First, we must remember that the nearer a planet is to the sun the
greater is the amount of heat and light that it receives, the variation
being proportional to the inverse square of the distance. The earth's
distance from the sun being 93,000,000 miles, while Mercury's is only
36,000,000, it follows, to begin with, that Mercury gets, on the
average, more than six and a half times as much heat from the sun as the
earth does. That alone is enough to make it seem impossible that Mercury
can be the home of living forms resembling those of the earth, for
imagine the heat of the sun in the middle of a summer's day increased
six or seven fold! If there were no mitigating influences, the face of
the earth would shrivel as in the blast of a furnace, the very stones
would become incandescent, and the oceans would turn into steam.

Still, notwithstanding the tremendous heat poured upon Mercury as
compared with that which our planet receives, we can possibly, and for
the sake of a clearer understanding of the effects of the varying
distance, which is the object of our present inquiry, find a loophole to
admit the chance that yet there may be living beings there. We might,
for instance, suppose that, owing to the rarity of its atmosphere, the
excessive heat was quickly radiated away, or that there was something in
the constitution of the atmosphere that greatly modified the effective
temperature of the sun's rays. But, having satisfied our imagination on
this point, and placed our supposititious inhabitants in the hot world
of Mercury, how are we going to meet the conditions imposed by the
rapid changes of distance--the swift fall of the planet toward the sun,
followed by the equally swift rush away from it? For change of distance
implies change of heat and temperature.

It is true that we have a slight effect of this kind on the earth.