7. _The mass must have possessed an excessive temperature._—There is ample evidence, Mr. Lockyer thinks, to show that the temperature of the solar nebula was as great as that of the sun at the present time. But I think it is extremely probable that, in some of its stages, the nebula had a very much higher temperature than that now possessed by the sun. There must, during the sifting period, have been complete chemical dissociation, so as to keep the metals and the metalloids uncombined, and thus allow the elements to arrange themselves according to their densities. The nebula hypothesis, remarks Mr. Lockyer, “is almost worthless unless we assume very high temperatures, because, unless you have heat enough to get perfect dissociation, you will not have that sorting out which always seems to follow the same law.” 8. _Gravitation could, under no possible condition, have generated the amount of heat required by the nebular hypothesis._—The nebular hypothesis does not profess to account for the origin of nebulæ. It starts with matter existing in space in the nebulous condition, and explains how, by condensation, suns, planets &c. are formed out of it. In fact, it begins at the middle of a process: it begins with this fine, attenuated material in the process of being drawn together and condensed under the influence of attraction, and professes to explain how, as the process goes on, a solar system necessarily results. To simplify our inquiry we shall confine our attention to the solar nebula, and consider in the first place how far condensation may be regarded as a sufficient source of heat. A. _Condensation._—The heat which our nebula could have derived from condensation up to the time that Neptune was detached from the mass, no matter how far the outer circumference of the mass may have originally extended beyond the orbit of that planet, could not have amounted to over 1/7,000,000 of a thermal unit (772 foot-pounds) for each cubic foot. It is perfectly obvious that this amount could not have produced the dissociation required; and without the required dissociation Neptune could never have been formed. Further, it is physically impossible that the materials of which our solar system are composed could have existed in the gaseous state in a cool condition prior to condensation. Unless possessed of great heat, even hydrogen could not exist in stellar space in the gaseous form; and far less could carbon, iron, platinum, &c. Before Neptune could have been formed the whole of the materials of the system must have possessed heat, not only sufficient to reduce them to the gaseous state, but sufficient to produce complete dissociation. But by no conceivable means could gravitation have conferred this amount of heat by the time that the mass had condensed to just within the limits of the orbit of Neptune.