At a distance from the sun the nebulous agglomerations which constitute a comet preserve a spherical or globular form, a certain indication that their molecules obey the preponderating action of the nucleus. This form would be preserved if no foreign influence interfered to derange their mutual positions or to disturb the general equilibrium.

But the comet, when approaching its perihelion, is subjected more and more to the attractive power of the sun, whose enormous mass suffices to change the spherical form of the cometary nebula, to render it more and more ellipsoidal, and finally to carry away beyond the sphere of the attraction of the nucleus whole strata of the nebulosity. This is proved beyond a doubt, as we have seen, by the analysis of M. Roche. In addition to the action of the solar mass there is likewise the action of radiated heat from the sun, which determines changes of great importance : the emission of vaporous matter from the nucleus, luminous jets, aigrettes, and successive concentric envelopes. If the tails of comets, as everything leads us to believe, are material realities, and not simple visual effects; if they are molecules detached from the nebulosity and projected far beyond it by a repulsive force, we may say that, having passed beyond the preponderating action of the nucleus, they have for the moment become foreign to the comet itself, which has thus suffered a portion, however small, of its matter or its mass to escape.