In his opening address to the Mathematical and Physical Section of the
British Association, Sir William Thomson affirmed, “with almost perfect
certainty, that, whatever may be the relative densities of rock, solid
and melted, or at about the temperature of liquefaction, it is, I
think, quite certain that cold solid rock is denser than hot melted
rock; and no possible degree of rigidity in the crust could prevent
it from breaking in pieces and sinking wholly below the liquid lava,”
and that “this process must go on until the sunk portions of the crust
build up from the bottom a sufficiently close-ribbed skeleton or frame,
to allow fresh incrustations to remain bridged across the now small
areas of lava-pools or lakes.”[11]
This would doubtless be the case if the material of the earth were
chemically homogeneous or of equal specific gravity throughout, and if
it were chemically inert in reference to its superficial or atmospheric
surroundings. But such is not the case. All we know of the earth shows
that it is composed of materials of varying specific gravities, and
that the range of this variation exceeds that which is due to the
difference between the theoretical internal heat of the earth and its
actual surface temperature.
We know by direct experiment that these materials, when fused together,
arrange themselves according to their specific gravities, with the
slight modification due to their mutual diffusibilities. If we take a
mixture of the solid elements of which the earth, so far as we know it,
is composed, fused them, and leave them exposed to atmospheric action,
what will occur?
The heavy metals will sink, the heaviest to the bottom, the lighter
metals (_i.e._, those that we call the metals of the earths, because
they form the basis of the earth’s superficial crust) will rise along
with the silicon, etc., to the surface; these and the silicon will
oxidize and combine, forming silicates, and with a sufficient supply
of carbonic acid, some of them, such as calcium, magnesium, etc.,
will form carbonates when the temperature sinks below that of the
dissociation of such compounds.
The scoria thus formed will float upon the heavy metals below and
protect them from cooling by resisting their radiation; but if in
the course of contraction of this crust some fissures are formed
reaching to the melted metals below, the pressure of the floating
solid will inject the fluid metal upwards into these fissures to a
height corresponding to the flotation depth of the solid, and thus
form metallic veins permeating the lower strata of the crust. I need
scarcely add that this would rudely but fairly represent what we know
of the earth.
But it may be objected that I only describe an imaginary experiment.
This is true as regards the whole of the materials united in a single
fusion. Nobody has yet produced a complete model with platinum and gold
in the centre, and all the other metals arranged in theoretical order
with the oxidized, silicated, and carbonated crust outside; but with a
limited number of elements this has been done, is being done daily, on
a scale of sufficient magnitude to amply refute Sir William Thomson’s
description of a fused earth solidifying from the centre outwards.
This refutation is to be seen in our blast furnaces, refining furnaces,
puddling furnaces, Bessemer ladles, steel melting-pots, cupels, foundry
crucibles; in fact, in almost every metallurgical operation down to the
simple fusion of lead or solder in a plumber’s ladle, with its familiar
floating crust of dross or oxide.
As an example I will, on account of its simplicity, take the open
hearth finery and the refining of pig-iron. Here a metallic mixture of
iron, silicon, carbon, sulphur, etc., is simply fused and exposed to
the superficial action of atmospheric air. What is the result?
Oxidation of the more oxidizable constituents takes place, and
these oxides at once arrange themselves according to their specific
gravities. The oxidized carbon forms atmospheric matter and rises above
all as carbonic acid, then the oxidized silicon, being lighter than
the iron, floats above that, and combines with aluminium or calcium
that may have been in the pig and with some of the iron; thus forming
a silicious crust closely resembling the predominating material of the
earth’s crust.
When the oxidation in the finery is carried far enough, the melted
material is tapped out into a rectangular basin or mould, usually
about 10 feet long and about 3 feet wide, where it settles and cools.
During this cooling the silica and silicates—_i.e._, the rock
matter—separate from the metallic matter and solidify on the surface
as a thin crust, which behaves in a very interesting and instructive
manner. At first a mere skin is formed. This gradually thickens, and
as it thickens and cools becomes corrugated into mountain chains and
valleys much higher and deeper, in proportion to the whole mass, than
the mountain chains and valleys of our planet. After this crust has
thickened to a certain extent volcanic action commences. Rifts, dykes,
and faults are formed by the shrinkage of the metal below, and streams
of lava are ejected. Here and there these lava streams accumulate
around their vent and form insulated conical volcanic mountains with
decided craters, from which the eruption continues for some time. These
volcanoes are relatively far higher than Chimborazo. The magnitude of
these actions varies with the quality of the pig-iron.
The open hearth finery is now but little used, but probably some are
to be seen at work occasionally in the neighborhood of Glasgow, and I
am sure that Sir William Thomson will find a visit to one of them very
interesting. Failing this, he may easily make an experiment by tapping
into a good-sized “cinder bogie” some melted pig-iron from a pudding
furnace (taking it just before the iron “comes to nature”), and leaving
the melted mixture to cool slowly and undisturbed.
The cinder of the blast furnace, which in like manner floats on the top
of the melted pig-iron, resembles still more closely the prevailing
rock-matter of the earth, on account of the larger proportion, and the
varied compounds, of earth-metals it contains.
For the volcanic phenomena alone he need simply watch what occurs
when in the ordinary course of puddling the cinder is run into a
large bogie, and the bogie is left to cool standing upright. I need
scarcely add that these phenomena strikingly illustrate and confirm Mr.
Mallett’s theory of earthquakes, volcanoes, and mountain-formation.
In merely passing through an iron-making district one may see the
results of what I have called the volcanic action, by simply observing
the form of those oyster-shaped or cubical blocks of cinder that are
heaped in the vicinity of every blast furnace that has been at work for
some time. Radial ridges or consolidated miniature lava-streams are
visible on the exposed face of nearly, if not quite all of these. They
were ejected or squeezed up from below while the mass was cooling, when
the outer crust had consolidated but the inner portion still remained
liquid. Many of these are large enough, and sufficiently well-marked,
to be visible from a railway carriage passing a cinder heap near the
road.[12]