[PROFESSOR HUXLEY has kindly read, and aided the Compilers and the Editor of the Nineteenth Centwry witl;!. his advice upon, the following article.] NATURE OF THE INNER EARTH. IT is not only to the geologist, to the - physicist, and to the astronomer that speculations as to the probable nature of the interior of the earth are full of interest. So fascinating a subject appeals to a circle of inquirers far outside the pale of the special sciences. Every thoughtful man naturally feels curious to know something about the nature of the innermost parts of this earth on which we dwell. Is our globe a stony sphere, solid to its very core? Or is it made up of a hollow shell, with a mass of molten matter within? Or is there nothing but compressed gas inside the hollow sphere? Or, finally, is there a solid crust on the outside and a solid nucleus in the center, separated from each other by an intermediate layer of liquid? Each of these views, in turn, has found its advocates; and each has been supported by arguments of more or less weight. As direct observation of the earth's interior is manifestly impossible, except to a depth which is utterly insignificant in comparison with the magnitude of the earth, all reasoning on this subject must needs be based on evidence of an indirect kind. The arguments which have been advanced are drawn principally from the figure of the earth, from its mean density, from the increase of temperature which is observed on descending to accessible depths, and especially from the widely occurring phenomena of vulcanicity. A noteworthy contribution to the subject from the volcanic side has recently been made by Herr Siemens, whose investigations will be found recorded in a paper recently published in the monthly reports of the Berlin Academy. * In seeking an explanation of the phenomena which he witnessed during' a visit to Vesuvius last May, the author has been led to some general studies in vulcanology which have far more than local interest. At the time of his visit steam, or other vapor, was being ejected in explosive puffs from the cone in the center of the great crater. Sharp explosions succeeded each other at tolerably regukr intervals of two or threeseconds, and gaverise to rotating rings, which, widening as they rose into the air, formed a crown of vapor around the summit of the mountain. It is hy no means easy to explain how such rapidly recurring explosions, with the accompanying jets of steam, could be produced. Assuming that steam or gas may be .suddenly generated at great depths, it might fairly be expected that its ejection would be accompanied by the outflow of much lava; and that after each explosion sufficient time must be given for the accumulation of fresh lava in the chimney of the volcano before the next expulsion could occur. It may be suggested, indeed, that as water at a very high temperature is dissociated into its components, the magma or molten rock beneath the volcano might contain an explosive mixture of oxygen and hydrogen gases; then, on any considerable diminution of pressure, these gases would recombine and again form water. It is, however, highly improbable that, under the enormous pressure to which the magma must be subjected - anything like dissociation should occur; for the author's own experiments have shown that a mixture of oxygen and hydrogen, when subjected to a very high pressure, will explode. Dismissing, then, the idea of dissociation, the author is driven to the conclusion that hydrogen gas, or it may be combustible compounds of hydrogen, rise from below, and, mingling with atmospheric oxygen, form an explosive mixture which is burnt in the upper part of the volcanic ehimney. From the large quantity of steam generated by the explosions, it is probable that hydrogen is theprincipal combustible constituent of the gases, but it is not easy to decide whether the hydrogen exists in a free state, or combined with sulphur, carbon, and other elements. The presence of much sulphurous acid gas among the products renders it likely, however, that sulphureted hydrogen is one of the burning gases. That water and, perhaps, hydrogen should be contained in * “ Physikalisch-mechanieche Betrachtungen, veranlasst dutch eine Beobachtung der Thiitigkeit der Vesuvs im Mai 1878.” Monatsbericht der k. preussischen Akademie derWWenschajten zu Berlin, 1878, pp. 558-582. the magma, whence the volcanic products arise, appears highly probable on the well-known nebular hypothesis. It is generally conceded that the nearest approach which has yet been made to a rational explanation of the formation of our earth is to be found in the bold hypothesis which was conceived by Kant and elaborated by Laplace. On this assumption the earth and all the other planetary bodies have resulted from the condensation of nebulre. Thousands of these faintly luminous cloud-like bodies have been detected in the heavens, and the spectroscope has shown that some of them contain glowing hydrogen - On the condensation of a nebula, by attraction of its particles, great heat would necessarily be developed. Chemical.forces would then come into play during the contraction, and such compounds would be formed as were capable of existing under the given conditions of temperature and pressure. On increase of pressure - by contraction, and on reduction of heat by radiation, a liquid magma would eventually be formed. It is only from the outer portion of this molten mass, where the pressure is least, that the steam and other vapors and gases could directly escape; while at great depths they would be retained, either dissolved in the liquid mass or intimately mingled with the magma. Against the assumption that hydrogen and other combustible gases have been retained in- the magma, it will, of course, be objected that no hydrogen is found m our atmosphere; but that, on the contrary, the existence of free oxygen shows that this latter element must have been in excess when the chemical compounds were m course of formation. It must be remembered, however, that the solar atmosphere contains a large-proportion of hydrogen, and that enormous volumes of this gas exist in the red flames which are shot forth from the sun. The sun evidently represents the central portion of the nebula from which the solar system took birth; and the existence of free hydrogen at the present time m this orb may suggest the former existence of an excess of this element throughout the entire system. Although oxygen now forms a large proportion of our atmosphere, this may not always have been the case. It is conceivable, indeed, that during the early stages of the earth' s history the oxygen may have existed wholly in a state of combination, and may have been set free as atmospheric oxygen at a later period. But we know too little about the influence of powerful pressure and intense temperature II modifying chemical attraction, to admit of profitable speculation on such a subject. By continued cooling of the molten globe, a separation of its components would probably occur, according to their . relative weights. It is not to be supposed that the spheroid of igneous liquid would be homogeneous throughout; indeed it is possible that different parts of the same nebula ' may vary in constitution. Those compounds which were specifically heavier would be attracted toward the interior of the viscous sphere, while the less dense substances might remain nearer to the outside; thus the acid silicates might 'be' separated from, and float upon, the denser basic silicates. Whether the solidification would commence at the outside or at the center of the refrigerating globe, is a point 011 which many a lance has been broken. If a mass of molten metal be allowed to cool, it is well known that a crust soon forms over the surface, while the interior may remain for some time in a liquid state: this is seen equally in casting a leaden bullet and in the lajgest foundry work. It has, therefore, not unnaturally been argued that a ' crust would form on the surface of. the cooling globe, and that the interior might remain in a molten condition even to the present day. It is necessary, however, to examine the arguments which have been advanced against this view. It is now thirty yeais since Professor James Thomson announced on theoretical considerations, that if a body expand during solidification, its melting point must be lowered by pressure. This sagacious inference was afterward confirmed experimentally by his brother, now Sir William Thomson, who showed that the melting . point of ice was lowered in the way suggested; at the same time he pointed out that if the substance contracted during solidification its melting point ought to be raised—a prediction which- was confirmed by the experiments of Professor Bunsen, of Heidelberg, and of the late Mr. Hopkins, of Cambridge, whose investigations extended to such substances as wax and stearine, sulphur and spermaceti. From such experiments it has been concluded that our ordinary silicious rocks would have their melting points elevated by increase of pressure; in other words, they would require more heat to keep them in a molten state, if they were subjected to great pressure in the interior of the earth, than if they were in a state of fusion at the surface. It is clear, therefore, that in such a case, pressure and he:! directly oppose each other; the former tending to prevent and the latter tending to promote fusion. Whether the rocks be solid or liquid at a given depth must consequently depend on which of these two powers gains the ascendency. Supposing that the surface of the cooling globe were locally solidified, the solid portions might be again fused as they descended to regions of higher temperature, and the globe might thus be kept in a liquid condition until it became sufficiently viscous to prevent the subsidence of the solidified portions, when a solid crust would permanently form on the exterior, inclosing a fluid mass within. But if the solidified portions, as they sank in the molten mass, had their fusing point greatly raised by the increased pressure to which they were subjected in their deeper-seated position, then it is possible that they might retain their solid condition even at the very center of the globe. In this event the process of solidification would begin at the center, and gradualfy tend outward, until a solid, or nearly solid, spheroid was ultimately produced. It will be observed that this discussion hinges on the question whether the molten rock would contract .on solidification, and, if so, to what extent. Sir William Thomson based his calculations on the experiments of Bischof, which went to show that solid rocks are about 20 per cent, denser than ' the same material in a molten state. ,Mr. Mallet's experiments on blast-furnace slags show, however, that these silicates contract only to about 6 per cent. during solidification. Herr Siemens seeks to explain the difference between these results by an appeal to some interesting experiments conducted by his brother, Friedrich Siemens, at his bottle glass works in Dresden. He found that if the glass be perfectly fused to a thin liquid and be then allowed to cool, it rapidly contracts until it acquires a plastic or viscous condition; but on further cooling of this viscous material, the contraction is greatly diminished; and as the temperature continues to fall, the amount of contraction becomes less and less. In fact, at the very moment of solidification, it is likeiy that a slight expansion occurs. It appears, therefore, that when such a liquid as a molten vitreous silicate acquires solidity, the greater part of the observed contraction occurs during the transition to the plastic state. Hence the author argues that Sir W. Thomson's calcula tions based on Bischof's experiments are inadmissible. and that they go to prove, not that the earth must in consequence of pressure be solid to its center, but simply that the interior has become plastic or viscous. According to Sir W. Thomson's views. volcanoes must be fed from local accumulations of lava, probably from pockets of liquefied or partially liquefied matter which exist here and there at varying depths beneath the surface. Herr Siemens insists on the mechanical difficulty of explaining how . under such conditions, the lava could be forced upward to the surface. He also exposes the geological difficulty of accounting, on the hypothesis-of a solid globe, for the formation of the many thousand feet of alternating sedimentary deposits which are spread over the surface of the earth. On these and on other grounds, the author is led to reject the assumption of a solid or nearly solid earth and to fall back on the hypothesis of a liquid or a viscous mass inclosed in a crust of moderate thickness. To explain the ascent of lava from the interior of the earth to the crater of a volcano it is assumed that the highly fusible alkaline and hydrous lavas have a density which is below that of the solid crust or of the viscous silicates with which they are associated. These lighter silicates find their way into narrow ramifying channels or other cavities in the earth' s ciust; and when communication is established between these cavities and the surface, a column of liquid lava is forced up the canal by hvdro- static pressure. The force with which the lava rises in the pipe will be much increased by the expulsion of steam and various gases which are associated with the molten material, and are released from this association by diminution of pressure. Whether the lava is poured out at the surface or not will depend on the quantity of molten matter which rises in the chimney, on its specific gravity, on the proportion of gas and of water which it contains, and especially on the altitude of the volcano. Many very lofty voleanoes ej'ect no liquid lava, since hydrostatic equilibrium is secured before the column rises into the crater. On this principle, too, it may possibly be explained why most active volcanoes are situated either in or near to the sea. In concluding this paper Herr Siemens briefly refers to the necessity of making another assumption in order to explain, on the hypothesis of a liquid sphere with a comparatively thin crust, the great elevation of many continental areas and the gradual upheaval of large tracts of country at the present day. The difference in height between the plateau of Central Asia and the bottom of the Pacific Ocean is at least 12,000 meters, representing a difference of pressure on the magma of about one thousand atmospheres. In order to attain, under such conditions, the requisite hydrostatic equilibrium, it seems necessary to assume a difference of density between the rocks which constitute the constituents and those which form the floor of the ocean, the latter being, of course, the denser. It is possible, however, that the semi-fluid masses which occur below the solid crust have such a thickness and such a density as to compensate for this difference of pressure. Secular elevation would then follow as a local consequence of such difference. In connection with this subject it may be pointed out that in measuring the great Indian arc of the meridian, which stretches from Cape Comorin to the Himalayas, it was found that tlhe density of the rocks under and in the neighborhood of the Himalayas is less than in the plains to the south. A mas of matter like that of a mountain will, of course, exert an attractive action upon the plumb line, and will tend to pull it out of the perpendicular. Archdeacon Pratt calculated the extent of this deflection in the case of the Himalayas, but observation showed that the actual deviation was very much less than his computation; thus suggesting that the matter in these mountains, or in their neighborhood, has a lower density than that of the rocks of the plains. It has also been found in geodesical surveys that gravity at coast stations is generally greater than at the corresponding continental stations. Indeed, Archdeacon Pratt remarks, in his “Figure of the Earth,” that “the density of the crust beneath the mountains must be less than that below the plains, and still less than that below the ocean bed." It is also interesting to note that the Astronomer Royal. in delivering a popular lecture last year at Cockermouth, expressed himself in similar terms: “If one might presume on such a point, I should say that the high parts of the earth are made of something light. The heavy dense parts are those covered by considerable quantities of water, and they have sunk deep into the center of lava in which I conceive all things to be resting."* In this lecture Sir George Airy adds the great weight of his authority to the view that the center of the earth is still, to a great extent, in a condition of igneous fluidity. “ I do think,” he says, “that a large proportion of the central part of the earth is fluid and hot, and I think that upon this there are resting divers classes of something like solid matter." From what has been advanced in the preceding pages it will have been gathered that the present tendency among most men of science seems in the direction of a return to the old-fashioned views according to which the earth has a moderately thin crust which rests on a spheroid of molten matter in a more or less viscous condition- THE MOLECULAR CONSTITUTION OF MATTER. It is a great stride to descend from speculations on the nature of the interior of the earth to speculations on the molecular constitution of matter. But the remarkable researches which Mr. Crookes has recently submitted to the Royal Society deserve the earliest possible notice, since they open up a new field of scientific inquiry whichhas already led to unexpected results. • On the passage of a spark from an induction coil through a highly rarefied gas, such as that in a common vacuum tube, a dark space is observed around the negative pole. It would appear that the intense molecular vibration set up in the metal forming this pole excites a molecular disturbance in the surrounding medium, and in the ease of a highly attenuated gas the area of disturbance may extend to a considerable distance from the electrified surface. By connecting an ingeniously constructed radiometer with the inducto- rium, in such a way as to make the movable fly play the part of the negative pole, it was found that, on exhausting * “On the Probable Condition of the Interior of the Earth.” By Sir George Airy, K.C,B., F.R.S., etc. “Transactions of the Cumberland Ac- sociation for the Advancement of Literature and Science.” Edited by J. Clifton Ward. Part III., 1878, p. 43. t On this subject attention may be called to It valuable paper by the Rev. Osmond Fisher, “On the Inequalities of tha Faith's Surface as produced by Lateral Pressure upon the Hypothesis of a Liquid Substratum,” “Cambridge Phil. Trans.,” vol. xii. part ii.; to C»pt. Button's “Critical Observations on Theories of the Earth's Phvsical Evolution,” thi , the Penn _, Philadelphia, May and June, 1876; Oeol. Mas , 1876. pp. 323, 370; and to a paper by the late Mr. David Forbes in the Geol. Mag.. October, 1867. * , On the Illumination of Lines of Molecular Pressure, and the Trajectory of Molecules.” By William Crookes, F.R.S., etc. “ Proceedings of the Royal Society,” vol. =viii., No. 191, p. 103.the vessel, the metallic faces of the vanes became enveloped in a halo of velvety violet light, while the opposite sides of the vanes remained obscure. As the pressure was reduced by continued exhaustion, the luminosity became separated from the metal by a dark space; and on continuing to exhaust, this dark space extended to the glass walls of the vessel, against which it appeared to become flattened, the rotation of the fly meanwhile being very rapid. In order to understand the principle on which Mr. Crookes seeks to explain these phenomena, it is necessary to refer briefly to the modern view of the constitution of gases known as the kinetic theory. According to this theory any given volume of gas contains a vast number of molecules, or material particles, moving in all directions with astounding rapidity, and therefore coming at every instant into contact with one another. Between successive encounters the molecule is supposed to move in a rectilinear path; but as the collisions succeed eaclt other with great rapidity, probably numbering millions in a second, the free path must, as a rule, be excessively small. During rarefaction the number of particles in the given space is, of course, reduced, and, therefore, the chances of collision are lessened. At a very high degree of exhaustion, such as obtains in these vacuum- tubes, the space is so little crowded that the molecular encounters are comparatively few, and the mean free path is therefore larger. According to Mr. Crookes' view, the electrified molecules of the residual medium in the tube rebound with great velocity from the negative pole, and in this way keep back the more slowly moving molecules which are advancing toward that pole. At the border of the dark space collisions occur, and the arrest of velocity gives rise to luminous effects. It is obvious, therefore, that the thickness of the dark space around the pole may be taken as the measure of the length of the free path between successive hits. By continued exhaustion the length of path may be made to exceed the distance between the fly of the electric radiometer and the sides of the glass vessel which incloses the instrument. There will consequently then be no luminosity produced until the molecules impinge against the glass, whereby their energy is suddenly checked. It is found that when the molecular rays are brought to a focus, in a tube at a high degree of exhaustion, the particles on impact with the glass develop a beautiful phosphorescence. A soft German gla®, which was chiefly used in Mr. Crookes' experiments, gave a fine greenish-yellow light. This phosphorescent light is to be distinguished from the ordinary luminosity observed in vacuum tubes; such tubes give, for example, different spectra according to the nature of the residual gas, while the phosphorescence in these highly rarefied media gives a continuous spectrum of the same kind, whatever be the nature of the gas. Further experiments led to the interesting discovery that the stream of molecules from the electrified surface is highly sensitive to magnetic influence, and may be deflected in one direction or another by means of a magnet. The rays which pass from the negatively electrified body to the glass surface are spoken of by the author as “ rays of molecular light” or “rays of emissive light.” These “ rays,” however, are simply streams of molecules passing from the excited body, which are invisible until, falling upon a suitable screen, their effects are manifested by the luminosity of this screen. A bullet which strikes a target may become red-hot; but the trajectory of the bullet could not, on that ground, be properly called a “ray “ of light or heat. It is conceivable that, at the very high degree of exhaustion attained in these tubes, the mean free path of the rapidly moving molecules may become so long that “ the hits in a given time may be disregarded in comparison to the misses.” ' In one- experiment it was found that a number of molecules sufficient to excite the green phosphorescence had been projected to a distance of 102 millimeters without being stopped by collision. In highly rarefied media the properties which are peculiar to gases must, therefore, be reduced to a minimum, and it is even conjectured that the media may pass into an ultra-gaseous state in which new properties come into play. To borrow Mr. Crookes' concluding words: “ The phenomena in these exhausted tubes reveal to physical science a new world—a world where matter exists in a fourth state, where the corpuscular theory of light holds good, and where light does not always move in a straight line, but where we can never enter, and in which we must be content to observe and experiment from the outside." (To be continued.)
This article was originally published with the title "Recent Science"