WHAT is meant by the physical or chemical constitution of a celestial body, or of any luminary whatever, whether star or sun, planet or moon; or, as we are treating of comets only, what is meant by the physical or chemical constitution of a comet?

We have presented for our consideration a question the nature of which is easily explained and not less easily understood; but it is one that the best-informed of astronomers would find it difficult to answer in its full integrity.

By comparison with the bodies that we see on the surface of the earth and with the terrestrial globe itself, considered as a whole, we shall proceed to explain what is meant by the physico-chemical constitution of a comet.

The earth is a globe, more accurately, a spheroid, whose form and dimensions are perfectly defined and well known, at all events as far as concerns its solid crust, the atmosphere that surrounds it, and the rocks and strata near its surface.

Comets are, from all these points of view, their movements alone excepted, conspicuously different from the earth and the rest of the planets.