316 Stoney— Of Atmospheres upon Planets and Satellites. 
temperature 7, can escape from B as freely as helium does from the Earth at a 
temperature of — 66°C. This and all lighter gases will escape. 
To determine what density of gas will be imprisoned by B as firmly as water 
is by the Earth, we proceed in a similar way. Here 
yy 
WT OEE? (18) 
and the rest of the work is the same as before, giving as its result the value of 
p,/T’, where p; is the density of a gas which will find it as difficult to escape from 
B as water does from the Earth. It and all denser gases will be retained. 
The investigation leaves uncertain the fate of gases whose density lies 
between p, and p». 
CuapterR VI.—Of the Moon. 
When we turn to the Moon, we find the conditions to be such that it can rid 
itself of an atmosphere with much ease. Upon the Moon 
ry (its radius), = 1738 km. 
Fy (the ratio of its mass to that of the Earth), . = 0:01235 
P (its period of rotation), ; : : . = 2,360,591 seconds. 
Calculating 2’, the least velocity which would enable a missile to quit the Moon 
by the equations in the last chapter, we find it to be about 2°38 km./sec., while 
on the Earth it is 11:015 km./sec., which, by the help of the rotation of the Earth 
and possible storm, may be, under favourable circumstances, furnished by a re- 
lative projectile velocity of 10°5 km./sec. Accordingly, more massive molecules 
can disengage themselves from the Moon with the same facility with which helium 
can leave the Earth, if p, their molecular mass, is greater than that of helium, in 
the ratio of the square of 10°5 to the square of 2°38, ze. if the molecules are 19°5 
times heavier than those of helium, or, which is the same thing, 39 times heavier 
than those of hydrogen. Accordingly, hydrogen sulphide, with a molecular mass 
17 times that of hydrogen, oxygen with a molecular mass of 16, nitrogen with a 
molecular mass of 14, and the vapour of water with a molecular mass of 9, will 
hurry away. They will all escape with greater facility than hydrogen does from 
the Earth. A like fate will befall argon with a molecular mass of 20, carbon 
dioxide with its molecular mass of 22, carbon disulphide with its molecular mass 
of 38, and all others of the gases emitted by volcanoes, or from fissures, of which 
the vapour density is less than 39. These will escape with greater promptness 
than does helium from the Earth. 
