Part 23
But, according to Faraday’s explanation, the strength and quickness of the regelation must also go hand in hand with the magnitude of the pressure employed. Helmholtz rightly dwells upon the fact that the appressed surfaces are usually not perfectly congruent--that they really touch each other in a few points only, the pressure being, therefore, concentrated. Now the effect of pressure exerted on two pieces of ice at a temperature of 0° C. is not only to lessen the thickness of the liquid film between the pieces, but also to flatten out the appressed points, and thus to spread the film over a greater space. On both theories, therefore, the strength and quickness of the regelation ought to correspond to the magnitude of the pressure.
The difficulty referred to above is thus stated by Helmholtz: ‘In the explanation given by Faraday, according to which the regelation is caused by a contact action of ice and water, I find a theoretic difficulty. By the freezing of the water a very sensible quantity of heat would be set free; and it does not appear how this is to be disposed of.’
On the part of those who accept Faraday’s explanation, the answer here would be that the free heat is diffused through the adjacent ice. But against this it will doubtless be urged that ice already at a temperature of 0° C. cannot take up more heat without liquefaction. If this be true under all circumstances, Faraday’s explanation must undoubtedly be given up. But the essence of that explanation seems to be that the interior portions of a mass of ice require a higher temperature to dissolve them than that sufficient to cause fusion at the surface. When therefore two moist surfaces of ice at the temperature 0° are pressed together, and when, in virtue of the contact action assumed by Faraday, the film of water between them is frozen, the adjacent ice (which is now in the interior, and not at the surface as at first) is in a condition to withdraw by conduction, and without prejudice to its own solidity, the small amount of heat set free. Once granting the contact action claimed by Faraday, there seems to be no difficulty in disposing of the heat rendered sensible by the freezing of the film.
When the year is advanced, and after the ice imported into London has remained a long time in store, if closely examined, parcels of liquid water will be found in the interior of the mass. I enveloped ice containing such water-parcels in tinfoil, and placed it in a freezing mixture until the liquid parcels were perfectly congealed. Removing the ice from the freezing mixture, I placed it, covered by its envelope, in a dark room, and found, after a couple of hours’ exposure to a temperature somewhat over 0° C., the frozen parcels again liquid. _The heat which fused this interior ice passed through the firmer surrounding ice without the slightest visible prejudice to its solidity._ But if the freezing temperature of the ice-parcels be 0° C., then the freezing temperature of the mass surrounding them must be higher than 0° C., which is what the explanation of Faraday requires.
In a quotation at p. 389 I have attached to the description of a precaution taken by Professor Helmholtz the query ‘why?’ He states that water freed of its air sinks, without freezing, to a temperature far below 0° C.; while when a piece of ice is in the water it cannot so sink in temperature, but is invariably deposited in the solid form at 0° C. This surely proves ice to possess a special power of solidification over water. It is needless to say that the fact is general--that a crystal of any salt placed in a saturated solution of the salt always provokes crystallisation. Applying this fact to the minute film of water enclosed between two appressed surfaces of ice, it seems to me in the highest degree probable that the contact action of Faraday will set in, that the film will freeze and cement the pieces of ice together.[41]
[41] Both Professor Helmholtz and I have since agreed to consider the physical cause of regelation an open question.
Apart from the present discussion, the following observation is perhaps worth recording: It is well known that ice during a thaw disintegrates so as to form rude prisms whose axes are at right angles to the planes of freezing. I have often observed this action on a large scale during the winters that I spent as a student on the banks of the Lahn. The manner in which these prisms are in some cases formed is extremely interesting. On close inspection, a kind of cloudiness is observed in the interior of a mass of apparently perfect ice. Looked at through a strong lens, this cloudiness appears as striæ at right angles to the planes of freezing, and when the direction of vision is across these planes the ends of the striæ are apparent. The spaces between the striæ are composed of clear unclouded ice. When duly magnified, the objects which produce the striæ turn out to be piles of minute liquid flowers, whose planes are at right angles to the direction of the striæ.
* * * * *
Since writing the above, I have been favoured with a copy of a discourse delivered by Professor De la Rive, at the opening of the forty-ninth meeting of the Société Helvétique, which assembled in 1865 at Geneva. From this admirable _résumé_ of our present knowledge regarding glaciers I make the following extract, which, together with those from the lecture of Helmholtz, will show sufficiently how the subject is now regarded by scientific men: ‘Such, gentlemen,’ says M. De la Rive, ‘is a description of the phenomena of glaciers, and it now remains to explain them, to consult observation, and deduce from it the fundamental character of the phenomena. Observation teaches us that gravity is the motive force, and that this force acts upon a solid body--ice--imparting to it a slow and continuous motion. What are we to conclude from this? That ice is a solid which possesses the property of flowing like a viscous body--a conclusion which appears very simple, but which was nevertheless announced for the first time hardly five-and-twenty years ago by one of the most distinguished philosophers of Scotland, Professor James D. Forbes. This theory, for it truly is a theory, basing itself on facts as numerous as they are well observed, enunciates the principle that ice possesses the characteristic properties which belong to plastic bodies. Although he did not directly prove it, to Professor Forbes belongs not the less the great merit of insisting on the plasticity of ice, before Faraday, in discovering the phenomenon of regelation, enabled Tyndall to prove that the plasticity was real, at least partially.
‘The experiment of Faraday is classical in connexion with our subject. It consists, as you know, in this, that if two morsels of ice be brought into contact in water, which may be even warm, they freeze together. Tyndall immediately saw the application of Faraday’s experiment to the theory of glaciers; he comprehended that, since pieces of ice could thus solder themselves together, the substance might be broken, placed in a mould, compressed, and thus compelled to take the form of the cavity which contained it. A wooden mould, for example, embraces a spherical cavity; placing in it fragments of ice and squeezing them, we obtain an ice sphere; placing this sphere in a second mould with a lenticular cavity and pressing it, we transform the sphere into a lens. In this way we can impart any form whatever to ice.
‘Such is the discovery of Tyndall, which may well be thus named, particularly in view of its consequences. For all these moulds magnified become the borders of the valley in which a glacier flows. Here the action of the hydraulic press which has served for the experiments of the laboratory is replaced by the weight of the masses of snow and ice collected on the summits, and exerting their pressure on the ice which descends into the valley. Supposing, for example, between the spherical mould and the lenticular one, a graduated series of other moulds to exist, each of which differs very little from the one which precedes and from that which follows it, and that a mass of ice could be made to pass through all these moulds in succession, the phenomenon would then become continuous. Instead of rudely breaking, the ice would be compelled to change by insensible degrees from the spherical to the lenticular form. It would thus exhibit a plasticity which might be compared to that of soft wax. But ice is only plastic under _pressure_; it is not plastic under _tension_: and this is the important point which the vague theory of plasticity was unable to explain. While a viscous body, like bitumen or honey, may be drawn out in filaments by tension, ice, far from stretching in this way, breaks like glass under this action. These points well established by Tyndall, it became easy for him to explain the mechanism of glaciers, and by the aid of an English geometer, Mr. William Hopkins, to show how the direction of the crevasses of a glacier are the necessary consequences of its motion.’
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I have quite recently had a mould constructed for me by Mr. Becker,[42] and yesterday (November 16, 1865) made with it an experiment which, on account of the ease with which it may be performed, will interest all those who care about exhibiting in a striking and instructive manner the effects of regelation. The mould is shown in fig. 12. It consists of two pieces of cast iron, A B C and D F G, slightly wedge-shaped and held together by the iron rectangle R E which is slipped over them. The inner face of A B C is shown in fig. 13. In it is hollowed out a semiring M N, with a semicylindrical passage O leading into it. The inner face of D F G is similarly hollowed out, so that when both faces are placed together, as in fig. 12, they enclose a ring 4 inches in external diameter, from M to N, and 3/4 of an inch in thickness, with the passage O, 1 inch in diameter, into which fits the polished iron plug P. At _q_ and _r_, fig. 13, are little pins which, fitting into holes corresponding to them, keep the slabs A B C and D F G from sliding over each other.
[42] I am continually indebted to this able mechanism for prompt and intelligent aid in the carrying out of my ideas.
The mould being first cooled by placing it for a short time in a mixture of ice and water, fragments of ice are stuffed into the orifice O and driven down with a hammer by means of the plug P. The bruised and broken ice separates at _x_, one portion going to the right, the other to the left. Driving the ice thus into the mould, piece after piece, it is finally filled. By removing the rectangle R E, the two halves of the mould are then separated, and a perfect ring of ice is found within. Two such rings soldered by regelation at _a_ are shown in fig. 14. It would be easy thus to construct a chain of ice. An hydraulic press may of course be employed in this experiment, but it is not necessary; with the hammer and plug beautiful rings of ice are easily obtained by the regelation of the crushed fragments.
I have now to add the description of an experiment which suggested itself to my ingenious friend Mr. Duppa, when he saw the ice-rings just referred to, and which was actually executed by him yesterday (the 16th) in the laboratory of the Royal Institution. Pouring a quantity of plaster of paris into a proper vessel, an ice-ring was laid upon the substance, an additional quantity of the cement being then poured over the ring. The plaster ‘set,’ enclosing the ring within it: the ring soon melted, leaving its perfect matrix behind. The mould was permitted to dry, and, molten lead being poured into the space previously occupied by the ice, a leaden ring was produced. Now ice can be moulded into any shape: statuettes, vases, flowers, and innumerable other ornaments can be formed from it. These enclosed in cement, in the manner suggested by Mr. Duppa, remain intact sufficiently long to enable the cement to set around them; they afterwards melt and disappear, leaving behind them perfect plaster moulds, from which casts can be taken.
V.
_CLOUDS._
From every natural fact invisible relations radiate, the apprehension of which imparts a measure of delight; and there is a store of pleasure of this kind ever at hand for those who have the capacity to turn natural appearances to account. It is pleasant, for example, to lie on one’s back upon a dry green slope and watch the clouds forming and disappearing in the blue heaven. A few days back the firmament was mottled with floating cumuli, from the fringes of which light of dazzling whiteness was reflected downwards, while the chief mass of the clouds lay in dark shadow. From the edge of one large cloud-field stretched small streamers, which, when attentively observed, were seen to disappear gradually, and finally to leave no trace upon the blue sky. On the opposite fringe of the same cloud, and beyond it, small patches of milky mist would appear, and curdle up, so as to form little cloudlets as dense apparently as the large mass beside which they were formed. The counter processes of production and consumption were evidently going on at opposite sides of the cloud. Even in the midst of the serene firmament, where a moment previously the space seemed absolutely void, white cloud-patches were formed, their sudden appearance exciting that kind of surprise which might be supposed to accompany the observation of a direct creative act.
These clouds were really the indicators of what was going on in the unseen air. Without them no motion was visible; but their appearance and disappearance proved not only the existence of motion, but also the want of homogeneity in the atmosphere. Though we did not see them, currents were mingling, possessing different temperatures and carrying different loads of invisible watery vapour. We know that clouds are not true vapour, but vapour precipitated by cold to water. We know also that the amount of water which the air can hold in the invisible state depends upon its temperature; the higher the temperature of the air, the more water will it be able to take up. But, when a portion of warm air, carrying its invisible charge, is invaded by a current of low temperature, the chilled vapour is precipitated, and a cloud is the consequence. In this way two parcels of moist air, each of which taken singly may be perfectly transparent, can produce by their mixture an opaque cloud. In the same way a body of clear humid air, when it strikes the cold summit of a mountain, may render that mountain ‘cloud-capped.’
An illustration of this process, which occurred some years ago in a Swedish ball-room, is recounted by Professor Dove. The weather was clear and cold, and the ball-room was clear and warm. A lady fainted, and air was thought necessary to her restoration. A military officer present tried to open the window, but it was frozen fast. He broke the window with his sword, the cold air entered, and _it snowed in the room_. A minute before this all was clear, the warm air sustaining a large amount of moisture in a transparent condition. When the colder air entered, the vapour was first condensed and then frozen. The admission of cool air even into our London ball-rooms produces mistiness. Mountain-chains are very effective in precipitating the vapour of our south-westerly winds; and this sometimes to such an extent as to produce totally different climates on the two sides of the same mountain-group. This is very strikingly illustrated by the observations of Dr. Lloyd on the rainfall of Ireland. Stations situated on the south-west side of a mountain-range showed a quantity of rain far in excess of that observed upon the north-east side. The winds in passing over the mountains were drained of their moisture, and were afterwards comparatively dry.
Two or three years ago I had an opportunity of witnessing a singular case of condensation at Mortain in Normandy. The tourist will perhaps remember a little chapel perched upon the highest summit in the neighbourhood. A friend and I chanced to be at this point near the hour of sunset. The air was cloudless, and the sun flooded the hillsides and valleys with golden light. We watched him as he gradually approached the crest of a hill, behind which he finally disappeared. Up to this point a sunny landscape of exquisite beauty was spread before us, the atmosphere being very transparent; but now the air seemed suddenly to curdle into mist. Five minutes after the sun had departed, a dense fog filled the valleys and drifted in fleecy masses up the sides of the hills. In an incredibly short time we found ourselves enveloped in local clouds so dense as to render our retreat a matter of some difficulty.
In this case, before the sun had disappeared the air was evidently nearly saturated with transparent vapour. But why did the vapour curdle up so suddenly when the sun departed? Was it because the withdrawal of his beams rendered the _air_ of the valleys colder, and thus caused the precipitation of the moisture diffused through the air? No. We must look for an explanation to a more direct action of the sun upon the atmospheric moisture. Let me explain. The beams which reach us from the sun are of a very composite character. A sheaf of white sunbeams is composed of an infinitude of coloured rays, the resultant effect of all upon the eye being the impression of whiteness. But though the colours, and shades of colour, which enter into the composition of a sunbeam are infinite, for the sake of convenience we divide them into seven, which are known as the prismatic colours.
The beams of the sun, however, produce _heat_ as well as light, and there are different _qualities_ of heat in the sunbeam as well as different qualities of light--nay, there are copious rays of heat in a sunbeam which give no light at all, some of which never even reach the retina at all, but are totally absorbed by the humours of the eye. Now, the same substance may permit rays of heat of a certain quality to pass freely through it, while it may effectually stop rays of heat of another quality. But in all cases the heat stopped is expended in heating the body which stops it. Now, water possesses this _selecting power_ in an eminent degree. It allows the blue rays of the solar beam to pass through it with facility, but it slightly intercepts the red rays, and absorbs with exceeding energy the _obscure rays_; and those are the precise rays which possess the most intense heating power.
We see here at once the powerful antagonism of the sun to the formation of visible fog, and we see, also, how the withdrawal of his beams may be followed by sudden condensation, even before the air has had any time to cool. As long as the solar beams swept through the valleys of Mortain, every particle of water that came in their way was reduced to transparent vapour by the heat which the particle itself absorbed; or, to speak more strictly, in the presence of this antagonism precipitation could not at all occur, and the atmosphere remained consequently clear.[43] But the moment the sun withdrew, the vapour followed, without opposition, its own tendency to condense, and its sudden curdling up was the consequence.
[43] At this time I was brooding over experiments on the absorption of radiant heat by aqueous vapour.
With regard to the _air_, its temperature may not only have remained sensibly unchanged for some time after the setting of the sun, but it may have actually become warmer through the heat set free by the act of condensation. It was not, therefore, the action of cold air upon the vapour which produced the effect, but it was the withdrawal of that solar energy which water has the power to absorb, and by absorbing to become dissipated in true vapour.
I once stood with a friend upon a mountain which commands a view of the glacier of the Rhone from its origin to its end. The day had been one of cloudless splendour, and there was something awful in the darkness of the firmament. This deepening of the blue is believed by those who know the mountains to be an indication of a humid atmosphere. The transparency, however, was wonderful. The summits of Mont Cervin and the Weisshorn stood out in clear definition, while the mighty mass of the Finsteraarhorn rose with perfect sharpness of outline close at hand. As long as the sun was high there was no trace of fog in the valleys, but as he sloped to the west the shadow of the Finsteraarhorn crept over the snow-fields at its base. A dim sea of fog began to form, which after a time rose to a considerable height, and then rolled down like a river along the flanks of the mountain. On entering the valley of the Rhone, it crossed a precipitous barrier, down which it poured like a cataract; but long before it reached the bottom it escaped from the shadow in which it had been engendered, and was hit once more by the direct beams of the sun. Its utter dissipation was the consequence, and though the billows of fog rolled on incessantly from behind, the cloud-river made no progress, but disappeared, as if by magic, where the sunbeams played upon it. The conditions were analogous to those which hold in the case of a glacier. Here the ice-river is incessantly nourished by the mountain snow: it moves down its valley, but does not advance in front. At a certain point the consumption by melting is equal to the supply, and here the glacier ceases. In the case before us the cloud-river, nourished by the incessant condensation of the atmospheric vapour, moved down its valley, but ceased at the point where the dissipating action of the sunbeams equalled the supply from the cloud-generator behind.
VI.
_KILLARNEY._
The total amount of heat which the sun sends annually to the earth is invariable, and hence if any portion of the earth’s surface during any given year be colder than ordinary, we may infer with certainty that some other portion of the surface is then warmer than ordinary. The port of Odessa owes its importance to a case of atmospheric compensation of this kind. Forty or fifty years ago, Western Europe received less than its normal amount of heat; the missing sunbeams fell upon the East, and Odessa became, to some extent, the granary from which the hungry West was fed. The position it then assumed it has since maintained. The atmosphere is the grand distributor of heat. It has its cold and warm currents--vast aërial rivers, which chill or cheer according to the proximate sources from which they are derived. In this present year 1860 the British Isles appear to lie near the common boundary of two such currents--the limit, however, shifting so as to cause both to pass over us in swift succession. Near this boundary line the atmospheric currents mingle, and the copious aqueous precipitation which we now observe is the result.