In that case the light pressure will prevail over
the attraction of gravitation, and propel the attenuated matter away
from the sun in the teeth of its attraction. The earth itself would be
driven away if, instead of consisting of a solid globe of immense
aggregate mass, it were a cloud of microscopic particles. The reason
is that the pressure varies in proportion to the surface of the body
acted upon, while the gravitational attraction is proportional to the
volume, or the total amount of matter in the body. But the surface of
any body depends upon the square of its diameter, while the volume
depends upon the cube of the diameter. If, for instance, the diameter
is represented by 4, the surface will be proportional to 4 ? 4, or 16,
and the volume to 4 ? 4 ? 4, or 64; but if the diameter is taken as 2,
the surface will be 2 ? 2, or 4, and the volume 2 ? 2 ? 2, or 8. Now,
the ratio of 4 to 8 is twice as great as that of 16 to 64. If the
diameter is still further decreased, the ratio of the surface to the
volume will proportionally grow larger; in other words, the pressure
will gain upon the attraction, and whatever their original ratio may
have been, a time will come, if the diminution of size continues, when
the pressure will become more effective than the attraction, and the
body will be driven away.
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