forming in this manner will depend upon the rate at which
it aggregates and the velocity with which the planetismals' fall into
it, and this velocity will increase with its mass and consequent force
of gravity. In the early stages of a planet's growth it will probably
remain cold, the small amount of heat produced by each impact being lost
by radiation before the next one occurs; and with a small and slowly
aggregating planet this condition will prevail till it approaches its
full size. Then only will its gravitative force be sufficient to cause
incoming matter to fall upon it with so powerful an impact as to produce
intense heat. Further, the compressive force of a small planet will be a
less effective heat-producing agency than in the case of a larger one.

The earth we know has acquired a large amount of internal heat, probably
sufficient to liquefy its whole interior; but Mars has only one-ninth
part the mass of the earth, and it is quite possible, and even probable,
that its comparatively small attractive force would never have liquefied
or even permanently heated the more central portions of its mass. This
being admitted, I suggest the following course of events as quite
possible, and not even improbable, in the case of this planet. During
the whole of its early growth, and till it acquired nearly its present
diameter, its rate of aggregation was so slow that the planetismals
falling upon it, though they might have been heated and even partially
liquefied by the impact, were never in such quantity as to produce any
considerable heating effect on the whole mass, and each local rise of
temperature was soon lost by radiation. The planet thus grew as a solid
and cold mass, compacted together by the impact of the incoming matter
as well as by its slowly increasing gravitative force. But when it had
attained to within perhaps 100, perhaps 50 miles, or less, of its
present diameter, a great change occurred in the opportunity for further
growth. Some large and dense swarm of meteorites, perhaps containing a
number of bodies of the size of the asteroids, came within the range of
the sun's attraction and were drawn by it into an orbit which crossed
that of Mars at such a small angle that the planet was able at each
revolution to capture a considerable number of them. The result might
then be that, as in the case of the earth, the continuous inpour of the
fresh matter first heated, and later on liquefied the greater part of it