terests in common. The army, to which we are so largely indebted, yet which, despite the great national movement in 1813, had retained agood deal of its squire-like (junkerlich) exclusiveness during the prolonged peace, has drawn nearer to the rest of the nation since our political revival, and tends more and more to become amalgamated with it. It is now the common school of all Germans, where the youth of all the educated classes meet together, first as volunteers, next as officers of the reserve, and finally as officers of the Land wehr; and, unless I am greatly mistaken, this citizen-soldier is destined to become the type of German culture, as the country gentleman has become that of English. Especially is this likely to be the case should admission to the volunteer service again be restricted to the educated, and those only who have passed through the highest school-classes be accepted, and should the officers' corps in the standing army continue, as during the last fifteen years, to be more and more recruited from the middle classes. If it has hitherto been the official, with his habits, sometimes formal, sometimes off-hand, who predominated and gave the tone in German society, that position is now from day to day passing more irrevocably into the hands of the independent merchant and manufacturer, who is also an officer in the national army, and on whose excess of nonchalance soldiery discipline acts as a wholesome check, while the starchness of his military bearing is advantageously corrected by the freedom of civil life.

Yet these are all merely external matters. As the free atmosphere of a scientific culture and ideal spirit breathed by our officials at the university, is the cause of their great superiority to the clerks of the French bureaucracy; so their presence in the army brings our youth together in the service of something higher, of something which transcends the narrow interests of their everyday life; and this it is that, properly speaking, crowns the whole civilization. This military training, it is true, only aims at making good Germans of our sons; but they ought to be brought up to be human beings as well This our colleges (Gymnasien), our technical, commercial, and cadet-schools do not do, or rather have left off doing; they train them to be merchants, professors, engineers, and soldiers, things which ought to be left to special schools, apprenticeship, or life itself. This is the thing we must guard against as the greatest danger which menaces German culture. It will only be when all the sons of the educated, no matter what career they may afterward adopt, are once more obliged to sit on the same benches, to share the same pastimes, to derive their intellectual nourishment from the same source, that we shall again have a right to think and talk about a German society. Only then can we attain that social unity of which we all feel the want, as we have acquired our literary unity by hard work, and our political unity by the force of arms.-Contemporary Review.

haunts. Even to less sanguine man the prospect is a cheering one; but while we gaze and listen

Medio de fonte leporum
Surgit amari aliquid-

the flashing crystals vanish from our path, and dirt resumes its sway. One

short half-hour of vernal sunshine suffices to dispel those fragile ice-sheets, whose thickness may be estimated at about a quarter of an inch.

And a quarter of an inch of ice is our present starting point. We have to imagine such a film of ice continuous over the whole surface of the globe. And we have further to carry out in imagination the suggestion which the shepherd, according to the old ballad, once offered to King John, as to the speediest method of making a complete circuit of the world; to wit,

Ride with the sun, and ride with the same
Until next morning he riseth again;

We

so that we may have the opportunity of seeing our ice-sheet dissolved, at least along one parallel of latitude. should then have realized one practical measure of the annual thermal loss of the planet on which we dwell. The heat radiated by the earth into space in one year has been computed to be, at the present time, as much as would be required to melt a quarter of an inch of ice covering the whole of its surface.*

Viewed in this light, the loss seems to be but infinitesimal. Half an hour of sunshine, or even the application of the palm of the hand for a few seconds, will produce in any one spot as much heat as finds its way there through the earth's crust in 365 days. But if, on the other hand, we look at it collectively, and imagine the ice whose supposed liquefaction is our present measure gathered together into a mass, it proves to be no less than 777 cubic miles of ice. We may therefore in thought portray an enormous glacier or iceberg, a mile broad and a mile high, extending in one straight unbroken line, say, from London to Vienna. And when we have cal

*"Chemical Denudation in Relation to Geological Time," by T. Mellard Reade, C.E., F.G.S., p. 34.

+Chemical Denudation in Relation to Geological Time," by T. Mellard Reade, C.E., F.G.S., p. 34. .

The

That the interior of the earth is the seat of intense heat is a familiar truth. Volcanic phenomena give us Mining exocular demonstration of it. periences, moreover, have furnished us with an almost uniform rate at which the heat increases, and this is generally computed to be about 1° F. for every 55 feet of descent. But mining experiences are necessarily very limited. deepest mine in England, that of the Rosebridge Colliery near Wigan, takes us down only 2445 feet and to a temperature not much exceeding 90° F. It is hot enough to make the work exceedingly trying to the miners, but that is all. This, however, is (so to speak) scarcely traversing the earth's epidermis. But if we may assume a unform increase of heat in descending, the temperature at a depth of 50 miles may be expressed in figures as 4800° F. In other words, at less than an eighth of the distance which lies between the circumference of the earth and its centre, the heat would be about twenty-two times the heat of boiling water at the sea level. Proportionate figures might of course express the heat at greater depths still, but figures fail to convey any idea to the mind of that which must necessarily transcend all imagination. Suffice it to say, that in a descending series we must eventually come to a heat so great that no substance with which we are acquainted could, under any conditions which we can imagine, exist in it in either solid or fluid form. And we conclude, therefore, that if the earth's centre be not itself in a gaseous condition (and there is reason to think that it may not be so), there must be a gaseous zone somewhere between a solid centre and a solid circumference.

"Facilis descensus Averni" is proverbially treated as a truism. But if the classic authors are to be our guides, and if in the centre of our planet Acherontian shades and Elysian fields are to be localized, there will be found practical difficulties of access which might well discourage even so unsubstantial a personage as a ghost. Nor can the allpowerful imagination accomplish the descent with any approach to ease. The distance we may suppose to be nearly 4300 miles; but along a line of this length connecting the surface of the earth with its centre, we may safely assume that conditions would vary greatly, and (since heat and pressure have to be balanced one against the other) probally by no means uniformly. We can measure the power of pressure upon the surface, but in the nether depths its power is in part open to conjecture, nor can we say how soon we may reach a debateable zone, at which the expansiveness of heat may overcome the compressive force of gravitation. Nor, again, could we venture to expect to find that zone itself always at an uniform depth. Here and there it seems to approach the surface. The volcano is nature's safety-valve, and the cavernous rumble of the earthquake warns us that there are imprisoned gases beneath our feet which pressure but imperfectly prevents from escaping. Upon other ground, also, it is quite evident that our experience, limited as it is to the surface of the earth, may tend to mislead us in regard to what lies beneath the surface; for if pressure increased uniformly with depth, the average density of the earth would be much greater than what upon astronomcial data we know it to be. The earth, as a whole, is about 5 times as heavy as it would be if it were entirely composed of water; or, technically expressed, the density of water is 1, and the mean density of the globe is 5. But 5 is only about double the density of rock matter upon the surface; whereas, if nothing but steadily increasing pressure be supposed it would vastly exceed this. There is therefore only one possible explanation. Heat, intense heat, somewhere or other, overcomes pressure and converts every thing into gas; and if it were in our power to try experiments, and to feed

the subterranean crucible with the most intractable substances-asbestos, fireproof safes, or what we will-all would there share the same fate-instant evan

escence.

Figures would be useless to express the intensity of such a heat, and comparisons would be but feeble. But it is evident that there must be some analogy between steam-true gaseous invisible steam, not the condensed vapor which so often passes under the name of steam

and these vaporiform rocks. And it may be asked, If a heat of 212° F. suffices to convert water into a gas which will propel an engine, not without some risk of a boiler bursting, what must be both the heat imprisoned and the power of pressure which girds it in, when rock matter (which, if condensed, would measure doubtless many hundred millions of cubic miles) is thereby kept throughout the ages in a vaporous form, and, like the steam in the trusty loco. motive, is the servant and not the master still?

But, as we have already said, this subterranean heat is escaping slowly yet incessantly; and that which once escapes never returns. One practical consequence, moreover, of this lessening of the internal heat of the globe is a reduction in its size. The weight of the world continues the same. The materials of which it was at first composed alone enter into its composition now, if we except a trifling accretion of meteorites, insufficient to make any calculable difference. There has, it is true, been a constant re-arrangement of the constituent particles, but the particles remain the same. And we know by the shrunken and wrinkled condition of the older rocks how important a part contraction, consequent upon cooling, has played in the re-moulding of the earth's surface. Year by year she has parted with a certain amount of heat, and as a consequence she has shrunk, and she is still shrinking, into slowly narrowing dimensions. But if geological time be as great as geologists claim, it may be thought that surely earth ought ere this to have cooled completely. That she has not done so is entirely due to the marvellous properties of rock-matter as a non-conductor of heat. The cooled surface of the globe is to the inner fires

as the eider-down quilt to the sleeper. Moreover, radiation through rock matter, necessarily a very slow process at all times, tends to become slower with the lapse of ages; for in proportion as the heat escapes, so does the cooled crust grow in bulk. If earth be growing older and colder, yet does she weave for herself an ever-thickening mantle and hug the closer the caloric whose escape she cannot altogether prevent.

The molten rock which volcanoes eject furnishes a good illustration of this non-conducting property. It is a wellknown fact that the outer surface of a lava stream soon cools sufficiently for a man to walk over it. But for years afterward the heat will continue to be in tense at a depth of a few feet. Poles can be thrust into fused rock by men standing upon the cooled surface above it.

Water poured into cracks will spirt fiercely up again in jets of steam, and cigars may be lit in crevices around which the moss grows, or the fern frond nestles, or the wild crocus is forced into premature bloom.

But perhaps no more curious and conclusive instance can be cited than one which has been recorded by the late Sir C. Lyell,* who tells us how a portion of a glacier upon the slopes of Etna was actually prevented from melting by the incandescent lava which had flowed over it from some vent in the higher regions of the volcano. And the anomaly is thus accounted for : A shower of volcanic dust some ten feet thick seems to have first fallen upon the ice, and so good a non-conductor of heat did this dust bed prove, that the fiery lava stream afterward flowed over it, not only not melting the subjacent ice, but actually preserving it by the exclusion of solar heat. The glacier, thus strangely preserved, supplied the Catanians with ice during at least thirty summers; and in this position, if it had not been disturbed, it might have endured as long as the snows upon the summit of Mont Blanc, or the frozen billows of a Palæo crystic sea.

The hint thus given by Nature seems to have been adopted, as the same author tells us, by the shepherds of the

*

And thus it comes to pass that, with the intensest heat at proportionately no great distance beneath us, we shiver in the northern blast. Commerce and invention are taxed to the utmost to provide us with a little artificially produced caloric, while, Tantalus-like, we realize that just beyond our reach there is a practically exhaustless supply. Who knows but that the next trimph of science and engineering skill-surely one which would be no more wonderful than the telegraph or the telephone-might be a conductor of subterranean heat ramifying like the gas-pipes of a city into every house, and superseding the use of fuel ! Nous verrons. But in the meantime our present subject leads to an inquiry exactly the converse of this. We have to explain, not how heat may be got out of the earth, but how in the first instance it ever got in.

An apple dumpling has ere now been represented as a phenomenon suggestive of philosophic inquiry. How an entire apple could have found its way into the centre of an unbroken but fragile crust certainly might be a problem demanding solution from one by whom an apple dumpling had never been otherwise regarded than as one of the delectable things which are charmed into existence by the magic sound of the dinner bell. The apple certainly could not have been placed there after the crust had assumed the conditions familiar to the eye and experience of the quasi-philosopher, who would reasonably and logically premise that the apple must be a something anterior to the paste. Upon exactly the same principle may we assume that Earth's inner fires must have been anterior to the cooled crust which has gathered round them; for after what we have seen of the non-conducting properties of this crust, we shall not be ready to suppose that heat from solar rays or from any other exterior source could penetrate so far. It is plain, therefore,

that our planet, or, to take a larger view, all the planets, were once in an incandescent state throughout, and that they are now found to be in various stages of cooling.

And this brings us to the nebular hypothesis," whose origin, like the birthplace of Homer, is a subject of controversy. Some, and especially some Frenchmen, claim it for Laplace; some, and especially some Englishmen, assign it to Sir W. Herschel; while Prof. Hæckel,* writing from a German point of view, traces it back to some half-forgotten utterances of Kant. Certain, however, is it that little was heard of it until about the year 1811, when it became in this country intimately associated with the name of the elder Herschel, by whom a nebula in Orion was supposed to exhibit the same gaseous conditions as those out of which our solar system is now believed to have been developed. If a planetary system may be thus accounted for by phenomena which observation shows to be still in existence in another part of the universe, the fact is one of great interest and importance.

This celebrated and now generally accepted hypothesis assumes as its starting-point that the whole of our solar system, from the sun, its present centre, to the orbit of what is now Neptune, or even perhaps to a circumference more distant still, was originally one vast nebula, such as those which may now be observed in the heavens, associated with more than one constellation. At some distant period of the past (it supposes) a condensation of this nebula began while a spiral motion of the whole, due to stellar attraction, and still, probably, in force, caused it to rotate. Two sets of changes consequently commenced. Outer rings became detached from the nebula, while condensation and gravitation caused each of these to assume in turn the form of a sphere, which, obeying the impetus already given to it, circled along what we may now call its orbit. Thus the superior planets, as some think, came into existence first, then Earth, then the inferior planets. But after the birth of

*"History of Creation," by Prof. Hæckel, vol. i. chap. i.

Mercury, the youngest of our family of planets, no more rings were given off, but the condensed remains of the quondam nebula, driven with fierce energy against one another and toward a common centre, produced the huge fire-ball which we call the sun.

So well does this hypothesis account for observed phenomena, and so long has it now stood the test of criticism, that its truth may be regarded as little short of demonstrated. Its application to our present subject is manifold. To begin with, it accounts for the spherical forms of the planets. The sphere is the natural and necessary form for condensing matter to assume, when no modifying conditions are present. We see this in such familiar instances as the raindrop or the dew-drop, both of which are vapors condensed into spheres. Any fluid, moreover, if just so much of it be allowed to pass over the rim of a vessel as will suffice to overcome cohesion, instantly assumes a spherical form, and we call it, by the synonym of ordinary parlance, a drop." And this it does by virtue of that same law of molecular attraction which produced the round world, and each of its associated spheres.

We have, however, to account not only for a sphere, but for an intensely heated sphere. And condensation is a vera causa for this. It is a well-known fact that, as a rule, the more rapid the condensation, the more fierce will be the evolution of heat. Familiar experiments in chemistry and physics might be cited to show this. But for our present purpose the rain-drop will again suffice us. As slow condensation of aqueous vapor gives us the gentle shower, so does its rapid condensation produce the thunderbolt.

The

This, however, upon our hypothesis, would not have been the only source of heat in a nascent planet. The very forces of attraction which moulded the sphere, must also have been accompanied by a fierce evolution of heat. very same cause which first produced and now sustains the solar fires-those fires which are unaccountable upon any theory of combustion-must have made each planet to come into existence aglow with heat-in fact, a mimic sun.

A popular tradition has ventured to suggest that the law of gravitation was