CAUSES OF EARTHQUAKES AND VOLCANOES.
Intimate connection between the causes of volcanoes and
earthquakes--Supposed original state of fusion of the
planet--Universal fluidity not proved by spheroidal figure of the
earth--Attempt to calculate the thickness of the solid crust of the
earth by precessional motion--Heat in mines increasing with the
depth--Objections to the supposed intense heat of a central
fluid--Whether chemical changes may produce volcanic heat--Currents
of electricity circulating in the earth's crust.
It will hardly be questioned, after the description before given of the
phenomena of earthquakes and volcanoes, that both of these agents have,
to a certain extent, a common origin; and I may now, therefore, proceed
to inquire into their probable causes. But first, it may be well to
recapitulate some of those points of relation and analogy which lead
naturally to the conclusion that they spring from a common source.
The regions convulsed by violent earthquakes include within them the
site of all the active volcanoes. Earthquakes, sometimes local,
sometimes extending over vast areas, often precede volcanic eruptions.
The subterranean movement and the eruption return again and again, at
irregular intervals of time, and with unequal degrees of force, to the
same spots. The action of either may continue for a few hours, or for
several consecutive years. Paroxysmal convulsions are usually followed,
in both cases, by long periods of tranquillity. Thermal and mineral
springs are abundant in countries of earthquakes and active volcanoes.
Lastly, hot springs situated in districts considerably distant from
volcanic vents have been observed to have their temperature suddenly
raised, and the volume of their water augmented, by subterranean
movements.
All these appearances are evidently more or less connected with the
passage of heat from the interior of the earth to the surface; and where
there are active volcanoes, there must exist, at some unknown depth
below, enormous masses of matter intensely heated, and, in many
instances, in a constant state of fusion. We have first, then, to
inquire, whence is this heat derived?
It has long been a favorite conjecture, that the whole of our planet was
originally in a state of igneous fusion, and that the central parts
still retain a great portion of their primitive heat. Some have
imagined, with the late Sir W. Herschel, that the elementary matter of
the earth may have been first in a gaseous state, resembling those
nebulae which we behold in the heavens, and which are of dimensions so
vast, that some of them would fill the orbits of the remotest planets of
our system. The increased power of the telescope has of late years
resolved the greater number of these nebulous appearances into clusters
of stars, but so long as they were confidently supposed to consist of
aeriform matter it was a favorite conjecture that they might, if
concentrated, form solid spheres; and it was also imagined that the
evolution of heat, attendant on condensation, might retain the materials
of the new globes in a state of igneous fusion.
Without dwelling on such speculations, which can only have a distant
bearing on geology, we may consider how far the spheroidal form of the
earth affords sufficient ground for presuming that its primitive
condition was one of universal fluidity. The discussion of this question
would be superfluous, were the doctrine of original fluidity less
popular; for it may well be asked, why the globe should be supposed to
have had a pristine shape different from the present one?--why the
terrestrial materials, when first called into existence, or assembled
together in one place, should not have been subject to rotation, so as
to assume at once that form which alone could retain their several parts
in a state of equilibrium?
Let us, however, concede that the statical figure may be a modification
of some other pre-existing form, and suppose the globe to have been at
first a perfect and quiescent sphere, covered with a uniform ocean--what
would happen when it was made to turn round on its axis with its present
velocity? This problem has been considered by Playfair in his
Illustrations, and he has decided, that if the surface of the earth, as
laid down in Hutton's theory, has been repeatedly changed by the
transportation of the detritus of the land to the bottom of the sea, the
figure of the planet must in that case, whatever it may have been
originally, be brought at length to coincide with the spheroid of
equilibrium.[742] Sir John Herschel also, in reference to the same
hypothesis, observes, "a centrifugal force would in that case be
generated, whose general tendency would be to urge the water at every
point of the surface to _recede_ from the _axis_. A rotation might
indeed be conceived so swift as to flirt the whole ocean from the
surface, like water from a mop. But this would require a far greater
velocity than what we now speak of. In the case supposed, the _weight_
of the water would still keep it _on_ the earth; and the tendency to
recede from the axis _could_ only be satisfied therefore by the water
leaving the poles, and flowing towards the equator; there heaping itself
up in a ridge, and being retained in opposition to its weight or natural
tendency towards the centre by the pressure thus caused. This, however,
could not take place without laying dry the polar regions, so that
protuberant land would appear at the poles, and a zone of ocean be
disposed around the equator. This would be the first or immediate
effect. Let us now see what would afterwards happen if things were
allowed to take their natural course.
"The sea is constantly beating on the land, grinding it down, and
scattering its worn-off particles and fragments, in the state of sand
and pebbles, over its bed. Geological facts afford abundant proof that
the existing continents have all of them undergone this process even
more than once, and been entirely torn in fragments, or reduced to
powder, and submerged and reconstructed. Land, in this view of the
subject, loses its attribute of fixity. As a mass it might hold together
in opposition to forces which the water freely obeys; but in its state
of successive or simultaneous degradation, when disseminated through the
water, in the state of sand or mud, it is subject to all the impulses of
that fluid. In the lapse of time, then, the protuberant land would be
destroyed, and spread over the bottom of the ocean, filling up the lower
parts, and tending continually to remodel the surface of the solid
nucleus, in correspondence with the _form of equilibrium_. Thus after a
sufficient lapse of time, in the case of an earth in rotation, the polar
protuberances would gradually be cut down and disappear, being
transferred to the equator (as being _then_ the _deepest sea_), till the
earth would assume by degrees the form we observe it to have--that of a
flattened or _oblate_ ellipsoid.
"We are far from meaning here to trace the process _by which_ the earth
really assumed its actual form; all we intend is to show that this is
the form to which, under a condition of a rotation on its axis, it must
_tend_, and which it would attain even if originally and (so to speak)
perversely constituted otherwise."[743]
In this passage, the author has contemplated the superficial effects of
aqueous causes only; but neither he nor Playfair seem to have followed
out the same inquiry with reference to another part of Hutton's system;
namely, that which assumes the successive fusion by heat of different
parts of the solid earth. Yet the progress of geology has continually
strengthened the evidence in favor of the doctrine that local variations
of temperature have melted one part after another of the earth's crust,
and this influence has perhaps extended downwards to the very centre.
If, therefore, before the globe had assumed its present form, it was
made to revolve on its axis, all matter to which freedom of motion was
given by fusion, must before consolidating have been impelled towards
the equatorial regions in obedience to the centrifugal force. Thus lava
flowing out in superficial streams would have its motion retarded when
its direction was towards the pole, accelerated when towards the
equator; or if lakes and seas of lava existed beneath the earth's crust
in equatorial regions, as probably now beneath the Peruvian Andes, the
imprisoned fluid would force outwards and permanently upheave the
overlying rocks. The statical figure, therefore, of the terrestrial
spheroid (of which the longest diameter exceeds the shortest by about
twenty-five miles), may have been the result of gradual and even of
existing causes, and not of a primitive, universal, and simultaneous
fluidity.[744]
Experiments made with the pendulum, and observations on the manner in
which the earth attracts the moon, have shown that our planet is not an
empty sphere, but, on the contrary, that its interior, whether solid or
fluid, has a higher specific gravity than the exterior. It has also been
inferred, that there is a regular increase in density from the surface
towards the centre, and that the equatorial protuberance is continued
inwards; that is to say, that layers of equal density are arranged
elliptically, and symmetrically, from the exterior to the centre. These
conclusions, however, have been deduced rather as a consequence of the
hypothesis of primitive and simultaneous fluidity than proved by
experiment. The inequalities in the moon's motion, by which some have
endeavored to confirm them, are so extremely slight, that the opinion
can be regarded as little more than a probable conjecture.
The mean density of the earth has been computed by Laplace to be about
5-1/2, or more than five times that of water. Now the specific gravity
of many of our rocks is from 2-1/2 to 3, and the greater part of the
metals range between that density and 21. Hence some have imagined that
the terrestrial nucleus may be metallic--that it may correspond, for
example, with the specific gravity of iron, which is about 7. But here a
curious question arises in regard to the form which materials, whether
fluid or solid, might assume, if subjected to the enormous pressure
which must obtain at the earth's centre. Water, if it continued to
decrease in volume according to the rate of compressibility deduced from
experiment, would have its density doubled at the depth of ninety-three
miles, and be as heavy as mercury at the depth of 362 miles. Dr. Young
computed that, at the earth's centre, steel would be compressed into
one-fourth, and stone into one-eighth of its bulk.[745] It is more than
probable, however, that after a certain degree of condensation, the
compressibility of bodies may be governed by laws altogether different
from those which we can put to the test of experiment; but the limit is
still undetermined, and the subject is involved in such obscurity, that
we cannot wonder at the variety of notions which have been entertained
respecting the nature and conditions of the central nucleus. Some have
conceived it to be fluid, others solid; some have imagined it to have a
cavernous structure, and have even endeavored to confirm this opinion by
appealing to observed irregularities in the vibrations of the pendulum
in certain countries.
An attempt has recently been made by Mr. Hopkins to determine the least
thickness which can be assigned to the solid crust of the globe, if we
assume the whole to have been once perfectly fluid, and a certain
portion of the exterior to have acquired solidity by gradual
refrigeration. This result he has endeavored to obtain by a new solution
of the delicate problem of the processional motion of the pole of the
earth. It is well known that while the earth revolves round the sun the
direction of its axis remains very nearly the same, _i. e._ its
different positions in space are all nearly parallel to each other. This
parallelism, however, is not accurately preserved, so that the axis,
instead of coming exactly into the position which it occupied a year
before, becomes inclined to it at a very small angle, but always
retaining very nearly the same inclination to the plane of the earth's
orbit. This motion of the pole changes the position of the equinoxes by
about fifty seconds annually, and always in the same direction. Thus the
pole-star, after a certain time, will entirely lose its claim to that
appellation, until in the course of somewhat more than 25,000 years the
earth's axis shall again occupy its present angular position, and again
point very nearly as now to the pole-star. This motion of the axis is
called _precession_. It is caused by the attraction of the sun and moon,
and principally the moon, on the protuberant parts of the earth's
equator; and if these parts were solid to a great depth, the motion thus
produced would differ considerably from that which would exist if they
were perfectly fluid, and incrusted over with a thin shell only a few
miles thick. In other words, the disturbing action of the moon will not
be the same upon a globe all solid and upon one nearly all fluid, or it
will not be the same upon a globe in which the solid shell forms
one-half of the mass, and another in which it forms only one-tenth.
Mr. Hopkins has, therefore, calculated the amount of precessional motion
which would result if we assume the earth to be constituted as above
stated; _i. e._ fluid internally, and enveloped by a solid shell; and he
finds that the amount will not agree with the observed motion, unless
the crust of the earth be of a certain thickness. In calculating the
exact amount some ambiguity arises in consequence of our ignorance of
the effect of pressure in promoting the solidification of matter at high
temperatures. The hypothesis least favorable for a great thickness is
found to be that which assumes the pressure to produce no effect on the
process of solidification. Even on this extreme assumption the thickness
of the solid crust must be nearly _four hundred miles_, and this would
lead to the remarkable result that the proportion of the solid to the
fluid part would be as 49 to 51, or, to speak in round numbers, there
would be nearly as much solid as fluid matter in the globe. The
conclusion, however, which Mr. Hopkins announces as that to which his
researches have finally conducted him, is thus expressed: "Upon the
whole, then, we may venture to assert that the minimum thickness of the
crust of the globe, which can be deemed consistent with the observed
amount of precession, cannot be less than one-fourth or one-fifth of the
earth's radius." That is from 800 to 1000 miles.[746]
It will be remarked, that this is a _minimum_, and any still _greater_
amount would be quite consistent with the actual phenomena; the
calculations not being opposed to the supposition of the general
solidity of the entire globe. Nor do they preclude us from imagining
that great lakes or seas of melted matter may be distributed through a
shell 400 or 800 miles thick, provided they be so inclosed as to move
with it, whatever motion of rotation may be communicated by the
disturbing forces of the sun and moon.
_Central heat._--The hypothesis of internal fluidity calls for the more
attentive consideration, as it has been found that the heat in mines
augments in proportion as we descend. Observations have been made, not
only on the temperature of the air in mines, but on that of the rocks,
and on the water issuing from them. The mean rate of increase,
calculated from results obtained in six of the deepest coal mines in
Durham and Northumberland, is 1 degrees Fahr. for a descent of forty-four
English feet.[747] A series of observations, made in several of the
principal lead and silver mines in Saxony, gave 1 degrees Fahr. for every
sixty-five feet. In this case, the bulb of the thermometer was
introduced into cavities purposely cut in the solid rock at depths
varying from 200 to above 900 feet. But in other mines of the same
country, it was necessary to descend thrice as far for each degree of
temperature.[748]
A thermometer was fixed in the rock of the Dolcoath mine, in Cornwall,
by Mr. Fox, at the great depth of 1380 feet, and frequently observed
during eighteen months; the mean temperature was 68 degrees Fahr., that
of the surface being 50 degrees, which gives 1 degrees for every
seventy-five feet.
Kupffer, after an extensive comparison of the results in different
countries, makes the increase 1 degrees F. for about every thirty-seven
English feet.[749] M. Cordier announces, as the result of his
experiments and observations on the temperature of the interior of the
earth, that the heat increases rapidly with the depth; but the increase
does not follow the same law over the whole earth, being twice or three
times as much in one country as in another, and these differences are
not in constant relation either with the latitudes or longitudes of
places.[750] He is of opinion, however, that the increase would not be
overstated at 1 degrees Cent. for every twenty-five metres, or about 1
degrees F. for every forty-five feet.[751] The experimental well bored
at Grenelle, near Paris, gave about 1 degrees F. for every sixty English
feet, when they had reached a depth of 1312 feet.
Some writers have endeavored to refer these phenomena (which, however
discordant as to the ratio of increasing heat, appear all to point one
way) to the condensation of air constantly descending from the surface
into the mines. For the air under pressure would give out latent heat,
on the same principle as it becomes colder when rarefied in the higher
regions of the atmosphere. But, besides that the quantity of heat is
greater than could be supposed to flow from this source, the argument
has been answered in a satisfactory manner by Mr. Fox, who has shown,
that in the mines of Cornwall the ascending have generally a higher
temperature than the descending aerial currents. The difference between
them was found to vary from 9 degrees to 17 degrees F.; a proof that,
instead of imparting heat, these currents actually carry off a large
quantity from the mines.[752]
If we adopt M. Cordier's estimate of 1 degrees F. for every 45 feet of
depth as the mean result, and assume, with the advocates of central
fluidity, that the increasing temperature is continued downwards, we
should reach the ordinary boiling point of water at about two miles
below the surface, and at the depth of about twenty-four miles should
arrive at the melting point of iron, a heat sufficient to fuse almost
every known substance. The temperature of melted iron was estimated at
21,000 degrees F., by Wedgwood; but his pyrometer gives, as is now
demonstrated, very erroneous results. Professor Daniell ascertained that
the point of fusion is 2786 degrees F.[753]
[Illustration: Fig. 92.
Section of the earth, in which the breadth of the outer boundary line
represents a thickness of 25 miles; the space between the circles,
including the breadth of the lines, 200 miles.]
According to Mr. Daniell's scale, we ought to encounter the internal
melted matter before penetrating through a thickness represented by
that of the outer circular line in the annexed diagram (fig. 92);
whereas, if the other or less correct scale be adopted, we should meet
with it at some point between the two circles; the space between them,
together with the lines themselves, representing a crust of 200 miles in
depth. In either case, we must be prepared to maintain that a
temperature many times greater than that sufficient to melt the most
refractory substances known to us, is sustained at the centre of the
globe; while a comparatively thin crust, resting upon the fluid, remains
unmelted; or is even, according to M. Cordier, increasing in thickness,
by the continual addition of new internal layers solidified during the
process of refrigeration.
The mathematical calculations of Fourier, on the passage of heat through
conducting bodies, have been since appealed to in support of these
views; for he has shown that it is compatible with theory that the
present temperature of the surface might coexist with an intense heat at
a certain depth below. But his reasoning seems to be confined to the
conduction of heat through solid bodies; and the conditions of the
problem are wholly altered when we reason about a fluid nucleus, as we
must do if it be assumed that the heat augments from the surface to the
interior, according to the rate observed in mines. For when the heat of
the lower portion of a fluid is increased, a circulation begins
throughout the mass, by the ascent of hotter, and the descent of colder
currents. And this circulation, which is quite distinct from the mode in
which heat is propagated through solid bodies, must evidently occur in
the supposed central ocean, if the laws of fluids and of heat are the
same there as upon the surface.
In Mr. Daniell's experiments for obtaining a measure of the heat of
bodies at their point of fusion, he invariably found that it was
impossible to raise the heat of a large crucible of melted iron, gold,
or silver, a single degree beyond the melting point, so long as a bar of
the respective metals was kept immersed in the fluid portions. So in
regard to other substances, however great the quantities fused, their
temperature could not be raised while any solid pieces immersed in them
remained unmelted; every accession of heat being instantly absorbed
during their liquefaction. These results are, in fact, no more than the
extension of a principle previously established, that so long as a
fragment of ice remains in water, we cannot raise the temperature of the
water above 32 degrees F.
If, then, the heat of the earth's centre amount to 450,000 degrees F.,
as M. Cordier deems highly probable, that is to say, about twenty times
the heat of melted iron, even according to Wedgwood's scale, and upwards
of 160 times according to the improved pyrometer, it is clear that the
upper parts of the fluid mass could not long have a temperature only
just sufficient to melt rocks. There must be a continual tendency
towards a uniform heat; and until this were accomplished, by the
interchange of portions of fluid of different densities, the surface
could not begin to consolidate. Nor, on the hypothesis of primitive
fluidity, can we conceive any crust to have been formed until _the
whole_ planet had cooled down to about the temperature of incipient
fusion.
It cannot be objected that hydrostatic pressure would prevent a tendency
to equalization of temperature; for, as far as observations have yet
been made, it is found that the waters of deep lakes and seas are
governed by the same laws as a shallow pool; and no experiments indicate
that solids resist fusion under high pressure. The arguments, indeed,
now controverted, always proceed on the admission that the internal
nucleus is in a state of fusion.
It may be said that we may stand upon the hardened surface of a
lava-current while it is still in motion,--nay, may descend into the
crater of Vesuvius after an eruption, and stand on the scoriae while
every crevice shows that the rock is red-hot two or three feet below us;
and at a somewhat greater depth, all is, perhaps, in a state of fusion.
May not, then, a much more intense heat be expected at the depth of
several hundred yards, or miles? The answer is,--that until a great
quantity of heat has been given off, either by the emission of lava, or
in a latent form by the evolution of steam and gas, the melted matter
continues to boil in the crater of a volcano. But ebullition ceases when
there is no longer a sufficient supply of heat from below, and then a
crust of lava may form on the top, and showers of scoriae may then
descend upon the surface, and remain unmelted. If the internal heat be
raised again, ebullition will recommence, and soon fuse the superficial
crust. So in the case of the moving current, we may safely assume that
no part of the liquid beneath the hardened surface is much above the
temperature sufficient to retain it in a state of fluidity.
It may assist us in forming a clearer view of the doctrine now
controverted, if we consider what would happen were a globe of
homogeneous composition placed under circumstances analogous, in regard
to the distribution of heat, to those above stated. If the whole planet,
for example, were composed of water covered with a spheroidal crust of
ice fifty miles thick, and with an interior ocean having a central heat
about two hundred times that of the melting point of ice, or 6400
degrees F.; and if, between the surface and the centre, there was every
intermediate degree of temperature between that of melting ice and that
of the central nucleus--could such a state of things last for a moment?
If it must be conceded, in this case, that the whole spheroid would be
instantly in a state of violent ebullition, that the ice (instead of
being strengthened annually by new internal layers) would soon melt, and
form part of an atmosphere of steam--on what principle can it he
maintained that analogous effects would not follow, in regard to the
earth, under the conditions assumed in the theory of central heat?[754]
M. Cordier admits that there must be tides in the internal melted ocean;
but their effect, he says, has become feeble, although originally, when
the fluidity of the globe was perfect, "the rise and fall of these
ancient land tides could not have been less than from thirteen to
sixteen feet." Now, granting for a moment, that these tides have become
so feeble as to be incapable of causing the fissured shell of the earth
to be first uplifted and then depressed every six hours, still may we
not ask whether, during eruptions, the lava, which is supposed to
communicate with a great central ocean, would not rise and fall sensibly
in a crater such as Stromboli, where there is always melted matter in a
state of ebullition?
_Whether chemical changes may produce volcanic heat._--Having now
explained the reasons which have induced me to question the hypothesis
of central heat as the primary source of volcanic action, it remains to
consider what has been termed the chemical theory of volcanoes. It is
well known that many, perhaps all, of the substances of which the earth
is composed are continually undergoing chemical changes. To what depth
these processes may be continued downwards must, in a great degree, be
matter of conjecture; but there is no reason to suspect that, if we
could descend to a great distance from the surface, we should find
elementary substances differing essentially from those with which we are
acquainted.
All the solid, fluid, and gaseous bodies known to us consist of a very
small number of these elementary substances variously combined: the
total number of elements at present known is less than sixty; and not
half of these enter into the composition of the more abundant inorganic
productions. Some portions of such compounds are daily undergoing
decomposition, and their constituent parts being set free are passing
into new combinations. These processes are by no means confined to
minerals at the earth's surface, and are very often accompanied by the
evolution of heat, which is intense in proportion to the rapidity of the
combinations. At the same time there is a development of electricity.
The spontaneous combustion of beds of bituminous shale, and of refuse
coal thrown out of mines, is generally due to the decomposition of
pyrites; and it is the contact of air and water which brings about the
change. Heat results from the oxidation of the sulphur and iron, though
on what principle heat is generated, when two or more bodies having a
strong affinity for each other unite suddenly, is wholly unexplained.
_Electricity a source of volcanic heat._--It has already been stated,
that chemical changes develop electricity; which, in its turn, becomes a
powerful disturbing cause. As a chemical agent, says Davy, its silent
and slow operation in the economy of nature is much more important than
its grand and impressive operation in lightning and thunder. It may be
considered, not only as directly producing an infinite variety of
changes, but as influencing almost all which take place; it would seem,
indeed, that chemical attraction itself is only a peculiar form of the
exhibition of electrical attraction.[755]
Now that it has been demonstrated that magnetism and electricity are
always associated, and are perhaps only different conditions of the same
power, the phenomena of terrestrial magnetism have become of no ordinary
interest to the geologist. Soon after the first great discoveries of
Oersted in electro-magnetism, Ampere suggested that all the phenomena of
the magnetic needle might be explained by supposing currents of
electricity to circulate constantly in the shell of the globe in
directions parallel to the magnetic equator. This theory has acquired
additional consistency the farther we have advanced in science; and
according to the experiments of Mr. Fox, on the electro-magnetic
properties of metalliferous veins, some trace of electric currents seems
to have been detected in the interior of the earth.[756]
Some philosophers ascribe these currents to the chemical action going on
in the superficial parts of the globe to which air and water have the
readiest access; while others refer them, in part at least, to
thermo-electricity excited by the solar rays on the surface of the earth
during its rotation; successive parts of the atmosphere, land, and sea
being exposed to the influence of the sun, and then cooled again in the
night. That this idea is not a mere speculation, is proved by the
correspondence of the diurnal variations of the magnet with the apparent
motion of the sun; and by the greater amount of variation in summer than
in winter, and during the day than in the night. M. de la Rive, although
conceding that such minor variations of the needle may be due to
thermo-electricity, contends that the general phenomena of terrestrial
magnetism must be attributed to currents far more intense; which, though
liable to secular fluctuations, act with much greater constancy and
regularity than the causes which produce the diurnal variations.[757]
The remark seems just; yet it is difficult to assign limits to the
accumulated influence even of a very feeble force constantly acting on
the whole surface of the earth. This subject, however, must evidently
remain obscure, until we become acquainted with the causes which give a
determinate direction to the supposed electric currents. Already the
experiments of Faraday on the rotation of magnets have led him to
speculate on the manner in which the earth, when once it had become
magnetic, might produce electric currents within itself, in consequence
of its diurnal rotation.[758] We have seen also in a former chapter (p.
129) that the recent observations of Schwabe, 1852, have led Col. Sabine
to the discovery of a connection between certain periodical changes,
which take place in the spots on the sun, and a certain cycle of
variations in terrestrial magnetism. These seem to point to the
existence of a solar magnetic period, and suggest the idea of the sun's
magnetism exerting an influence on the mass of our planet.
In regard to thermo-electricity, I may remark, that it may be generated
by great inequalities of temperature, arising from a partial
distribution of volcanic heat. Wherever, for example, masses of rock
occur of great horizontal extent, and of considerable depth, which are
at one point in a state of fusion (as beneath some active volcano); at
another, red-hot; and at a third, comparatively cold--strong
thermo-electric action may be excited.
Some, perhaps, may object, that this is reasoning in a circle; first to
introduce electricity as one of the primary causes of volcanic heat, and
then to derive the same heat from thermo-electric currents. But there
must, in truth, be much reciprocal action between the agents now under
consideration; and it is very difficult to decide which should be
regarded as the prime mover, or to see where the train of changes, once
begun, would terminate. Whether subterranean electric currents if once
excited might sometimes possess the decomposing power of the voltaic
pile, is a question not perhaps easily answered in the present state of
science; but such a power, if developed, would at once supply us with a
never-failing source of chemical action from which volcanic heat might
be derived.
_Recapitulation._--Before entering, in the next chapter, still farther
into the inquiry, how far the phenomena of volcanoes and earthquakes
accord with the hypothesis of a continued generation of heat by chemical
action, it may be desirable to recapitulate, in a few words, the
conclusions already obtained.
1st. The primary causes of the volcano and the earthquake are, to a
great extent, the same, and must be connected with the passage of heat
from the interior to the surface.
2dly. This heat has been referred, by many, to a supposed state of
igneous fusion of the central parts of the planet when it was first
created, of which a part still remains in the interior, but is always
diminishing in intensity.
3dly. The spheroidal figure of the earth, adduced in support of this
theory, does not of necessity imply a universal and simultaneous
fluidity, in the beginning; for supposing the original figure of our
planet had been strictly spherical--which, however, is a gratuitous
assumption, resting on no established analogy--still the statical figure
must have been assumed, if sufficient time be allowed, by the gradual
operation of the centrifugal force, acting on the materials brought
successively within its action by aqueous and igneous causes.
4thly. It appears, from experiment, that the heat in mines increases
progressively with their depth; and if the ratio of increase be
continued uniformly from the surface to the interior, the whole globe,
with the exception of a small external shell, must be fluid, and the
central parts must have a temperature many times higher than that of
melted iron.
5thly. But the theory adopted by M. Cordier and others, which maintains
the actual existence of such a state of things, seems wholly
inconsistent with the laws which regulate the circulation of heat
through fluid bodies. For, if the central heat were as intense as is
represented, there must be a circulation of currents, tending to
equalize the temperature of the resulting fluids, and the solid crust
itself would be melted.
6thly. Instead of an original central heat, we may, perhaps, refer the
heat of the interior to chemical changes constantly going on in the
earth's crust; for the general effect of chemical combination is the
evolution of heat and electricity, which in their turn become sources of
new chemical changes.