history of a spiral nebula according to the Planetesimal Hypothesis.
Two suns, perhaps extinguished ones, have drawn near together, and an
explosive outburst has occured in one or both. The second phase calls
for a more agile exercise of the imagination.

To simplify the case, let us suppose that only one of the tugging suns
is seriously affected by the strain. Its vast wings produced by the
outburst are twisted into spirals by their rotation and the contending
attractions exercised upon them, as the two suns, like battleships in
desperate conflict, curve round each other, concentrating their
destructive energies. Then immense quantities of débris are scattered
about in which eddies are created, and finally, as the sun that caused
the damage goes on its way, leaving its victim to repair its injuries
as it may, the dispersed matter cools, condenses, and turns into
streams of solid particles circling in elliptical paths about their
parent sun. These particles, or fragments, are the ``planetesimals''
of the theory. In consequence of the inevitable intersection of the
orbits of the planetesimals, nodes are formed where the flying
particles meet, and at these nodes large masses are gradually
accumulated. The larger the mass the greater its attraction, and at
last the nodal points become the nuclei of great aggregations from
which planets are shaped.

This, in very brief form, is the Planetesimal Hypothesis which we are
asked to substitute for that based on Laplace's suggestion as an
explanation of the mode of origin of the solar system; and the
phenomena of the spiral nebulæ are appealed to as offering evident
support to the new hypothesis. We are reminded that they are
elliptical in outline, which accords with the hypothesis; that their
spectra are not gaseous, which shows that they may be composed of