FOOTNOTES:

HEAT CONSUMED IN MOLECULAR WORK.

This heat is entirely expended ininterior work,[A]and does nothing towards augmenting the temperature; the water is at the temperature of the ice which produced it, both are 32°; and the steam is at the temperature of the water which produced it, both are 212°. The whole of the heat is consumed in producing the change of aggregation; I say "consumed," not hidden or "latent" in either the water or the steam, but absolutely non-existent as heat. The molecular forces, however, which the heat has sacrificed itself to overcome are able to reproduce it; the water in freezing and the steam in condensing give out the exact amount of heat which they consumed when the change of aggregation was in the opposite direction.

At a temperature of several degrees below its freezing point ice is much harder than at 32°. I have more than once cooled a sphere of the substance in a bath of solid carbonic acid and ether to a temperature of 100° below the freezing point. During the time of cooling the ice crackled audibly from its contraction, and afterwards it quite resisted the edge of a knife; while at 32° it may be cut or crushed with extreme facility. The cold sphere was subjected topressure; it broke with the detonation of a vitreous body, and was taken from the press a white opaque powder; which, on being subsequently raised to 32° and again compressed, was converted into a pellucid slab of ice.

ICE NEAR THE MELTING POINT.

But before the temperature of 32° is quite attained, ice gives evidence of a loosening of its crystalline texture. Indeed the unsoundness of ice at and near its melting point has been long known. Sir John Leslie, for example, states that ice at 32° isfriable; and every skater knows how rotten ice becomes before it thaws. M. Person has further shown that the latent heat of ice, that is to say, the quantity of heat necessary for its liquefaction, is not quite expressed by the quantity consumed in reducing ice at 32° to the liquid state. The heat begins to be rendered latent, or in other words the change of aggregation commences, a little before the substance reaches 32°,—a conclusion which is illustrated and confirmed by the deportment of melting ice under pressure.

ROTTEN ICE AND SOFTENED WAX.

In reference to the above result Professor Forbes writes as follows:—"I have now to refer to a fact ... established by a French experimenter, M. Person, who appears not to have had even remotely in his mind the theory of glaciers, when he announced the following facts, viz.—'That ice does not pass abruptly from the solid to the fluid state; that it begins tosoftenat a temperature of 2° Centigrade below its thawing point; that, consequently, between 28° 4' and 32° of Fahr. ice is actually passing through various degrees of plasticity within narrower limits, but in the same manner that wax, for example, softens before it melts.'" The "softening" here referred to is the "friability," of Sir J. Leslie, and what I have called a "loosening of the texture." Let us suppose the Serpentine covered by a sheet of pitch so smooth and hard as to enable a skater to glide over it; and which is afterwards gradually warmed until it begins to bend under his weight,and finally lets him through. A comparison of this deportment with that of a sheet of ice under the same circumstances enables us to decide whether ice "passes through various degrees of plasticity in the same manner as wax softens before it melts." M. Person concerned himself solely with the heat absorbed, and no doubt in both wax and ice that heat is expended in "interior work." In the one case, however, the body is so constituted that the absorbed heat is expended in rendering the substance viscous; and the question simply is, whether the heat absorbed by the ice gives its molecules a freedom of play which would entitle it also to be called viscous; whether, in short, "rotten ice" and softened wax present the same physical qualities?

FOOTNOTES:[A]I borrow this term from Professor Clausius's excellent papers on the Dynamical Theory of Heat.

[A]I borrow this term from Professor Clausius's excellent papers on the Dynamical Theory of Heat.

There is one other point in connexion with the viscous theory which claims our attention. The announcement of that theory startled scientific men, and for two or three years after its first publication it formed the subject of keen discussion. This finally subsided, and afterwards Professor Forbes drew up an elaborate paper, which was presented in three parts to the Royal Society in 1845 and 1846, and subsequently published in the 'Philosophical Transactions.'

In the concluding portion of Part III. Professor Forbes states and answers the question, "How far a glacier is to be regarded as a plastic mass?" in these words:—"Were a glacier composed of a solid crystalline cake of ice, fitted or moulded to the mountain bed which it occupies, like a lake tranquilly frozen, it would seem impossible to admit such a flexibility or yielding of parts as shouldpermit any comparison to a fluid or semifluid body, transmitting pressure horizontally, and whose parts might change their mutual positions so that one part should be pushed out whilst another remained behind. But we know, in point of fact, that a glacier is a body very differently constituted. It is clearly proved by the experiments of Agassiz and others that the glacier is not a mass of ice, but of ice and water, the latter percolating freely through the crevices of the former to all depths of the glacier; and it is a matter of ocular demonstration that these crevices, though very minute, communicate freely with one another to great distances; the water with which they are filled communicates force also to great distances, and exercises a tremendous hydrostatic pressure to move onwards in the direction in which gravity urges it, the vast porous mass of seemingly rigid ice in which it is as it were bound up."

CAPILLARY HYPOTHESIS.

"Now the water in the crevices," continues Professor Forbes, "does not constitute the glacier, but only the principal vehicle of the force which acts on it, and the slow irresistible energy with which the icy mass moves onwards from hour to hour with a continuous march, bespeaks of itself the presence of a fluid pressure. But if the ice were not in some degree ductile or plastic, this pressure could never produce any the least forward motion of the mass. The pressure in the capillaries of the glacier can only tend to separate one particle from another, and thus produce tensions and compressionswithin the body of the glacier itself, which yields, owing to its slightly ductile nature, in the direction of least resistance, retaining its continuity, or recovering it by reattachment after its parts have suffered a bruise, according to the violence of the action to which it has been exposed."

I will not pretend to say that I fully understand this passage, but, taking it and the former one together, I thinkit is clear that the water which is supposed to gorge the capillaries of the glacier is assumed to be essential to its motion. Indeed, an extreme degree of sensitiveness has been ascribed to the glacier as regards the changes of temperature by which the capillaries are affected. In three succeeding days, for example, Professor Forbes found the diurnal summer motion of a point upon the Mer de Glace to increase from 15.2 to 17.5 inches a day; a result which he says he is "persuaded" to be due to the increasing heat of the weather at the time. If, then, the glacier capillaries can be gorged so quickly as this experiment would indicate, it is fair to assume that they are emptied with corresponding speed when the supply is cut away.

TEMPERATURE AT CHAMOUNI; WINTER 1859.

The extraordinary coldness of the weather previous to the Christmas of 1859 is in the recollection of everybody: this lowness of temperature also extended to the Mer de Glace and its environs. I had last summer left with Auguste Balmat and the Abbé Vueillet thermometers with which observations were made daily during the cold weather referred to. I take the following from Balmat's register.

Date.Minimum temperatureCentigrade.December16-15°"17-20"18-161/2"19-9"20-13"21-201/2"22-41/4December23-41/2°"24-61/2"25-2"26+2"27-3"28-101/2"29-6

The temperature at the Montanvert during the above period may be assumed as generally some degrees lower, so that for a considerable period, previous to my winter observations, the portion of the Mer de Glace near the Montanvert had been exposed to a very low temperature. I reachedthe place after the weather had become warm, but during my stay there the maximum temperature did not exceed -41/2° C. Considering therefore the long drain to which the glacier had been subjected previous to the 29th of December, it is not unreasonable to infer that the capillary supply assumed by Professor Forbes must by that time have been exhausted. Notwithstanding this, the motion of the glacier at the Montanvert amounted at the end of December to half its maximum summer motion.

BALMAT'S MEASUREMENTS.

The observations of Balmat which have been published by Professor Forbes[A]also militate, as far as they go, against the idea of proportionality between the capillary supply and the motion. If the temperatures recorded apply to the Mer de Glace during the periods of observation, it would follow that from the 19th of December 1846 to the 12th of April 1847 the temperature of the air was constantly under zero Centigrade, and hence, during this time, the gorging of the capillaries, which is due to superficial melting, must have ceased. Still, throughout this entire period of depletion the motion of the glacier steadily increased from twenty-four inches to thirty-four and a half inches a day. What has been here said of the Montanvert, and of the points lower down where Balmat's measurements were made, of course applies with greater force to the higher portions of the glacier, which are withdrawn from the operation of superficial melting for a longer period, and which, nevertheless, if I understand Professor Forbes aright, have their motionleast affectedin winter. He records, for example, an observation of Mr. Bakewell's, by which the Glacier des Bossons is shown to be stationary at its end, while its upper portions are moving at the rate of a foot a day. This surely indicates that, at those places where the glacier is longest cut off from superficial supply, the motion is least reduced, which would be a most strangeresult if the motion depended, as affirmed, upon the gorging of the capillaries.

BAKEWELL'S OBSERVATIONS.

The perusal of the conclusion of Professor Forbes's last volume shows me that a thought similar to that expressed above occurred to Mr. Bakewell also. Speaking of a shallow glacier which moved when the alleged temperature was so enormously below the freezing point that Professor Forbes regards the observation as open to question (in which I agree with him), Mr. Bakewell asks, "Is it possible that infiltrated water can have any action whatever under such circumstances?" The reply of Professor Forbes contains these words:—"I have nowhere affirmed the presence of liquid water to be asine quâ nonto the plastic motion of glaciers." This statement, I confess, took me by surprise, which was not diminished by further reading. Speaking of the influence of temperature on the motion of the Mer de Glace, Professor Forbes says, the glacier "took no real start until the frost had given way, and the tumultuous course of the Arveiron showed that its veins were again filled with the circulating medium to which the glacier, like the organic frame, owes its moving energy."[B]And again:—"It is this fragility precisely which, yielding to the hydrostatic pressure of the unfrozen water contained in the countless capillaries of the glacier, produces the crushing action which shoves the ice over its neighbour particles."[C]

HUXLEY'S OBSERVATIONS.

After the perusal of the foregoing paragraphs the reader will probably be less interested in the question as to whether the assumed capillaries exist at all in the glacier. According to Mr. Huxley's observations, they do not.[D]During the summer of 1857 he carefully experimented with coloured liquids on the Mer de Glace and its tributaries, and in no case was he able to discover these fissuresin the sound unweathered ice. I have myself seen the red liquid resting in an auger-hole, where it had lain for an hour without diffusing itself in any sensible degree. This cavity intersected both the white ice and the blue veins of the glacier; and Mr. Huxley, in my presence, cut away the ice until the walls of the cavity became extremely thin, still no trace of liquid passed through them. Experiments were also made upon the higher portions of the Mer de Glace, and also on the Glacier du Géant, with the same result. Thus the very existence of these capillaries is rendered so questionable, that no theory of glacier-motion which invokes their aid could be considered satisfactory.

FOOTNOTES:[A]'Occ. Pap.,' p. 224.[B]'Phil. Trans.,' 1846, p. 137, and 'Occ. Pap.,' p. 138.[C]'Occ. Pap.,' p. 47.[D]'Phil. Mag.,' 1857, vol. xiv., p. 241.

[A]'Occ. Pap.,' p. 224.

[B]'Phil. Trans.,' 1846, p. 137, and 'Occ. Pap.,' p. 138.

[C]'Occ. Pap.,' p. 47.

[D]'Phil. Mag.,' 1857, vol. xiv., p. 241.

In the 'Transactions' of the Royal Society of Edinburgh for 1849 is published a very interesting paper by Prof. James Thomson of Queen's College, Belfast, wherein he deduces, as a consequence of a principle announced by the French philosopher Carnot, that water, when subjected to pressure, requires a greater cold to freeze it than when the pressure is removed. He inferred that the lowering of the freezing point for every atmosphere of pressure amounted to .0075 of a degree Centigrade. This deduction was afterwards submitted to the test of experiment by his distinguished brother Prof. Wm. Thomson, and proved correct. On the fact thus established is founded Mr. James Thomson's theory of the "Plasticity of Ice as manifested in Glaciers."

STATEMENT OF THEORY.

The theory is this:—Certain portions of the glacier are supposed first to be subjected to pressure. This pressure liquefies the ice, the water thus produced being squeezed through the glacier in the direction in which it can most easily escape. But cold has been evolved by the act of liquefaction, and, when the water has been relieved from the pressure, it freezes in a new position. The pressure being thus abolished at the place where it was first applied, new portions of the ice are subjected to the force; these in their turn liquefy, the water is dispersed as before, and re-frozen in some other place. To the succession of processes here assumed Mr. Thomson ascribes the changes of form observed in glaciers.

This theory was first communicated to the Royal Society through the author's brother, Prof. William Thomson, and is printed in the 'Proceedings' of the Society for May, 1857. It was afterwards communicated to the British Associationin Dublin, in whose 'Reports' it is further published; and again it was communicated to the Belfast Literary and Philosophical Society, in whose 'Proceedings' it also finds a place.

On the 24th of November, 1859, Mr. James Thomson communicated to the Royal Society, through his brother, a second paper, in which he again draws attention to his theory. He offers it in substitution for my views as the best argument that he can adduce against them; he also controverts the explanations of regelation propounded by Prof. James D. Forbes and Prof. Faraday, believing that his own theory explains all the facts so well as to leave room for no other.

DIFFICULTIES OF THEORY.

But the passage in this paper which demands my chief attention is the following:—"Prof. Tyndall (writes Mr. Thomson), in papers and lectures subsequent to the publication of this theory, appears to adopt it to some extent, and to endeavour to make its principles co-operate with the views he had previously founded on Mr. Faraday's fact of regelation." I may say that Mr. Thomson's main thought was familiar to me long before his first communication on the plasticity of ice appeared; but it had little influence upon my convictions. Were the above passage correct, I should deserve censure for neglecting to express my obligations far more explicitly than I have hitherto done; but I confess that even now I do not understand the essential point of Mr. Thomson's theory,—that is to say, its application to the phenomena of glacier motion. Indeed, it was the obscurity in my mind in connexion with this point, and the hope that time might enable me to seize more clearly upon his meaning, which prevented me from giving that prominence to the theory of Mr. Thomson which, for aught I know, it may well deserve. I will here briefly state one or two of my difficulties, and shall feel very grateful to have them removed.

IMPROBABLE DEDUCTION.

Let us fix our attention on a vertical slice of ice transverse to the glacier, and to which the pressure is applied perpendicular to its surfaces. The ice liquefies, and, supposing the means of escape offered to the compressed water to be equal all round, it is plain that there will be as great a tendency to squeeze the water upwards as downwards; for the mere tendency to flow down by its own gravity becomes, in comparison to the forces here acting on the water, a vanishing quantity. But the fact is, that the ice above the slice is more permeable than that below it; for, as we descend a glacier, the ice becomes more compact. Hence the greater part of the dispersed water will be refrozen on that side of the slice which is turned towards the origin of the glacier; and the consequence is, that, according to Mr. Thomson's principle, the glacier ought to move up hill instead of down.

I would invite Mr. Thomson to imagine himself and me together upon the ice, desirous of examining this question in a philosophic spirit; and that we have taken our places beside a stake driven into the ice, and descending with the glacier. We watch the ice surrounding the stake, and find that every speck of dirt upon it retains its position; there is no liquefaction of the ice that bears the dirt, and consequently it rests on the glacier undisturbed. After twelve hours we find the stake fifteen inches distant from its first position: I would ask Mr. Thomson how did it get there? Or let us fix our attention on those six stakes which M. Agassiz drove into the glacier of the Aar in 1841, and found erect in 1842 at some hundreds of feet from their first position:—how did they get there? How, in fine, does the end of a glacier become its end? Has it been liquefied and re-frozen? If not, it must have beenpusheddown by the very forces which Mr. Thomson invokes to produce his liquefaction. Both the liquefaction, as far as it exists, and the motion, are products of the same cause.In short, this theory, as it presents itself to my mind, is so powerless to account for the simplest fact of glacier-motion, that I feel disposed to continue to doubt my own competence to understand it rather than ascribe to Mr. Thomson an hypothesis apparently so irrelevant to the facts which it professes to explain.

Another difficulty is the following:—Mr. Thomson will have seen that I have recorded certain winter measurements made on the Mer de Glace, and that these measurements show not only that the ice moves at that period of the year, but that it exhibits those characteristics of motion from which its plasticity has been inferred; the velocity of the central portions of the glacier being in round numbers double the velocity of those near the sides. Had there been any necessity for it, this ratio might have been augmented by placing the side-stakes closer to the walls of the glacier. Considering the extreme coldness of the weather which preceded these measurements, it is a moderate estimate to set down the temperature of the ice in which my stakes were fixed at 5° Cent. below zero.

REQUISITE PRESSURE CALCULATED.

Let us now endeavour to estimate the pressure existing at the portion of the glacier where these measurements were made. The height of the Montanvert above the sea-level is, according to Prof. Forbes, 6300 feet; that of the Col du Géant, which is the summit of the principal tributary of the Mer de Glace, is 11,146 feet: deducting the former from the latter, we find the height of the Col du Géant above the Montanvert to be 4846 feet.

Now, according to Mr. Thomson's theory and his brother's experiments, the melting point of ice is lowered .0075° Centigrade for every atmosphere of pressure; and one atmosphere being equivalent to the pressure of about thirty-three feet of water, we shall not be over the truth if we take the height of an equivalent column of glacier-ice, of a compactness the mean of those which it exhibits uponthe Col du Géant and at the Montanvert respectively, at forty feet. The compactness of glacier ice is, of course, affected by the air-bubbles contained within it.

ACTUAL PRESSURE INSUFFICIENT.

If, then, the pressure of forty feet of ice lower the melting point .0075° Centigrade, it follows that the pressure of a column 4846 feet high will lower it nine-tenths of a degree Centigrade. Supposing, then, theunimpeded thrust of the whole glacier, from the Col du Géant downwards, to be exerted on the ice at the Montanvert; or, in other words, supposing the bed of the glacier to be absolutely smooth and every trace of friction abolished, the utmost the pressure thus obtained could perform would be to lower the melting point of the Montanvert ice by the quantity above mentioned. Taking into account the actual state of things, the friction of the glacier against its sides and bed, the opposition which the three tributaries encounter in the neck of the valley at Trélaporte, the resistance encountered in the sinuous valley through which it passes; and finally, bearing in mind the comparatively short length of the glacier, which has to bear the thrust, and oppose the latter by its own friction merely;—I think it will appear evident that the ice at the Montanvert cannot possibly have its melting point lowered by pressure more than a small fraction of a degree.

The ice in which my stakes were fixed being -5° Centigrade, according to Mr. Thomson's calculation and his brother's experiments, it would require 667 atmospheres of pressure to liquefy it; in other words, it would require the unimpeded pressure of a column of glacier-ice 26,680 feet high. Did Mont Blanc rise to two and a half times its present height above the Montanvert, and were the latter place connected with the summit of the mountain by a continuous glacier with its bed absolutely smooth, the pressure at the Montanvert would be rather under that necessaryto liquefy the ice on which my winter observations were made.

MEASUREMENTS APPLY TO SURFACE.

If it be urged that, though the temperature near the surface may be several degrees below the freezing point, the great body of the glacier does not share this temperature, but is, in all probability, near to 32°, my reply is simple. I did not measure the motion of the ice in the body of the glacier; nobody ever did; my measurements refer to the ice at and near the surface, and it is this ice which showed the plastic deportment which the measurements reveal.

Such, then, are some of the considerations which prevent me from accepting the theory of Mr. Thomson, and I trust they will acquit me of all desire, to make his theory co-operate with my views. I am, however, far from considering his deduction the less important because of its failing to account for the phenomena of glacier motion.

POSSIBLE MOULDING OF ICE.

Broadly considered, two classes of facts are presented to the glacier-observer; the one suggestive of viscosity, and the other of the reverse. The former are seen wherepressurecomes into play, the latter wheretensionis operative. By pressure ice can be moulded to any shape, while the same ice snaps sharply asunder if subjected to tension. Were the result worth the labour, ice might be moulded into vases or statuettes, bent into spiral bars, and, I doubt not, by the proper application of pressure, aropeof ice might be formed and coiled into aknot. But not one of these experiments, though they might be a thousandfold more striking than any ever made upon a glacier, would in the least demonstrate that ice is really a viscous body.

Fig. 30. Moulds used in experiments with ice.

I have here stated what I believe to be feasible. Let me now refer to the experiments which have been actually made in illustration of this point. Two pieces of seasoned box-wood had corresponding cavities hollowed in them, so that, when one was placed upon the other, a lenticularspace was enclosed.aandb,Fig. 30, represent the pieces of box-wood with the cavities in plan:crepresents their section when they are placed upon each other.

ACTUAL MOULDING OF ICE.

Asphereof ice rather more than sufficient to fill the lenticular space was placed between the pieces of wood and subjected to the action of a small hydraulic press. The ice was crushed, but the crushed fragments soon reattached themselves, and, in a few seconds, a lens of compact ice was taken from the mould.

Fig. 31. Moulds used in experiments with ice.

This lens was placed in a cylindrical cavity hollowed out in another piece of box-wood, and represented atc,Fig. 31; and a flat piece of the wood was placed over the lens as a cover, as atd. On subjecting the whole to pressure, the lens broke, as the sphere had done, but the crushed mass soon re-established its continuity, and in less than half a minute a compact cake of ice was taken from the mould.

Fig. 32. Moulds used in experiments with ice.

In the following experiment the ice was subjected to a still severer test:—A hemispherical cavity was formed in one block of box-wood, and upon a second block a hemispherical protuberance was turned, smaller than the cavity, so that, when the latter was placed in the former, a space of a quarter of an inch existed between the two.Fig. 32represents a section of the two pieces of box-wood; the brass pinsa,b, fixed in the slabg h, and entering suitable apertures in the mouldi k, being intended to keep the two surfaces concentric. A lump of ice being placed in the cavity, the protuberance was brought down upon it, and the mould subjected to hydraulic pressure: after a short interval the ice was taken from the mould as asmooth compactcup, its crushed particles having reunited, and established their continuity.

ICE MOULDED TO CUPS AND RINGS.

Fig. 33. Moulds used in experiments with ice.

To make these results more applicable to the bending of glacier-ice, the following experiments were made:—A block of box-wood,m,Fig. 33, 4 inches long, 3 wide, and 3 deep, had its upper surface slightly curved, and a groove an inch wide, and about an inch deep, worked into it. A corresponding plate was prepared, having its under surface part of a convex cylinder, of the same curvature as the concave surface of the former piece. When the one slab was placed upon the other, they presented the appearance represented in section atn. A straight prism of ice 4 inches long, an inch wide, and a little more than an inch in depth, was placed in the groove; the upper slab was placed upon it, and the whole was subjected to the hydraulic press. The prism broke, but, the quantity of ice being rather more than sufficient to fill the groove, the pressure soon brought the fragments together and re-established the continuity of the ice. After a few seconds it was taken from the mould a bent bar of ice. This bar was afterwards passed through three other moulds of gradually augmenting curvature, and was taken from the last of them asemi-ringof compact ice.

The ice, in changing its form from that of one mould to that of another, was in every instance broken and crushed by the pressure; but suppose that instead of three moulds three thousand had been used; or, better still, suppose the curvature of a single mould to change by extremely slow degrees; the ice would then so gradually change its form that no rude rupture would be apparent. Practically the ice would behave as aplasticsubstance; and indeedthis plasticity has been contended for by M. Agassiz, in opposition to the idea of viscosity. As already stated, the ice, bruised, and flattened, and bent in the above experiments, was incapable of being sensibly stretched; it was plastic to pressure but not to tension.

A quantity of water was always squeezed out of the crushed ice in the above experiments, and the bruised fragments were intermixed with this and with air. Minute quantities of both remained in the moulded ice, and thus rendered it in some degree turbid. Its character, however, as to continuity may be inferred from the fact that the ice-cup, moulded as described, held water without the slightest visible leakage.

SOFTNESS OF ICE DEFINED.

Ice at 32° may, as already stated, be crushed with extreme facility, and glacier-ice with still more readiness than lake-ice: it may also be scraped with a knife with even greater facility than some kinds of chalk. In comparison with ice at 100° below the freezing point, it might be popularly calledsoft. But its softness is not that of paste, or wax, or treacle, or lava, or honey, or tar. It is the softness of calcareous spar in comparison with that of rock-crystal; and although the latter is incomparably harder than the former, I think it will be conceded that the term viscous would be equally inapplicable to both. My object here is clearly to define terms, and not permit physical error to lurk beneath them. How far this ice, with a softness thus defined, when subjected to the gradual pressures exerted in a glacier, is bruised and broken, and how far the motion of its parts may approach to that of a truly viscous body under pressure, I do not know. The critical point here is that the ice changes its form, and preserves its continuity, during its motion, in virtue ofexternalforce.PRESSURE AND TENSION.It remains continuous whilst it moves, because its particles are kept in juxtaposition by pressure, and when this external prop is removed, and the ice, subjected totension, has to depend solely upon the mobility of its own particles to preserve its continuity, the analogy with a viscous body instantly breaks down.[A]

FOOTNOTES:[A]"Imagine," writes Professor Forbes, "a long narrow trough or canal, stopped at both ends and filled to a considerable depth with treacle, honey, tar, or any such viscid fluid. Imagine one end of the trough to give way, the bottom still remaining horizontal: if the friction of the fluid against the bottom be greater than the friction against its own particles, the upper strata will roll over the lower ones, and protrude in a convex slope, which will be propagated backwards towards the other or closed end of the trough. Had the matter been quite fluid the whole would have run out, and spread itself on a level: as it is, it assumes precisely the conditions which we suppose to exist in a glacier." This is perfectly definite, and my equally definite opinion is that no glacier ever exhibited the mechanical effects implied by this experiment.

[A]"Imagine," writes Professor Forbes, "a long narrow trough or canal, stopped at both ends and filled to a considerable depth with treacle, honey, tar, or any such viscid fluid. Imagine one end of the trough to give way, the bottom still remaining horizontal: if the friction of the fluid against the bottom be greater than the friction against its own particles, the upper strata will roll over the lower ones, and protrude in a convex slope, which will be propagated backwards towards the other or closed end of the trough. Had the matter been quite fluid the whole would have run out, and spread itself on a level: as it is, it assumes precisely the conditions which we suppose to exist in a glacier." This is perfectly definite, and my equally definite opinion is that no glacier ever exhibited the mechanical effects implied by this experiment.

FARADAY'S FIRST EXPERIMENT.

I was led to the foregoing results by reflecting on an experiment performed by Mr. Faraday, at a Friday evening meeting of the Royal Institution, on the 7th of June, 1850, and described in the 'Athenæum' and 'Literary Gazette' for the same month. Mr. Faraday then showed that when two pieces of ice, with moistened surfaces, were placed in contact, they became cemented together by the freezing of the film of water between them, while, when the ice was below 32° Fahr., and thereforedry, no effect of the kind could be produced. The freezing was also found to take place under water; and indeed it occurs even when the water in which the ice is plunged is as hot as the hand can bear.

A generalisation from this interesting fact led me to conclude that a bruised mass of ice, if closely confined, must re-cement itself when its particles are brought into contact by pressure; in fact, the whole of the experiments above recorded immediately suggested themselves to my mind as natural deductions from the principle established by Faraday. A rough preliminary experiment assured me that the deductions would stand testing; and the construction of the box-wood moulds was the consequence. We could doubtless mould many solid substances to any extent by suitable pressure, breaking the attachment of their particles, and re-establishing a certain continuity by the mere force of cohesion. With such substances, to which we should never think of applying the term viscous, we might also imitate the changes of form to which glaciers are subject: but, superadded to the mere cohesion which here comes into play, we have, in the case ofice, the actual regelation of the severed surfaces, and consequently a more perfect solid. In theIntroductionto this book I have referred to the production of slaty cleavage by pressure; and at a future page I hope to show that the lamination of the ice of glaciers is due to the same cause; but, as justly observed by Mr. John Ball, there is no tendency to cleave in thesoundice of glaciers; in fact, this tendency is obliterated by the perfect regelation of the severed surfaces.

RECENT EXPERIMENTS OF FARADAY.

Mr. Faraday has recently placed pieces of ice, in water, under the strain of forces tending to pull them apart. When two such pieces touch at a single point they adhere and move together as a rigid piece; but a little lateral force carefully applied breaks up this union with a crackling noise, and a new adhesion occurs which holds the pieces together in opposition to the force which tends to divide them. Mr. James Thomson had referred regelation to the cold produced by the liquefaction of the pressed ice; but in the above experiment all pressure is not only taken away, but is replaced by tension. Mr. Thomson also conceives that, when pieces of ice are simply placed together without intentional pressure, the capillary attraction brings the pressure of the atmosphere into play; but Mr. Faraday finds that regelation takes placein vacuo. A true viscidity on the part of ice Mr. Faraday never has observed, and he considers that his recent experiments support the view originally propounded by himself, namely, that a particle of water on a surface of ice becomes solid when placed between two surfaces, because of the increased influence due to their joint action.

HOW CRYSTALS ARE "NURSED."

In theIntroductionto this book I have briefly referred to the force of crystallization. To permit this force to exercise its full influence, it must have free and unimpeded action; a crystal, for instance, to be properly built, ought to be suspended in the middle of the crystallizing solution, so that the little architects can work all round it; or if placed upon the bottom of a vessel, it ought to be frequently turned, so that all its facets may be successively subjected to the building process. In this way crystals can benursedto an enormous size. But where other forces mingle with that of crystallization, this harmony of action is destroyed; the figures, for example, that we see upon a glass window, on a frosty morning, are due to an action compounded of the pure crystalline force and the cohesion of the liquid to the window-pane. A more regular effect is obtained when the freezing particles are suspended in still air, and here they build themselves into those wonderful figures which Dr. Scoresby has observed in the Polar Regions, Mr. Glaisher at Greenwich, and I myself on the summit of Monte Rosa and elsewhere.

Not only however in air, but in water also, figures of great beauty are sometimes formed. Harrison's excellent machine for the production of artificial ice is, I suppose, now well known; the freezing being effected by carrying brine, which had been cooled by the evaporation of ether, round a series of flat tin vessels containing water. The latter gradually freezes, and, on watching those vessels while the action was proceeding very slowly, I have seen little six-rayed stars of thin ice forming, and rising to thesurface of the liquid. I believe the fact was never before observed, but it would be interesting to follow it up, and to develop experimentally this most interesting case of crystallization.

DISSECTION OF ICE BY SUNBEAM.

The surface of a freezing lake presents to the eye of the observer nothing which could lead him to suppose that a similar molecular architecture is going on there. Still the particles are undoubtedly related to each other in this way; they are arranged together on this starry type. And not only is this the case at the surface, but the largest blocks of ice which reach us from Norway and the Wenham Lake are wholly built up in this way. We can reveal the internal constitution of these masses by a reverse process to that which formed them; we can send an agent into the interior of a mass of ice which shall take down the atoms which the crystallizing forces had set up. This agent is a solar beam; with which it first occurred to me to make this simple experiment in the autumn of 1857. I placed a large converging lens in the sunbeams passing through a room, and observed the place where the rays were brought to a focus behind the lens; then shading the lens, I placed a clear cube of ice so that the point of convergence of the rays might fall within it. On removing the screen from the lens, a cone of sunlight went through the cube, and along the course of the cone the ice became studded with lustrous spots, evidently formed by the beam, as if minute reflectors had been suddenly established within the mass, from which the light flashed when it met them. On examining the cube afterwards I found that each of these spots was surrounded by a liquid flower of six petals; such flowers were distributed in hundreds through the ice, being usually clear and detached from each other, but sometimes crowded together into liquid bouquets, through which, however, the six-starred element could be plainly traced. At first the edges of theleaves were unbroken curves, but when the flowers expanded under a long-continued action, the edges became serrated. When the ice was held at a suitable angle to the solar beams, these liquid blossoms, with their central spots shining more intensely than burnished silver, presented an exhibition of beauty not easily described. I have given a sketch of their appearance inFig. 34.

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