No one will pretend to allege that no gases can be produced in the shell of the sun, or to say anything against those formed in the inner half of it finding their way to the hollow centre, and going on increasing there till they were able to force their way out through the shell; that is, until their pressure was equal to the resistance offered by the gaseous body of the sun, or against their temperature increasing until it came to correspond to their density and most probably rising to a much higher degree. Such, then, must even now be the construction of the sun, as reduced to its present diameter and density. That is, a hollow sphere consisting of cosmic matter combined with gases and having a hollow centre filled with chemically formed gases or vapours.
Here it may be argued that the sun ceases to be a hollow sphere, but that is not so. The most that can be said about it is that it is a hollow sphere with the empty part filled up. It would only be in much the same condition as a hollow globe of iron filled with melted antimony or bismuth. Its construction would be in no way changed by the empty hollow being filled up, so long as its condition remained gaseous--not changed to liquid or solid. The only difference in our sphere would be that its density would virtually be the same from what we have called the region of greatest density to the centre, which would not only involve a greater distance of that region from the surface of the sphere, but another reduction of the above mentioned densities of the sun; for we cannot in any way imagine that the pressure in its interior can be less than many thousands of atmospheres.
Whatever may be the relative densities of the shell and the gases in the hollow, they will have no necessary effect upon the temperature of the latter, because, let the densities be what they may, the gases might be cooled down to absolute zero of temperature, or raised to any imaginary degree without any change being made in their weight as long as their volume was maintained the same. This has been proved by laboratory experiments almost as far as possible. Gases at very high degrees of pressure and consequent densities have been cooled down to not far from the absolute zero of temperature, while others under very low pressures have been heated up to nearly as great heat as the enclosing vessel would bear, without their weight being altered in either case; but in the sun there is a larger laboratory in which we can place no limit to pressure or temperature. We know, however, that pressures are required sufficiently great to blow out jet prominences with velocities of 100,000 miles per second or more, to heights 200,000 and even 350,000 miles above the photosphere; and if we knew what these pressures are we might be able to learn something about the minimum temperatures of the gases. To obtain these pressures we have--in the construction we are advocating--a real containing receptacle with sides 195,942 miles thick, in the outer half of which we have the compressing force, due to the gravitation of the whole mass of the sun acting at the centre, and over and above, both in it and the inner half, we have the cohesive force of the matter of which it is composed. In fact we have a sun whose construction we can understand, in which we have gases shut up without their expansive forces being impaired in any way, ready to be exerted with full energy whenever they are relieved from compression by any commotions in any part of the whole body, and taking their part in keeping the whole of the matter composing it in constant motion. How these commotions are produced it is not difficult to explain to a very considerable extent at least, but this we must leave over until we have reconstructed the original nebula, and shown how the solar system could be elaborated from it, almost exactly in the way conceived by Laplace in his nebular hypothesis. We shall then also be able to extend our exposition of what is to be learnt from our mode of construction, and to still further reduce our estimate of the mean density of the sun.