four and a half times the weight of water; but the actual weight of
the principal solid substances composing the outer crust is as two
and a half times the weight of water; and this, we know, if the globe
were solid and cold, should increase vastly towards the centre, water
acquiring the density of quicksilver at 362 miles below the surface,
and other things in proportion, and these densities becoming much
greater at greater depths; so that the entire mass of a cool globe
should be of a gravity infinitely exceeding four and a half times the
weight of water. The only alternative supposition is, that the
central materials are greatly expanded or diffused by some means; and
by what means could they be so expanded but by heat? Indeed, the
existence of this central heat, a residuum of that which kept all
matter in a vaporiform chaos at first, is amongst the most solid
discoveries of modern science, {42} and the support which it gives to
Herschel's explanation of the formation of worlds is most important.
We shall hereafter see what appear to be traces of an operation of
this heat upon the surface of the earth in very remote times; an
effect, however, which has long passed entirely away. The central
heat has, for ages, reached a fixed point, at which it will probably
remain for ever, as the non-conducting quality of the cool crust
absolutely prevents it from suffering any diminution.



THE EARTH FORMED--ERA OF THE PRIMARY ROCKS.



Although the earth has not been actually penetrated to a greater
depth than three thousand feet, the nature of its substance can, in