By some other theorists, who have felt the full force of this last objection, an attempt has been made to explain the phenomena of volcanoes by supposing that the globe is solid from its surface to its centre, but that the internal portions of the globe are at such a high temperature that they are only retained in a solid condition by the enormous pressure to which they are subjected. The central masses of the globe are thus regarded as being in an _actually_ solid, but in a _potentially_ liquid condition, and any local relief of pressure is at once followed by the conversion of solid to liquefied materials, in the district where the relief takes place, resulting in the manifestation of volcanic phenomena at the spot. It may be granted that this hypothesis better accords with the known facts of Vulcanology than any of those which we have previously described, but it is impossible to shut our eyes to the fact that not a few serious difficulties still remain. Thus it is based upon the assumption that the law of the elevation of the point of fusion by pressure is true at temperatures and pressures almost infinitely above those at which we are able to conduct observations; but neither experiment nor analogy warrant this conclusion, for the former shows that the elevation of the point of fusion by pressure goes on in a continually diminishing ratio, and the latter famishes us with the example of volatile liquids which, above their critical points, obstinately remain in a gaseous condition under the highest pressures. Nor is it easy upon this hypothesis to account for the very irregular distribution of temperatures within the earth's crust, as demonstrated by observations in mines, wells, and borings. The hypothesis further requires the assumption that, at such very moderate depths as are required for the reservoirs of volcanoes, the effects of pressure and temperature on the condition of rock-materials are so nicely balanced that the smallest changes at the surface lead to a disturbance of the equilibrium. [Sidenote: DIFFICULTIES NOT YET EXPLAINED.]