a 
preponderating downward motion, so that of the molecules emitted by the stratum more will pass down- 
wards than upwards. This state of things is unstable, and will not arrive at an equilibrium until either 
the density or the temperature is greater on the underside of the layer. If the density be greater, more 
molecules will fly into the stratum from beneath than from aboye: and if the temperature be greater the 
molecules will strike up into it, both more frequently and with greater speed. In the Earth’s atmosphere 
it is by a combination of both these that the equilibrium is maintained: both the temperature and the 
density decrease from the surface of the Earth upwards. 
“94, We have hitherto taken into account only those molecules which, after a collision, have arrived 
at the stratum from the side on which the collision took place. But besides these there will be a certain 
number of molecules which, having passed through the stratum from beneath, fall back into it without 
* Further information on this subject will be found in sections 22, 24, 25, 26, and in the footnote 
to section 93, of the paper here quoted. 
3B2 
