Reflecting on what we have written up till now, we see that the analysis of the nebular hypothesis we have made, which at first may have appeared to be unnecessary or even useless, has shown us and made us think over many details, of which we had only a vague notion previously. It has shown us that without condensation at or near the surface of the nebula--which we have pointed out must have been caused by its greatest mass being near that region, and which Laplace procured by endowing it with excessive heat--the various members of the solar system could not have been evolved from it in terms of the hypothesis. From it we have been able to learn, by means of the register of the acceleration of revolution from one planet to another, when, and for what reason, the nebula ceased to be able to throw off any planet nearer to the sun than the supposed Vulcan, or almost even so near. Finally, and not to go into greater detail, it has so far given us some ideas, that we had not before, of the internal structure of the sun, and has made us believe that a great deal may be learnt by attempting to find out what that structure really is. For this purpose, it appears to us that a careful examination into, and study of, the interior of the earth might be a great help, and to this we shall appeal, as we cannot think of any other process by which our object can be attained. This, therefore, we shall endeavour to do in the following chapters. CHAPTER VIII. PAGE 142 Inquiry into the Interior Construction of the Earth. What is really known of the exterior or surface. 143 What is known of the interior. 144 Little to be learned from Geology, which reaches very few miles down. 145 Various notions of the interior. 146 What is learnt from earthquake and volcanoes. Igno-aqueous fusion, liquid magma. 147 Generally believed that the earth consists of solid matter to the centre. Mean density. Surface density. 148 More detailed estimate of densities near the surface. 148 Causes of increased surface density after the crust was formed. 150 Calculations of densities for 9 miles deep, and from there to the centre forming Table IV. 151 Reflections on the results of the calculations. 151 Notion that the centre is composed of the heaviest metals. "Sorting-out" theory absurd. 152 Considerations as to how solid matter got to the centre. 153 Gravitation might carry it there, but attraction could not. 154 How the earth could be made out of cosmic matter, meteorites or meteors. THE INTERIOR OF THE EARTH AND ITS DENSITY.