The conditions of matter with which we are acquainted do not cover all the ground presented by these mysterious visitors.  We know a gas sixteen times as light as air, but hydrogen is vastly too heavy and dense; for we see the faintest star through thousands of miles of cometary matter; we know that water may become cloudy vapor, but a little of it obscures the vision.  Into what more ethereal, and we might almost say spiritual, forms matter 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?