Concrete Matter from Atomic Origins. 403 
experiments made about eighty years ago, found that melted 
granite, slate, and trachyte, all contract by more than ten 
per cent, in freezing ; and sixteen years ago, Carl Barus * 
found that diabase (a partially crystalline basaltic rock) is 
fourteen per cent, denser than melted diabase. He found the 
melting temperature of diabase to be about ]170°C. ; and the 
late Professor Roberts- Austen; by experiments which he 
kindly made at my request, found the melting temperatures 
of several different rocks under ordinary atmospheric pressure f 
to be as follows : — 
Melting-point. Error. 
Felspar 1520°C. ±30° 
Hornblende ... about 1400° „ 
Mica 1440° „ +30° 
Quartz 1775° ., . +15° 
Basalt about 880° '„ 
§ 14. Go back now to the end of §11, and consider the 
Earth after the gaseous stage of its evolution has ended in 
liquefaction at the centre. The temperature will sink 
throughout, in virtue of the automatic stirring, and of out- 
ward thermal conduction ; and the amount of material in 
the thoroughly liquid condition will increase until the whole 
globe becomes liquid, except a vaporous or gaseous atmo- 
sphere of comparatively small total mass, resting on the liquid 
all around its spherical surface. The density of this liquid 
increases from surface to centre, in its earlier stages probably 
only because of the greater pressure ; but ultimately also in 
consequence of subsidence of the denser chemical ingredients, 
after the convective currents have become too slack for 
thorough mixing up of all the materials. 
§ 15. Crystalline freezing may begin at the surface, 
because of the rapid loss of heat by radiation outwards, but 
each solid crystal sinks because its density is greater than 
that of the fluid in contact with it. In sinking, it is melted 
and redissolved by the hotter fluid below. This process goes 
on until the temperature at every part of the liquid is reduced 
to that at which some of the ingredients such as quartz, felspar, 
hornblende, mica, crystallize out of the liquid, under the 
hydrostatic pressure at the depth of the portion considered. 
This formation of crystals leaves a mother liquor consisting 
of ingredients which freeze at a lower temperature. At this 
stage the portions frozeu at the surface do not melt in sinking 
through the liquid, and they fall clown to the centre, or to the 
central nucleus if there is one. Thus the main rigidification 
* Phil. Mag. 1893, first half-year, pp. 173-175. 
f See Addendum to u The Age of the Earth as an Abode fitted for 
Life," Phil. Mag. 1899, first half-year, p. 89. 
