320 Stoney— Of Atmospheres upon Planets and Satellites. 
not attain a velocity 18 times that of mean square sufficiently often to enable the 
gas to escape from an atmosphere in appreciable quantities. 
We are accordingly now in a position to go beyond the statement made on 
p. 314. We may now say— 
1°. A velocity of 9:27 times that of mean square is attained by the molecules 
of a gas sufficiently often to enable helium to escape from the Earth. 
2°. A velocity 18 times that of mean square is so seldom attained that Venus 
has been able to retain its stock of water. 
3°. Since Venus can prevent the escape of water, the Earth, with its larger 
potential, is competent to retain its hold upon a gas of somewhat less density, 
viz., one whose density p = 7°43. 
Accordingly, as regards the Earth, we may come to the following conclusions: 
1°, Gases with a density of 2 or less than 2 can certainly escape from the Earth; 
2°, a gas witha density of 7:43, and all denser gases* are effectually imprisoned by 
the Earth; 3°, the information supplied by Venus, supplemented by our present 
chemical knowledge, does not determine what would be the fate of a gas, if there 
be such, whose density lies between 2 and 7:43. 
CuapTer I[X.—Of Mars. 
The case of Mars is one of exceptional interest. Using the data furnished 
by the Nautical Almanac, we find its radius to be 
y = 3372 km. 
As in the case of Mercury, its mass is not yet known with exactness. It has 
become better known since observations have been made on the elongations of 
its satellites, which seem to furnish the value 
m 
a= 01074. 
Its period of rotation is known, viz.: 88648 seconds; whence and from its 
radius we find 
w (the velocity at the equator due to rotation) = 239 m. /sec. 
* Ammonia NH;, and Methane CHj,, are a little above this limit, and therefore can neither of them 
escape. Ammonia is no doubt washed out of the Earth’s atmosphere by rain; but it is not easy to see 
what becomes of the methane. It seems unlikely on chemical grounds that it directly combines with 
oxygen, furnishing water and carbon dioxide. Possibly it meets with a trace of chlorine, and furnishes 
methyl chloride and hydrogen in the presence of sunshine ; or possibly it is nitro-methane that is formed. 
