MR. ROBERT MALLET ON VOLCANIC ENERGY. 
159 
to suggest that his calculation really proves nothing as to what may be the actual facts in 
nature, because the physical data which he has employed are not such as actually occur 
in nature. The law of compression with depth adopted was objected to by Dr. Young; 
and the assumption that the coefficient of contraction is the same for the entire globe, 
whether liquid or solid, hotter or colder, seems to vitiate the results. 
If a colder and consolidated crust possesses a greater density and a smaller coefficient 
of contraction than the liquid or solid but hotter matter beneath, by accretion and 
solidification from which the crust is gradually thickened, it is easy to see that such a 
relation may subsist between these quantities that the distance of the centre of gyration 
of the globe from its centre of figure may remain constant, either always or for very long 
periods, although the diameter of the earth as a whole may have diminished by con- 
traction ; and thus, since the moment of momentum must have remained unchanged, the 
angular velocity, upon which the length of the day depends, may have remained constant 
(or so nearly so that its variation may have been absolutely insensible) for 2000 years, 
or perhaps for a much longer period, notwithstanding that the world, as a whole, may 
have been losing heat all the time at a very sensible rate per annum, as seems to be the 
undeniable fact. 
46. Let us take up the train of phenomena of refrigeration from the period when we 
may suppose the whole globe a liquid spheroid in fusion, rotating upon an axis inclined to 
its orbit as now, losing heat by radiation, and receiving that of the sun. Whatever the 
rate of refrigeration generally, it must have been greatest towards the poles ; so that any 
solidified crust must have first formed about the poles and spread thence in two hemi- 
spherical sheets, getting thinner as they neared the equator, where they ultimately 
joined. From what we know by our mere furnace or laboratory experience of the 
effects of our highest temperatures on metals and on the materials that we must pre- 
sume constitute the mass of our globe, it is certain that, at temperatures exceeding their 
fusing-points, they become more and more liquid up to some not yet known limit, and 
that at points a good deal above those of fusion they are all reduced to mobile liquids 
of extremely small viscosity. 
47. On the other hand, it is equally certain that all metals, and such mixed materials as 
constitute rocks (acid and basic silicates), pass through a rapidly increasing phase of 
viscosity as they pass below the fusing-point on their Avay to ordinary solidity. It is 
this interval of rapidly increasing viscosity below the fusing-point and above that of 
complete solidification that enables platinum and iron &c. to be welded. This stage of 
viscosity in metals is very brief ; but in earthy mixtures or compounds, such as the 
acid or basic silicates, it is much more prolonged, and the increase of viscosity towards 
the inferior limits of temperature much greater than in metals. 
48. With the extreme fluidity of the molten spheroid at the exalted temperature that 
we must infer for it at the time of its first condensation from vapour, and after the first 
great chemical equilibrium of its elements (then entered into combinations analogous to 
those we now find in the globe so far as we know it), it is not conceivable but that 
