Complexity and extent of the question–The law of gravitation suffices to explain the movements of comets–Lacunse in the theory; acceleration of the motion of the comets of Encke and Faye–Origin of comets; their systems–Questions relative to their physical and chemical constitution–Form of atmospheres; birth and development of tails.

Let us glance back for a moment at the contents of the preceding chapters.

We there find many facts accumulated, observations both interesting and instructive, phenomena whose variations suggest reflections without limit concerning the nature of the bodies to which they relate. Nevertheless, do these collected facts permit a clear and certain reply to the simple question: What is a comet?

I say a simple question, for so, as a rule, it is thought to be by non-scientific people; but in reality there is no question more complex. In order to attempt to reply to it, or at least to relate what is known for certain about comets, and to pass in review the most probable conjectures on doubtful points, we must proceed methodically, and thus as it were divide the difficulty.

A first natural division of the subject is at once apparent, it seems to us, from the exposition of cometary phenomena which has been made in the preceding chapters.