“Part l] INTERNAL HEAT OF THE EARTH. 45 
composition. Dr. Pfaff indeed offers a calculation to show that the 
mean terrestrial density of 5°5 is not incompatible with the notion 
that the whole globe consists of materials of the same density as the 
rocks of the crust.* 
Analogies in the solar system, however, as well as the actual struc- 
ture of the rocky crust of the globe, suggest that heavier metallic 
ingredients possibly predominate in the nucleus. If the materials 
of the globe were once, as they are believed to have been, in a fluid 
condition, they would then be subject to an internal arrangement in 
accordance with their relative specific gravities. We may conceive 
that as in the case of the sun, as well as of the solar system generally 
(ante, p. 8), there would be, so long as internal mobility lasted, a 
tendency in the denser elements to gravitate towards the centre, in 
the lighter to accumulate outside. That a distribution of this nature 
has certainly taken place to some extent is evident from the structure 
of the envelopes and crust. It is what might be expected if the 
constitution of the globe resembles on a small scale the larger 
planetary system of which it forms a part. The existence even of a 
metallic interior has been inferred from the metalliferous veins which 
traverse the crust, and which are commonly supposed to have been 
filled from below.’ 
Evidence of Internal Heat.—In the evidence obtainable 
as to the former history of the earth, no fact is of more importance 
than the existence of a high temperature beneath the crust, which 
has now been placed beyond all doubt. This feature of the 
planet’s organization is made clear by the following proofs :— 
(1.) Volcanoes—In many regions of the earth’s surface openings 
exist from which steam and hot vapours, ashes and streams of molten 
rock are from time to time emitted. The abundance and wide 
diffusion of these openings, inexplicable by any mere local causes, 
must be regarded as indicative of a very high internal temperature. 
If to the still active vents of eruption we add those which have 
formerly been the channels of communication between the interior 
and the surface, there are probably few large regions of the globe 
where proofs of volcanic action cannot be found. Hverywhere we 
meet with masses of molten rock which have risen from below as if 
from some general reservoir. The phenomena of active volcanoes 
are fully discussed in Book III. Part I. 
(2.) Hot Springs—Where volcanic eruptions have ceased, 
evidence of a high internal temperature is still often to be found in 
springs of hot water which continue for centuries to maintain their 
1 Allgemeine Geologie als exacte Wissenschaft, p. 42. 
2 The late David Forbes suggested that the planet might be supposed to consist 
of three layers of uniform densities, enclosed. one within the other, the density increas- 
ing towards the centre in arithmetical progression. Allowing 2°5 as the specific gravity 
of the crust or outer layer, he assigned 12:0 or thereabouts as that of the middle layer, 
and supposed that the inner nucleus might possess one averaging 20°0. (Popular Science 
eee April, 1869.) Materials do not yet exist for any satisfactory conclusions on this 
subject. 
