hemispheres. Thus it is seen that everything
teaches us how little this which we call the
solid earth is worthy of the name— how thin
and fragile is the film enveloping the fluid portion
of the globe— and how promptly it would,
without doubt, be destroyed if it were not for
the five hundred and fifty-nine volcanoes distributed
over its surface acting as safety-valves,
and presenting outlets more or less free to the
action of the subterranean fires."

As a singular example of the confidence with
which a whole school of geologists but recently
enunciated their reasonings in the earth and
air, I translate the following from this author:
"The world generally forms a very exaggerated
idea of the thickness and solidity of the terrestrial
crust. Here are a few figures fitted to
convey somewhat more exact notions. The most
superficial layers of soil partake of the variations
of the temperature dependent upon the seasons
to a depth varying with the latitude, but never
considerable. Beyond this point the temperature
rises as we sink down; and experiments
many times repeated have shown that this increase
is at about an average of one degree for
every thirty-three metres. Let us take the
round number of thirty metres. The result is,
that at a depth of three thousand metres, or
three-fourths of a league from the surface, we
find already the temperature of boiling water.
Supposing that the heat increases uniformly at
a depth of twenty kilometres, we shall find six
hundred and sixty-six degrees, that is to say, a
heat which melts several of the fluids entering
into the composition of our rocks. Thus, at
about four post leagues from the surface, ought
to commence the incandescent mass which forms
nearly the whole of our globe. When compared
with the size of the earth, this thickness
represents about three millimetres for a globe of one
metre. In other words, it will be about equal
to the thickness of a sheet of (French) letter-paper
for one of those globes generally used in
geographical studies. When we bring the question
to these terms, we cease to be astonished
at the movements which may agitate this film;
and if we are surprised at anything, it is that
the earth is not more frequently the theatre
of upsets (bouleversements), which, although
frightful to us, would be scarcely felt over a
vast extent of our planet."

The principle from which these startling inferences
have been drawn has, however, been
much shaken by recent observations. The air,
it has been proved, does not grow colder by
regular degrees as we go up, and therefore it
may yet be found that the rocks do not, by
regular ratios, grow hotter as we quarry down.
Mr. Glaisher says the decrease of temperature
is 51 deg. Fahrenheit in twenty-five thousand
feet of elevation; two-fifths of the whole decrease
in five miles taking place on the first
mile. Probably the cause of greater cooling on
the first mile is, that the earth imbibes and
radiates the heat of the sun's rays, and the
aërial voyager finds the decrease of heat to be
greater at first because he then loses the influences
of the accumulations and the radiations.

But whilst it would be unwise to conclude
that we know accurately the rate at which the
heat of the earth increases downwards, the progress
of science appears to be continually confirming
the doctrine of central heat. A succession
of chemists has pursued the series of experiments
begun by Mitscherlich, and nearly every
mineral and metal in the crust of the earth has
been produced artificially by imitating the processes
of nature. Ebelman astonished the last
generation of reading people by making jewels.
Boracic acid enters into the composition of
several minerals, and forms thirty-one per cent
of alumina and thirty-nine of silica. This acid
Ebelman used as a solvent at a high temperature,
and then, evaporating the solvent, produced,
among other minerals, rubies, sapphires,
spinels, chrysoberyl, chrysolite, and chromate of
iron. He pounded emeralds, and then fusing
the dust with boracic acid and a little oxide of
chromium, reproduced, or rather made, new
emeralds. The crystals of the artificial chrysoberyls
were sufficiently large to have their
angles measured and to be tested, and they were
found to be identical with those of natural chrysoberyls.
Metals can be produced artificially, like
minerals, and even gold may be made at a cost of
double the price of the natural production.
M. Daubrée has recently extended considerably
the list of artificial minerals and metals. With
other minerals he has obtained quartz and felspar.
Clay and kaolin, having been previously
purified by washing, under this process produced
felspar with crystals of quartz. M. Babinet,
hearing felspar much spoken of when these experiments
were made known, remarked, "Felspar!
that is a very common rock indeed!"
"But," said some one, "we are speaking of
artificial felspar." "Artificial felspar! that is
an unique specimen in the world!"

The action of heat in the formation of primitive
rocks cannot be doubted in presence of
these experiments. But as if to show us how
far we are from the solution of these problems,
stones, unless we are to disbelieve a vast amount
of testimony, descend from the skies, from the
regions of inconceivably severe cold, consisting
of iron nickel, felspathic sand, silicious sand,
formed into octahedral crystals, resembling sand
after it has been a long time in a furnace, and
more or less melted, fused, and glazed at the
surface. These stones were for a long time
called air-stones, and now they are called meteor-stones,
but nobody has been able to prove clearly
what they are. Their new name is given to
them by those who suppose them to be shooting-stars,
always to be seen in the evening sky, but
especially in August and November. The Arabs
call these shooting-stars celestial crickets, and
certainly the comparison describes well their
apparent leaps in the lofty fields of blue among
the stars. Aërolites or meteorites have, it is
said, fallen in showers. Certain stones, now
preserved in museums, have, we are assured,
been seen to fall. Some writers imagine the