see that low density goes along with great size
and great velocity of rotation, and that high density goes along with
small size and small velocity of rotation. In the second place, we are
shown this relation still more clearly if we compare the extreme
instances--Saturn and Mercury. The special contrast of these two, like
the general contrast of the groups, points to the truth that low
density, like the satellite-forming tendency, is associated with the
ratio borne by centrifugal force to gravity; for in the case of Saturn
with his many satellites and least density, centrifugal force at the
equator is nearly 1/6th of gravity, whereas in Mercury with no satellite
and greatest density centrifugal force is but 1/360th of gravity.

There are, however, certain factors which, working in an opposite way,
qualify and complicate these effects. Other things equal, mutual
gravitation among the parts of a large mass will cause a greater
evolution of heat than is similarly caused in a small mass; and the
resulting difference of temperature will tend to produce more rapid
dissipation of heat. To this must be added the greater velocity of the
circulating currents which the intenser forces at work in larger
spheroids will produce--a contrast made still greater by the relatively
smaller retardation by friction to which the more voluminous currents
are exposed. In these causes, joined with causes previously indicated,
we may recognize a probable explanation of the otherwise anomalous fact
that the Sun, though having a thousand times the mass of Jupiter, has
yet reached as advanced a stage of concentration. For the force of
gravity in the Sun, which at his surface is some ten times that at the
surface of Jupiter, must expose his central parts to a pressure
relatively very intense; producing, during contraction, a relatively
rapid genesis of heat. And it is further to be remarked that, though the
circulating currents in the Sun have far greater distances to travel,
yet since his rotation is relatively so slow that the angular velocity
of his substance is but about one-sixtieth of that of Jupiter's
substance, the resulting obstacle to circulating currents is relatively
small, and the escape of heat far less retarded. Here, too, we may note
that in the co-operation of these factors, there seems a reason for the
greater concentration reached by Jupiter than by Saturn, though Saturn
is the elder as well as the smaller of the two; for at the s