Stonry— Of Atmospheres upon Planets and Sutellites. 325 
as light as two-thirds, nor Uranus any lighter than three-quarters of the density of 
hydrogen. On the whole, the probability seems to be that the atmosphere of 
Saturn is nearly the same as that of Jupiter; while the atmospheres of Uranus and 
Neptune more nearly approximate to that of the Earth, with perhaps the addition 
of any gases with densities less than 7°34 that may possibly have left the Earth 
when the Earth was hotter, and whose withdrawal from the Earth is perhaps 
what has left the gaps in the series of terrestrial elements which appear to exist 
between hydrogen and helium, and between helium and lithium. 
Cuarrer XII.—Of the Satellites and Minor Planets. 
We have no sufficient information as to the densities of any of these bodies. 
But the asteroids, or minor planets, which lie between the orbits of Mars and 
Jupiter, are all of them bodies so small that, even if they were as dense as osmium, 
iridium, or platinum, they could not retain their hold upon an atmosphere. The 
same may be said of the two satellites of Mars, of the new satellite of Jupiter, of 
most of the satellites of Saturn, and of the small bodies that make up the rings of 
Saturn. None of these can condense any atmosphere upon them. If there are 
molecules of gases travelling in their neighbourhood, they also are, each of them, 
an independent satellite. 
One satellite of Saturn and three of Jupiter are larger than our Moon ; and one 
other of Saturn and one of Jupiter, though smaller than the Moon, are not much 
smaller. We should need to know the densities of these bodies before we could 
speak with confidence about them. The presumption, however, is, that as their 
primaries are very much less dense than the Earth, so these satellites are probably 
less dense than the Moon. If so, they also, as well as the smaller satellites, must 
be devoid of atmosphere. 
We know too little about the satellites of Uranus and Neptune to venture upon 
any conclusion about them. The satellite of Neptune appears to be a body of 
considerable size, and, with some probability, it may have an atmosphere. 
Cuapter XIII.— What becomes of the Molecules that escape. 
The speed of the Earth in its orbit is about 830 km./sec. Now it follows, from 
the dynamics of potential, that the potential of the Sun at the distance of the 
Earth is represented by the square of this number if the Sun’s mass be measured 
in gravitational units. That is 
k =~ = 900, 
TRL 
where m is the mass of the Sun, and + the radius of the Earth’s orbit. 
