of the ice and water, freezes the water again in new positions, and thus a change of form, or plastic yielding of the mass of ice to the applied pressures, has occurred. The newly-formed ice is at first free from the stress of the applied forces, but the yielding of one part always leaves some other part exposed to the pressure, and that part, in its turn, melts and falls in temperature; and, on the whole, a continual succession goes on, of pressures being applied to particular parts-liquefaction occurring in those parts accompanied by evolution of cold,—dispersion of the water so produced in such directions as will relieve its pressure, and re-congelation, by the cold previously evolved, of the water on its being relieved from this pressure. The cycle of operations then begins again, for the parts re-congealed, after having been melted, must, in their turn, through the yielding of other parts, receive pressures from the applied forces, thereby to be again liquefied, and to proceed through successive operations as before. The succession of these processes must continue as long as the external forces tending to change of form remain applied to the mass of porous ice permeated by minute quantities of liquid water. The ice is thus shown to be incapable of opposing a permanent resistance to the pressures, and to be subject to gradual changes of form while they act on it; or, in other words, it has been shown to be possessed of the quality of plasticity. In the foregoing, I have supposed the ice under consideration to be porous, and to contain small quantities of liquid water diffused through its substance. Porosity and permeation by liquid water are generally understood, from the results of observations, and from numerous other reasons, to be normal conditions of glacier ice. It is not, however, necessary for the purposes of my explanation of the plasticity of ice at the freezing-point, that the ice should be, at the outset, in this condition; for even if we commence with the consideration of a mass of ice perfectly free from porosity, and free from particles of liquid water diffused through its substance, and if we suppose it to be kept in an atmosphere at or above 0° Centigrade, then, as soon as pressure is applied to it, pores occupied by liquid water must instantly be formed in the compressed parts, in accordance with the fundamental principle of the explanation which I have proposed-the lowering, namely, of the freezing- or melting-point, by pressure, and the cognate fact, that ice cannot exist at 0° Centigrade under a pressure exceeding that of the atmosphere. I would further wish to make it distinctly understood, that no part of the ice, even if supposed at the outset to be solid, or free from porosity, can resist being permeated by the water squeezed against it from such parts as may be directly subjected to the pressure; because, the very fact of that water being forced against any portions of the ice supposed to be solid, will instantly subject them to pressure, and so will cause melting to set in throughout their substance, thereby reducing them immediately to the porous condition. Thus it is a matter of indifference, as to whether we commence with the supposition of a mass of porous or of solid ice. Mr. Thomson then referred to an experiment made by Prof. Christie, late Secretary to the Royal Society, showing the plasticity of ice in small hand specimens, and also to more recent experiments by Prof. Tyndall to the same effect, and very interesting on account of the striking way in which they exhibit the phenomena. He also stated that another very important quality of ice was brought forward by Faraday in 1850 (see 'Athenæum,' No 1181). It was that two pieces of moist ice will consolidate into one on being laid in contact with one another, even in hot weather. The theory he had just propounded, he said, afforded a clear explanation of this fact as follows:-The two pieces of ice, on being pressed together at their point of contact, will, at that place, in virtue of the pressure, be in part liquefied and reduced in temperature, and the cold evolved in their liquefaction will cause some of the liquid film intervening between the two masses to freeze. It is thus evident, he added, that by continued pressure fragmentary masses of ice may be moulded into a continuous mass; and a sufficient reason is afforded for the reunion, found to occur in glaciers, of the fragments resulting from an ice cascade, and for the mending of the crevasses or deep fissures which result occasionally from their motion along their uneven beds. Secular Variations in Lunar and Terrestrial Motion from the influence of Tidal Action. By D. VAUGHAN, of Cincinnati, Ohio. Laplace concludes from his elaborate investigations, that the rotation of the earth is not affected by the occurrence of the tides; nor do his formulæ reveal any permanent alteration in the motion of the lunar orb which disturbs the repose of our oceans. These results, announced by so high an authority, might be received without a careful examination if the fundamental principles of natural philosophy did not discountenance the idea of an actual creation of power by lunar attraction. The tides constitute an important mechanical agent; and, could their whole force be rendered available, it would be found adequate to several hundred times the labour of the human population. So great an amount of motive power, whether appropriated to the great purposes of nature and art, or wasted in overcoming friction, cannot be produced without some expense; and my present object is to trace the change which it involves in the motions of the earth and the moon. As the extreme disproportion between the momentum of the oceanic waters and that of the planetary bodies is the chief source of error in these investigations, I shall commence by showing how the tidal action should operate, if the moon moved around the earth in an exact circle, situated in the plane of the equator, and not more than 34,000 miles in diameter. Her periodical revolution in this case would occupy nearly twelve hours, and the lunar day would be about twenty-four hours in length. The tidal action on the seas nearest to the moon would be almost twice as great as on those most distant; the former being about 5000 times, and the latter over 2500 times, the disturbing action now exerted by the moon on the watery domain. The aqueous appendage of our planet would in this case form two great moveable oceans, sustained on its opposite sides by the attraction of our satellite, and keeping pace with her movements. Without taking into consideration the oscillations of the solid part of the earth which might possibly occur in these circumstances, it is evident that there should be a general flow of the waters from west to east; and though the current may be alternately reversed in deep channels, the force propelling it in an eastern direction should always maintain the ascendency. A vast body of water, circulating around the earth from west to east, could not fail to accelerate its rotary motion; although the result would not be exhibited by the formulæ of Laplace. The moon in this case would sustain a loss of momentum to a more considerable extent. It is well known that the attraction of mountains modifies the direction of terrestrial gravity in their vicinity; and that a plumb-line on that part of the equator immediately west of the Andes would be slightly deflected to the east. In the case we have supposed, the direction of terrestrial gravity would experience a similar deflection at places in conjunction with the moon from the attraction of the excess of waters which swelled behind her. Accordingly, the lunar orb would be drawn, not directly to the earth's centre, but always to a point a little westward of it, and a constant loss of motion would be an inevitable consequence. It would be different if the earth could preserve an invariable form, for in that case its attraction on a satellite being always directed to the centre, or alternately deflected east and west of that point, the loss and gain of motion should be evenly balanced after one or many revolutions. Other investigations lead to the same conclusion. A satellite revolving just beyond the confines of our atmosphere, would alternately accelerate and retard the movements of one more distant; and physical astronomy shows that in our planetary systems a like periodicity results from the inequality of the times in which the several planets perform their revolutions. But as the tidewave rolls around the earth with the same mean angular velocity as the moon, their mutual action will not exhibit the periodicity which characterizes planetary disturbances. In the analytical solution of this problem, the equation depending on the difference of motion of the moon and the tide-wave would acquire by integration a divisor infinitely small; and this proves its secular character. If Laplace finds no such divisors, it is because all the modifications in the action of the moon on the waters of the ocean are not embraced in his investigations on the subject. Leaving the supposed case, we shall now pass to the actual condition of the agencies concerned in tidal phenomena on our globe. At her present distance the revolution of the moon occupies more time than the earth's period of rotation; and the tidal wave which has the greatest disturbing influence being always east of our satellite, must add to its velocity, while it retards that of the earth. We may remark, however, that the additional velocity imparted to the moon would give her a larger orbit, and increase the period of her revolution. Hence the orbital motion of the moon, as well as the rotary motion of the earth, sustain a loss depending on the difference of the tidal force on opposite sides of our globe, and so very insignificant, that some mil lions of years would be required to cause a reduction or one per cent. in the momenta of these vast bodies. I must however question the results of Laplace, who finds that the change in the length of the day has not amounted to the th part of a second during the last 2000 years. This conclusion is based on a comparison of ancient and modern eclipses; and the time of the earth's rotation is thus ascertained from the revolutions of the moon, making corrections for the disturbances operating on the latter body. But all the disturbing influences have not yet been taken into consideration; and as the one noticed in the present article operates on the earth and moon, we cannot regard either of these bodies as an infallible chronometer for measuring the vast ages of eternity. On the Light of Suns, Meteors, and temporary Stars. Modern science recognizes shooting-stars, fire-balls, and meteoric stones as bodies which enter our atmosphere from external space with immense velocities. From the great elevation at which these objects are fuminous, it has been inferred that their light has little or no dependence on aërial action; and indeed the presence of the air alone could not account for the greatness of the illumination which marks their approach to the earth, but ceases when they enter the dense stratum of the atmosphere. The diameter of many luminous meteors has been estimated at two or three thousand feet; and the globe of light which they exhibited must have been several million times greater than the largest meteoric stone yet found on the earth's surface. It is supposed that these brilliant exhibitions are produced by cosmical masses several hundred yards in diameter, which, in traversing the planetary regions, occasionally sweep through the verge of our atmosphere, and, after casting a few fragments on the earth, continue their course through space. But the idea that such wandering bodies should graze our planet so often, without ever striking it directly or falling to its surface, is too extravagant to be seriously entertained. It would be far more likely that, during a naval engagement, a ship should be almost touched by several thousand balls, without being ever struck by a single one. Moreover, there is not the slightest evidence that meteorites ever perform such remarkable feats of precision, or experience so many narrow escapes from a collision with the earth; for, instead of being observed departing into space, they suddenly disappear after their encounter with the air. The small amount of solid matter which falls to the ground on these occasions is justly regarded as inadequate to evolve so vast a body of light by acting on the rarefied air at great elevations; but our globe seems to be invested with an atmosphere of æther having far more wonderful properties. Astronomical investigations prove the existence of a rare medium pervading all space; and this subtle fluid cannot be wholly insensible to chemical forces, which alone could render it useful in nature's economy. Extreme rarity would, indeed, prevent it from undergoing any chemical change in the interplanetary regions; but it is compressed to a much greater density about the vast spheres by which space is tenanted. The atmospheres of this fluid enveloping the earth and the other large planets, are not sufficiently dense for chemical action, except in cases where they receive an additional pressure from meteoric bodies sweeping through them with wonderful rapidity. The evolution of light on such occasions depends, not only on the size and velocity of the falling mass, but also on the direction in which it approaches the planetary surface; and observation shows that the most brilliant meteors move very nearly parallel to the horizon. But around the sun a much stronger attractive force gives this ethereal fluid the compression necessary for a constant chemical action, and a steady development of light; while the realms of space furnish inexhaustible supplies of the luciferous matter, and impart perpetual brilliancy to the great luminary of our system. It is not possible that the self-luminous condition of the sun could be maintained by any combustible, or light-yielding matter; of which it is composed. From a comparison of the relative intensity of solar, lunar, and artificial light, as determined by Euler and Wollaston, it appears that the rays of the sun have an illuminating power equal to that of 14,000 candles, at a distance of one foot; or of 3500,000000,000000,000000,000000 candles, at a distance of 95,000,000 miles. It follows that the amount of light which flows from the solar orb could be scarcely produced by the daily combustion of 700 globes of tallow, each equal to the earth in magnitude. A sphere of combustible matter much larger than the sun itself should be consumed every ten years in maintaining its wonderful brilliancy, and its atmosphere, if pure oxygen, would be expended before a few days in supporting so great a conflagration. An illumination on so vast a scale could be kept up only by the inexhaustible magazine of æther disseminated through space, and ever ready to manifest its luciferous properties on large spheres, whose attraction renders it sufficiently dense for the play of chemical affinity. Accordingly, suns derive the power of shedding perpetual light, not from their chemical constitution, but from their immense mass and their superior attractive power. We thus obtain some definite knowledge respecting the stupendous magnitude of the fixed stars; and making due allowance for their density, we may confidently pronounce the smallest stellar body several thousand times greater than the globe we inhabit. This theory gives considerable support to the views which many astronomers maintain, on different grounds, in regard to the relative brilliancy of the stars; for it appears that, though the self-luminous occupants of space are not necessarily equal in size, they differ much less than we might anticipate from an acquaintance with the members of our planetary system. That the light of the sun is furnished, not by its solid or liquid matter, but by its luminous atmosphere, has been proved very conclusively from the observations with Arago's polarizing telescope. There is also evidence that this luciferous envelope is constantly replenished by supplies of æther from space. The sun's rotation assists in effecting this object by expelling the fluid from its equatorial regions, and thus creating a corresponding influx at its poles. A displacement by this means would evidently cause the solar atmosphere to advance constantly from its poles to its equator; and such a movement is indicated by the change in the position of the sun's spots, which, according to the observations of Peters for many years, are continually diminishing their heliocentric latitude. The progressive motion of the solar orb through space tends also to replenish its atmosphere with fresh material for the maintenance of its light; and the position of the large planets has some influence on the amount of æther which it receives from the celestial domain. The periodicity observed in the solar spots, and some changes exhibited by many variable stars, may be ascribed to an effect of this kind. But the result would be far more decided if a sun had large planets in its immediate vicinity; for the attraction of these bodies would alter the pressure on its æthereal atmosphere, and produce a corresponding variation in the development of its light. On this principle we may explain several phenomena connected with the variable stars; and I may remark, that Argelander regards many of their peculiarities as indicating, that planets revolving around some suns affect the generation of light in their photospheres. But a planet revolving in an orbit of the smallest size possible would be productive of more remarkable consequences. Sweeping through the æthereal atmosphere of the great central sphere, it would impart a sufficient degree of pressure for luciferous action; and exhibit, on a grand scale, the evolution of light which accompanies the visits of meteoric masses to the earth. From the great brilliancy of meteors which move in a horizontal direction, it is evident that a satellite revolving around a large globe, at a small distance above its surface, should be favoured with all the conditions necessary for a sublime meteoric illumination; and it is probable that some of the bright tenants of space may shine by light originating from such a cause. Indeed, the resistance of the space-pervading medium must constantly diminish the orbits of all satellites, and, after innumerable years, bring them into such a proximity with their central bodies that such grand meteoric phenomena would be almost inevitable. If space contain dark systems (as is generally believed), the central orb which presides over each of them would become luminous, when one of its planets was passing through the final state of existence. In a paper read at the last meeting of the American Association for the Advancement of Science, and published in the Proceedings' (pp. 111—113), I have shown that the stability of satellites could no longer exist if their orbits were reduced to a certain limit; and that the attraction of the primary body would render them incapable of preserving a planetary form. In like manner, a member of one of the dark systems of space, when brought too near its central orb, would be likewise doomed to suffer a dismemberment; and the fragments resulting from the mighty wreck would immediately scatter into separate orbits. Instead, therefore, of closing its planetary career as one vast meteor, the attendant should form a host of meteoric masses, and thus send forth far greater floods of light into space. But the frag ments, gradually assuming circular orbits, would ultimately form a ring similar to that around Saturn; and as this change advanced, the light should constantly decline until it ceased when the æther partook of the motion of the fragmentary host, and became almost insensible to their pressure. It is to occurrences of this kind, which must occasionally take place in the wide domains of creation, that we may ascribe the appearance of temporary stars, and in doing so, we obtain a satisfactory explanation of the various peculiarities which they exhibit. The existence, on our own sphere, of the æther which acts so important a part in the scene of celestial wonders is indicated by certain electrical phenomena. On its presence seems to depend the evolution of light attending the passage of electricity through the vacuum of an exhausted receiver, and the light of the aurora borealis appears to be evolved by electric action from the æthereal fluid, which arrives at the polar regions from space. It is only by this hypothesis that we can account for the effect of a shooting-star during an aurora, in lighting up certain parts of the vaults of heaven not previously illuminated (see Humboldt's Cosmos' on Aerolites). medium which fills space is not to be regarded as a mere impediment to planetary It thus appears that the subtle motion, but as a useful agent in the course of Nature's operations, and as indispensable to our existence as the appendages of air and water which roll around our planet. CHEMISTRY. On Ozone. By Professor T. Andrews. On the Heat of Combination of Acids and Bases. By Professor ANDREWS. On the Condition of Thames Water, as affected by London Sewage. The authors had, for a period of nearly six months, made consecutive weekly examinations, microscopical and chemical, of Thames water, taken at high and low tide from the middle of the stream at Greenwich. From their experiments it appeared that the pouring in of the contents of drains did not affect Thames water so seriously as was generally considered, inasmuch as the greater part of the sewage become destroyed by the innoxious processes of oxidation and vital development, while only a small portion underwent the process of putrefaction, properly so called. The amount of organic matter present did not appear to be any criterion of the offensiveness of the water, seeing that it existed for the most part in the state of living organisms. The authors found invariably a greater amount of organic matter in high, than in low water. On Urea as a Direct Source of Nitrogen to Vegetation. By CHARLES A. CAMERON, M.D. The author showed that nitrogen was as available as food for plants, when a constituent for urea, as in its ammoniacal combination; or, in other words, that urea, without being converted into ammonia, may be taken up into the organisms of plants, and there supply the necessary quantity of nitrogen. He described the experiments which led him to this conclusion, which were very elaborate, and were made on barley plants grown in non-nitrogenous soils, and in confined spaces supplied with air freed from ammonia. The following conclusions were deducible from the results of his experiments, viz,-1. That the perfect development of barley can take place, under |