ing the exterior envelope of a
comet, becomes inversely as the 2.5th power of the distances from the
comet's centre.[45] This being the law of the radial stream, it follows,
of course, that a comet's diameter is inversely as the force of the
radial stream. It must, however, be borne in mind, that we are speaking
of the atomic density, and not of density by compression; for this
cometary dust, which renders luminous the escaping ether of the nucleus,
must be far too much diffused to merit the name of an elastic fluid. May
not the concentric rings, which were so conspicuous in the comet of
1811, be owing to differences in the gravitating forces of such
particles, sifted, as it were, and thus arranged, according to some
ratio of the distances, by the centripulsive force of the electric coma,
leaving vacant intervals, through which the ether passed without
becoming luminous? This at least is the explanation given by our theory.
We may, indeed, consider it possible that the escaping ether, when very
intense, might be rendered luminous by passing into the surrounding
ether, and, as it became more diffused by radiation, at last become
invisible. In this case, as the law of radiation is as the squares of
the distances from the centre inversely, the rays would be more and more
bent at right angles, or apparently shortened, as the power of the
radial stream increased, and the apparent diameters of the coma would
be diminished faster than the ratio of the 2.5th power of the distances.
But whichever view we adopt, the diameter would again increase in the
same ratio on leaving the sun, if we make allowance for increase of
temperature, as well as for diminution of density, for the ordinary
distance of a comet's visibility. We, however, regard the change of
diameter, as due to both these nodes of action, as best agreeing with
the indications afforded by their tails.

From the preceding remarks, it results that the density of the particles
producing the nebulous envelope of a comet, renders the variations of
diameter only approximate to the law of the radial stream; a comet's own
electric energy, or the intensity of the escaping ether, may also modify
this expression, and many other causes may be suggested. That the radial
stream is the cause, in the way we have pointed out, is proved by the
positions of the major axis of the short-period comet, making frequently
nearly a right angle with the radius vector of the orbit in 1828. A soap