his present density of 1·43 weighing 89 pounds, each cubic foot must have possessed 8,900,000,000,000 foot-pounds. But when the mass was expanded sufficiently to occupy 274,000,000,000 times its original space (which it would do when it extended to the orbit of Neptune), the heat possessed by each cubic foot would then amount to only 32 foot-pounds. In point of fact it would not even amount to so much, for a quantity equal to upwards of 20,000,000 years’ heat would necessarily be consumed in work against gravity in the expansion of the mass, all of which would, of course, be given back in the form of heat as the mass contracted. During the nebulous condition, however, this quantity would exist in an entirely different form, so that only 19 out of the 32 foot-pounds per cubic foot generated by concussion would then exist as heat. The density of the nebula would be only 1/16,248,160 that of hydrogen at ordinary temperature and pressure. The 19 foot-pounds of heat in each cubic foot would thus be sufficient to maintain an excessive temperature; for there would be in each cubic foot only 1/440,000 of a grain of matter. But, although the _temperature_ would be excessive, the _quantity_ both of light and heat in each cubic foot would of necessity be small. The heat being only 1/71 of a thermal unit, the light emitted would certainly be exceedingly feeble, resembling very much the electric light in a vacuum-tube. VI. _Binary Systems._ The theory affords a rational explanation of the origin of binary stars. Binary stars, in so far as regards their motion, follow also, of course, as a consequence, from the gravitation theory. If two bodies come into grazing collision, “they will,” says Sir William Thomson, “commence revolving round their common centre of inertia in long elliptic orbits. Tidal interaction between them will diminish the eccentricities of their orbits, and, if continued long enough, will cause them to revolve in circular orbits round their centre of inertia.”[19] This conclusion was pointed out many years ago by Dr. Johnstone Stoney. [Footnote 19: _Proceedings of Royal Institution_, vol. xii. p. 15.] VII. _Sudden Outbursts of Stars._