_INTRODUCTORY_ 1

Remarkable popular interest in questions concerning
other worlds and their inhabitants--Theories of
interplanetary communication--The plurality of worlds in
literature--Romances of foreign planets--Scientific interest
in the subject--Opposing views based on telescopic and
spectroscopic revelations--Changes of opinion--Desirability
of a popular presentation of the latest facts--The natural
tendency to regard other planets as habitable--Some of the
conditions and limitations of the problem--The solar system
viewed from outer space--The resemblances and contrasts of
its various planets--Three planetary groups recognized--The
family character of the solar system


CHAPTER II

_MERCURY, A WORLD OF TWO FACES AND MANY CONTRASTS_ 18

Grotesqueness of Mercury considered as a world--Its
dimensions, mass, and movements--The question of an
atmosphere--Mercury's visibility from the earth--Its
eccentric orbit, and rapid changes of distance from the
sun--Momentous consequences of these peculiarities--A
virtual fall of fourteen million miles toward the sun
in six weeks--The tremendous heat poured upon Mercury
and its great variations--The little planet's singular
manner of rotation on its axis--Schiaparelli's astonishing