vided, that when the Indian army is employed for imperial purposes beyond the frontiers of India the cost shall be borne by England, and when for Indian purposes the cost shall be borne by India. There seems to be no room for doubt that the present war has been undertaken, in part at least, for imperial purposes, and, therefore, India cannot be legally called upon to bear its entire cost. It has, in fact, been most distinctly stated by the Prime Minister that the military expedition into Afghanistan was not simply an Indian war, but was undertaken for imperial purposes; for, in a speech which he made in the House of Lords on the 10th of December, he said: "This is not a question of the Khyber Pass merely, and of some small cantonments at Dakka or at Jellalabad. It is a question which concerns the character and the influence of England in Europe.' As no one would for a moment think of throwing upon India the entire cost of maintaining the influence and character of England in Europe, no other conclusion is possible, than that the advance of 2,000,000l., without interest, to India is intended to be England's contribution towards the expense of an expedition which has been undertaken in the interest of the two countries. This being the case, it will be desirable to explain the exact share of the expense which will be borne by England and India respectively. As the 2,000,000l., which England can borrow at 3 per cent., is to be repaid by seven equal annual instalments, and as the first instalment will become due at the end of next year, the amount which England will contribute by foregoing the interest on the loan is somewhat less than 320,000l. This sum, therefore, represents the amount which England will pay towards the expense of an expedition which, it is officially stated, will cost 2,600,000l., and which, in the opinion of almost all independent military authorities, will greatly exceed this amount. But, assuming that the official estimate should prove strictly correct, it appears that India will pay 2,280,000l. and England 320,000l. India, therefore, will contribute more than seven pounds for every pound that is contributed by England. It is scarcely credible that a proposal should have been brought forward which would lead to such a result. It is, perhaps, only fair to conclude that when the real nature of the scheme is understood it will be promptly abandoned. At any rate it is difficult to suppose that it will ever be sanctioned by Parliament. The English people, whatever may be their faults, have never been charged, even by their bitterest detractors, with meanness. But it is not easy to see how we can escape from such a charge, if, when an expedition has been undertaken, not simply in the interest of India, but to maintain the 'influence and character of England in Europe,' we compel the Indian people, whether they wish it or not, surrounded as they are with poverty and financial embarrassment, to pay more than seven times as much as is contributed by all the wealth of England.

HENRY FAWCETT.

RECENT SCIENCE.

(PROFESSOR HUXLEY has kindly read, and aided the Compilers and the Editor with his advice upon, the following article.)

Ir 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 centre, 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 vapour, was being ejected in explosive puffs from the cone in the centre of the great crater. Sharp explosions succeeded each other at tolerably regular

''Physikalisch-mechanische Betrachtungen, veranlasst durch eine Beobachtung der Thätigkeit der Vesuvs im Mai 1878.' Monatsbericht der k. preussischen Akademie der Wissenschaften zu Berlin, 1878, pp. 558-582.

intervals of two or three seconds, and gave rise to rotating rings which, widening as they rose into the air, formed a crown of vapour around the summit of the mountain. It is by 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 chimney. From the large quantity of steam generated by the explosions, it is probable that hydrogen is the principal 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 sulphuretted hydrogen is one of the burning gases.

That water and perhaps hydrogen should be contained in 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 nebulæ. 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 vapours 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 in 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 in 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 in 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 in 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 towards 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 centre of the refrigerating globe, is a point on 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 largest 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 years since Professor James Thomson announced, on theoretical considerations, that if a body expand during solidifica

tion, its melting point must be lowered by pressure. This sagacious inference was afterwards 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 siliceous 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 heat 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, enclosing a fluid mass within. But if the solidified portions, as they sånk 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 centre of the globe. In this event the process of solidification would begin at the centre, and gradually tend outwards, 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