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conclusion would be that the comet is a glowing gas, shining as incandescent gases do in our chemical laboratories. That such should be the case and the whole case seems impossible for two reasons. The comet cannot be hot enough to glow; and its light fades out to nothing as it recedes from the sun. The most likely conclusion seems to be that the action of the sun's rays causes a glow through some process which has not yet been made clear to us.

What seems certain is that the matter of which a bright comet is composed is volatile. When a bright comet is carefully scrutinised with a telescope, masses of vapour can be seen from time to time slowly rising from its head in the direction of the sun, then spreading out and moving away from the sun so as to form the tail. The latter is not an appendage which the comet carries as animals carry their tails, but is like a stream of smoke issuing from a chimney.

It frequently happens that when a comet is first discovered it has no tail at all. The latter begins to form when the sun is approached. The nearer the comet approaches the sun, and the greater the heat to which it is exposed, the more rapidly the tail develops. All this shows that the matter which composes a great comet is, in part volatile. When warmed by the heat of the sun it begins to evaporate, just as water would under the same circumstances. The steam or vapour thus arising is repelled by the sun, so as to form a stream of matter issuing from the comet.