is believed generally to act was there sufficiently explained, and it
only remains to see how it is theoretically extended to the particles
of matter supposed to constitute the Zodiacal Light. We know that
corpuscles, or ``fragments of atoms'' negatively electrified, are
discharged from hot bodies. Streams of these ``ions'' pour from many
flames and from molten metals; and the impact of the cathode and
ultra-violet rays causes them to gush even from cold bodies. In the
vast laboratory of the sun it is but reasonable to suppose that
similar processes are taking place. ``As a very hot metal emits these
corpuscles,'' says Prof. J. J. Thomson, ``it does not seem an
improbable hypothesis that they are emitted by that very hot body, the
sun.'' Let it be assumed, then, that the sun does emit them; what
happens next? Negatively charged corpuscles, it is known, serve as
nuclei to which particles of matter in the ordinary state are
attracted, and it is probable that those emitted from the sun
immediately pick up loads in this manner and so grow in bulk. If they
grow large enough the gravitation of the sun draws them back, and they
produce a negative charge in the solar atmosphere. But it is probable
that many of the particles do not attain the critical size which,
according to the principles before explained, would enable the
gravitation of the sun to retain them in opposition to the pressure of
the waves of light, and with these particles the light pressure is
dominant. Clouds of them may be supposed to be continually swept away
from the sun into surrounding space, moving mostly in or near the
plane of the solar equator, where the greatest activity, as indicated
by sunspots and related phenomena, is taking place. As they pass
outward into space many of them encounter the earth. If the earth,
like the moon, had no atmosphere the particles would impinge directly
on its surface, giving it a negative electric charge. But the presence
of the atmosphere changes all that, for the first of the flying