may be changed we
cannot tell. But if we conceive comets to be only gas, it would
expand indefinitely in the realms of space, where there is no force
of compression but its own. We might say that comets are composed
of small separate masses of matter, hundreds of miles apart; and,
looking through thousands of miles of them, we see light enough
reflected from them all to seem continuous. Doubtless that is sometimes
the case. But the spectroscope shows another state of things: it
reveals in some of these comets an incandescent gas--usually some
of the combinations of carbon. The conclusion, then, naturally is
that there are both gas and small masses of matter, each with an
orbit of its own nearly parallel to those of all the others, and
that they afford some attraction to hold the mass of intermingled and
confluent gas together. Our best judgment, then, is that the nucleus
is composed of separate bodies, or matter in a liquid condition,
capable of being vaporized by the heat of the sun, and driven off,
[Page 133] as steam from a locomotive, into a tail. Indications of
this are found in the fact that tails grow smaller at successive
returns, as the matter capable of such vaporization becomes
condensed. In some instances, as in that of the comet of 1843, the
head was diminished by the manufacture of a tail. On the other hand,
Professor Peirce showed that the nucleus of the comets of 1680,
1843, and 1858 must have had a tenacity equal to steel, to prevent
being pulled apart by the tidal forces caused by its terrible
perihelion sweep around the sun.

It is likely that there are great varieties of condition in different
comets, and in the same comet at times. We see them but a few days
out of the possible millions of their periodic time; we see them
only close to the sun, under the spur of its tremendous attraction
and terrible heat. This gives us ample knowledge of the path of
their orbit and time of their revolution, but little ground for
judgment of their condition, when they slowly round the uttermost
cape of their far-voyaging, in the terrible cold and darkness,
to commence their homeward flight. The unsolved problems are not
all in the distant sun and more distant stars, but one of them
is carried by us, sometimes near, sometimes far off; but our
acquaintance with the possible forms and conditions of matter is
too limited to enable us to master the difficulties.

_Will Comets strike the Earth?_

Very likely, since