Popular Science Monthly/Volume 5/August 1874/The Physics of Ice
|THE PHYSICS OF ICE.|
THREE-QUARTERS of a century ago a cargo of ice was obtained from a pond near the junction of Broadway and Canal Street, in New York, and sent to Charleston, South Carolina, in a vessel chartered by a gentleman of that city. But it was in 1805-'6 that Frederick Tudor, of Boston, inaugurated and laid the foundation of the now immense ice-trade of the United States by shipping, as a mercantile adventure, a cargo to St. Pierre, on the island of Martinique. This cargo, with several subsequent shipments to other West Indian ports, was largely unprofitable. The people to whom it was sent, unfamiliar with its use, knew little of its value.
In 1833, Mr. Tudor sent in the ship Tuscany the first cargo of ice from this country to Calcutta, and thus began the ice-trade of the United States with seaports of India. Several years after that event, the Hon. Edward Everett, then our minister to England, met in London a wealthy and eminent Hindoo, who cordially thanked the American people whom he represented for the great service they had done to his countrymen in shipping cargoes of ice to India. It is obvious that, in our zone of alternate heat and cold, ice is one of Winter's great benefactions. In the healthy preservation of food it is indispensable during summer's heat. In regions where little ice forms, the mountain-snows are economized. Some years since the supply on Mount Etna gave out, and a glacier buried beneath sand and lava was found, and worked as an ice-quarry to supply the necessities of the people. Ice of good quality is now produced by artificial methods, but it is our purpose to develop in this paper some of the physical properties and phenomena of ice, rather than its economic value.
Ice is simply water in a solid state. In ordinary conditions it begins to form at a temperature of 32° of Fahrenheit's thermometer, and this is the well-known freezing-point. Below it the molecules of water become fixed in the grasp of molecular force; above it they are separated by heat, and fall asunder, forming liquid water. But to this law are exceptions, in which water may be cooled many degrees below 32°, still remaining liquid. In glass vessels exposed in open air, water kept perfectly still has been reduced in temperature 15° below freezing, and in a vacuum much lower than this. In this condition there is a "tendency to freeze which is kept in check only by the difficulty of making a commencement," and the process begins by the slightest jar of the water. Fine particles of vapor, or mist, and water in fine capillary tubes, may remain unfrozen 20° or more below the freezing-point. Water thus cooled rises in temperature the instant crystallization begins, by liberation of its heat, and at 32° becomes solid. But, ice may be chilled to any attainable degree of cold. Prof. Tyndall reduced its temperature 100°, during which process it shrank in volume and became intensely hard. From the blow of a hammer it broke with a vitreous ring.
We often witness the fact that water will not freeze if in rapid motion, although it be much colder than the freezing-point; but in this case will freeze at the bottom where its motion is retarded, forming what is called "ground, or anchor ice." The sandy bottom beneath swiftly-flowing streams is sometimes frozen solid by radiation of its heat to the cold water flowing over it. But no ice will form at the bottom of a pond or lake if the water be at rest; it then forms upon the surface only. The particles of water, as they become chilled to near the freezing-point, expand, become lighter, and continually rise to the surface, where they solidify, forming a roof of ice. This phenomenon opens a most interesting chapter of physical science, and we will presently recur to it.
The freezing-point of water may be changed by pressure, that is, water under pressure will not solidify at a temperature of 32°; nor is it known how great a degree of cold it can resist if a corresponding degree of pressure be brought to bear upon it. The lowering of the freezing-point of water by pressure is one-seventieth of a degree Fahr. for a whole atmosphere. Under a pressure of several thousand atmospheres, ice has been liquefied at or near the temperature of zero; so
that the freezing-point was zero, instead of 32°. The tendency of pressure is, therefore, to keep water liquid, and to render it so after being frozen. The effect of pressure on a cube of transparent ice is well shown in Fig. 1. It is no longer transparent, but is traversed by hazy lines which come into view as the strain is applied. These hazy lines are portions which have become liquid. We have seen that freezing is a process in which water expands in volume. This implies change in its molecular structure, or that the molecules assume new positions. Perhaps the wonderful movement of particles around the poles of a magnet, illustrated by Fig. 2, may suggest the nature of the interior movements which occur in the crystallization of water. Certain it is that the molecules recede from each other and occupy more space than when they lay compacted in the liquid condition.
Water expands, in freezing, with a force that is practically irresistible, its increase in volume being about ten per cent. Flasks of copper and iron are broken by it. Rocks are split asunder and disintegrated, and from this cause the freezing of water plays an important part in geological changes. A bomb-shell filled with water and closed by an iron stopper was exposed to frost; in a little time the stopper
was driven out to a distance of several hundred feet, and a mass of ice protruded. In another case the shell burst, and a sheet of ice expanded around the crevice. These tremendous mechanical effects are shown in Fig. 3. In these and similar cases the water may not have been solidified, the tendency to freeze being held in check by pressure, but occurred the instant the pressure diminished.
The expansion of water at freezing is vastly important in its relations to life. Without this property, water in high latitudes would become permanently solid, and the aspect of Nature be one of lifeless desolation. Water, in cooling from a high temperature, contracts in volume, and the cooled particles sink until the mass is reduced throughout to a temperature about seven degrees above the freezing-point, when an important change takes place. Contraction of volume ceases, and expansion begins. The chilled particles remain at the surface from their lightness, and there solidify, while the water beneath, in its deeper portions, may be 7° warmer than the point of freezing. By this means a temperature of water in lakes is maintained adequate to the wants of life.
The structure of ice is crystalline, and the fundamental pattern of the crystals is six-rayed stars. But it is only in entire freedom of molecular motion that crystals attain perfection of symmetry. They form upon the surface of water, when the cold is severe, with great rapidity; but are modified in their arrangement or aggregation the instant the first crust is produced. The additions to the thickness of the ice are always at its underside, and the result is a prismatic form.
the prisms growing downward. These prisms are hexagonal in shape, and are so joined at their sides as to present an apparently homogeneous structure. That such is not the case, however, will be seen when we speak of the decay of ice. That the internal structure of ice is upon the stellate type is shown when a small cube of it is dissected by a beam of light. By the heat rays of the beam, the ice is decrystallized—its molecular architecture is taken down, and the result appears in stellate figures of exquisite beauty upon the screen. These figures are areas in the ice which have been liquefied by the beam, which thus throws on the screen an "image of its own work." In Fig. 5 we have magnified pictures of the crystalline structure of ice. They seem shadows of living objects, rivaling fern-leaf and blossom in delicacy and fairy-like beauty. Prof. Tyndall, the great classic on this subject, says, with reason, that, "in the estimation of science, ice bears the same relation to glass that an oratorio of Handel does to the cries of a market-place. The ice is order; the glass is confusion.... Nature lays her beams in music." In each complete flower is a little
disk. These are vacuous spots, caused by diminution of volume as the ice is converted to water at each point where a flower is produced.
Ice-structure is not impaired by the luminous rays of a beam, to which it is transparent, but by the dark or heat rays, to which it is opaque. These, arrested in their transition through it, expend their energy in taking asunder the molecules of which it is constructed. They become "our working anatomist," and reveal the interior and otherwise hidden form of ice architecture, shown in the ice-flowers of the figure. Had the heat-rays which destroyed the ice fallen into water, it would have been heated; but, falling into the cube of ice, and melting a portion of it, no change in temperature occurred. Ice melts at 32°, and the temperature of the water in which it floats is kept steadily at thai point until all of it has disappeared. The ices we consume, and the iced drinks for which we thirst in summer, are, at the freezing temperature, 68° colder than the internal organs with which they are brought into contact.
Ice, like other solids, may be cooled and warmed. That which Tyndall chilled 100° could be warmed steadily to the temperature of 32°, and a thermometer would indicate the change; but at that point the process is interrupted—the structure falls into pieces, and not until the mass is entirely liquid can the warming be resumed. From that point, however, it goes on until, at a temperature of 212°, it again ceases, and the molecules of water are separated into vapor.
But, in melting the ice by the dark or heat rays of the beam of light, a great quantity of heat was consumed, not in raising temperature, but in undoing what molecular force had done. To simply melt a pound of ice requires 142° of heat, that is, an amount which would raise the temperature of a pound of water 142°. Now, this is the equivalent of the molecular force exerted in solidifying the water, and the mechanical value of the two forces is the same. Expressed in figures, it is equal to lifting the same pound of ice 110,000 feet high. The mere melting of 20 pounds of ice, a quantity received daily by many families, is equivalent, in mechanical force, to lifting nearly 1,000 tons' weight a foot high, or to lifting two persons weighing 300 pounds 1,000 feet higher than the summit of Mount Washington. We may thus realize the enormous display of energy along the line where heat and molecular force contend for the mastery.
The transition of water to ice, and of ice to water, produces important changes in the temperature of surrounding objects. We are often made painfully sensible of the chilling influence of the atmosphere when its heat is rapidly abstracted in the melting of large masses of ice and snow. But the reverse of this takes place in freezing. The crystallization of water is attended with an elevation of temperature. The heat which vapor carries with it in its aërial journeys is liberated when those vapors are transformed into flakes of snow. The expression we often hear when a storm in winter is imminent, that "the cold is too great for snowing," is true enough. The air is made warmer when snow-flakes begin to form, and the temperature is higher than it would otherwise be while snowing continues. In this way the formation of ice and snow modifies and softens the temperature of arctic winters; and the blossoms which open with the spring-time are not more significant of milder airs than are those which are born of frost and vapor, and expand their petals to the winter's tempest. Snowflakes are stellate in form; the molecules of vapor in crystallizing aggregate in six-rayed figures, but in endless diversity of patterns. Captain Scoresby figured 96 of these, beautiful illustrations of which are shown in the plate. And Nature is profuse of these "frozen flowers." On mountains, and amid solitudes of the North unseen by man, she scatters them as she does those which waste their perfume in the desert. The delicate lace-like figures which follow the touch of frost on the window-pane are of the stellate type, but, being modified by disturbing influences, develop into gorgeous patterns.
Ice, when decayed, is weakened and becomes soft throughout, scarcely more compacted than snow. By rains and mild weather of spring, ice on our Northern lakes is thus impaired. In this condition it is said to be "honey-combed;" and, while yet many inches in thickness, and apparently solid, is unsafe to travel over. The foot of a horse will pass through it, displacing merely the portions beneath, and without fracture of the surrounding parts. This arises from the prismatic structure already noticed; and it is along the lines of adhesion of the prisms that the ice first yields to the invasion of heat. When thus weakened, it will sometimes disappear from the surface of a lake by a few hours of heavy storm, or, if any portion remain, it will be in the form of crystals, thoroughly permeated by water. So rapidly has it vanished in many instances from lakes, that its sinking was insisted on, but it is now known that it disintegrates and disappears by internal liquefaction.
A most interesting and important property of ice remains to be noticed. We refer to that by which it may be moulded into almost any form by pressure. Cubes of solid ice have been pressed into balls, cups, rings, and other shapes, showing its extraordinary plasticity. At a temperature of 32°, ice is by no means a rigid substance, but readily yields to pressure. Placed in the cavity of a mould (Fig. 6), it is broken into innumerable fragments as pressure is applied. It has
been shown, however, that, unless the crushing be sudden, the ice is not reduced to a granular or powdery mass, but maintains its cohesion while it undergoes change of form. During the process a portion of the ice becomes liquefied, and the water escapes, carrying with it the heat liberated in the liquefaction, but the portions remaining are moist, and at each point of contact directly adhere together by freezing of the moisture. This refreezing takes place throughout the mass,
molecule to molecule, particle to particle, mass to mass, and a ball or other figure of solid ice is the result. If there be no moisture, there can be no refreezing until moisture is produced or applied. Thus pieces of ice below the freezing temperature will not adhere because the surfaces are dry. The same is true of dry, granular snow in very cold weather—only by long-continued moulding and pressure in the hands can it be compacted. But in this case some liquefaction has been produced, and then the surfaces in contact freeze together. Snow, in the upper Alps, often covers gorges in the glaciers, and if moist can be trodden into bridges sufficiently compact to pass safely over.
This property of ice and snow Prof. Tyndall calls regelation. It was discovered by Faraday in 1850, who found that moist surfaces of ice adhered if brought together. This occurs under water as well as in the air; at summer heat, and beneath water so hot as to be painful to the hands. The phenomenon may be explained in this way: If we hold in our hands two cubes of ice, their outer surfaces are exposed to the atmosphere, and, if it be warm enough, some liquefaction at the surfaces takes place, and they become moist. Now, if the cubes be brought together, two of the outer surfaces become inner ones, and the moisture, chilled to the temperature of the ice, freezes, and the two cubes become one mass. It is because the molecules of ice may be continually crowded into new positions that the mass may be changed in form without its continuity being broken. A slab of ice placed in a suitable position will bend by its own weight. In this case the molecules throughout undergo gradually a change of position; but, if the stress be too rapidly applied, fracture occurs.
All the properties and phenomena of ice which we have considered, and in a marked degree that of regelation, are shown in the growth and movement of glaciers. In these we see the development of ice on its grandest scale. Equally in structure and form in molecular and molar motion they are an expression of energies that are irresistible and sublime. "To produce from aqueous vapor," observes Prof. Tyndall, "the little mass of snow which a child can carry, demands an exertion of energy competent to gather up the blocks of the largest stone avalanche I have ever seen, and pitch them to twice the height from which they fell." Who, then, shall estimate the potential energy of the great ice-rivers of the Alps, or of the glaciers of the Arctic Zone?
The motion of the Mer de Glace, and of other glaciers, is so slow as to be ascertained only by persistent observation, or by careful measurement. In 1827 a hut was erected by Huji on the glacier of the Unteraar for purposes of observation, but the hut was found to move down the valley. In fourteen years it was nearly a mile below its first position. In 1820 three mountain-guides were plunged by an avalanche into a gorge of a glacier on the side of Mont Blanc. After a burial of forty years in the ice, they were found several miles below the spot where they were lost. The velocity of a glacier depends chiefly upon the angle of slope over which it moves. The hut of Huji moved 336 feet in a year. But the motion in different portions of a glacier is very unequal, slow at the margins and at the bottom where friction retards progress, but may attain a velocity of three feet or more in a day at its line of most rapid flow. It has been estimated that the ice-sheet which covered New England at its greatest development in the glacial age may not have advanced more than a foot in a week, a mile in a century.
A question has arisen, Do glaciers slide upon their beds? It seems to be conceded that sliding takes place to a limited extent. It may occur where the uniform flow of the glacier is interrupted, and separation of its parts produces crevasses, as along its margins, and over an uneven bed. We are chiefly concerned, however, with the motion which has its origin in the physical properties of ice.
The flowing of a glacier may be quite independent of its sliding motion, if such it has. It flows because of its plasticity, its molecules undergoing incessant change of position as they do in ice under pressure, and regelation goes on throughout the mass. By these means its cohesion and continuity are maintained.
It is often stated that the temperature of the interior of a glacier must be much below the freezing-point. This is probably an error, the temperature throughout differing but little from 32°. The pressure, indeed, may be enormous, and portions of the ice be liquefied by it, but the water which is "ice-cold" escapes through innumerable fissures, and the freezing-point is not lowered by the pressure, as it would be if the water of liquefaction did not escape. The constant flowing of a glacier necessitates unceasing supply, and its source is found accordingly in that zone of elevation where snows accumulate. The snow-fall of which the glacier is born implies vapor clouds and condensation, and equally evaporation, the proximity of a warm climate and expanse of ocean. Hence it is inferred that cold and warm climates were contiguous during the age of glaciers, as they are at this period of their decline. Glaciers relieve the land of accumulating snows as streams do of excess of waters. But for these, mountains reaching above the line of perpetual frost would become buried, and the "ocean piled upon the land." But such a process has its limitations in the economy of Nature.
The snow which falls in great volume upon mountains is a dry powdery mass, and cannot be consolidated until some liquefaction has taken place. This quickly occurs. Through the clear air of great altitudes the sun's rays fall with intense power upon objects, even while the temperature is at freezing in the shade. Portions of the surface snows are thus melted, the under portions are moistened by the percolating waters, and regelation begins. The phenomenon of the snow-ball is here reproduced on a gigantic scale, differing in this: in the one case liquefaction is produced by pressure, in the other by solar heat. Gradually the under-layers become incipient glacier-ice.
Movement of the mass originates in its gravity, and the direction must be down the slope on which it lies. Many streams in this way blend into immense rivers of ice, often several hundred feet in depth. The snow when first consolidated is filled with air-bubbles, and is white and opaque. Its whiteness disappears by expulsion of the air-bubbles
from pressure as the glacier moves down the valley. At its termination the ice is transparent, and its exquisite tints of blue indicate the extreme minuteness of the reflecting surfaces which linger in it.
The physical properties of ice by which it flows need not be re-stated further than to mention that the opinion of Prof. Forbes, that it flows as a "viscous substance," is not accepted. Wax, we may say, is both plastic and viscous, and yields equally to pressure and to tension. Ice yields to pressure, but not to tension. It cannot be stretched. Glaciers maintain their cohesion under pressure as plastic bodies, but, wanting viscosity, they break into profound chasms where the tension is great. In ice, therefore, one property is wanting to render it a viscous substance. In Fig. 9 is beautifully shown the opening of crevasses on the margin of a glacier, where the flow is retarded by friction. A like phenomenon occurs when a glacier falls in a cascade over a precipice. Then chasms appear of startling
depths, and gigantic blocks of ice are thrown into the widest confusion. From the chaos come sounds which indicate what is going on below. The nether air is filled with echoes from the murmur or roar of water, the falling of bowlders, and crashing of ice. At the foot of the fall the broken fragments of ice are crowded together, become solid by regelation, and the mass moves on.
The terminus of a glacier may be many thousand feet below the limit of perpetual snow before its disintegration is complete. But the wonderful fabric falls at last, as heat destroys its molecular framework, and is lost in the turbid flood which forever pours from beneath its portals. But the sediment of the incipient rivers thus formed was no part of the crystalline structure, for crystallization casts out impurities, and gathers neither soil nor stain in its beautiful textures. The sediment arises from abrasion of solid matters held in the under-surface of the glacier upon the rocks of its bed. In Fig. 10 is shown the source of the Arveiron and the sublime view of the foot of the glacier whence it issues. Of this M. Rendu says, "It is a vast portico more than a hundred feet high, let into an immense façade, and surmounted by lofty pyramids of ice. Nothing is more astonishing than this work of the elements, of which Nature alone has conceived the plan, and achieved the construction."
Ice presents, under pressure, many phenomena of great interest other than those mentioned, and to which we can only refer. The prismatic or crystalline form, so beautifully developed in lake-ice, is more or less destroyed in glaciers by the unceasing fracture and regelation which takes place from pressure, and the mass assumes a granular structure. The same phenomenon occurs with ice in a mould. In glaciers are veins which reflect a deeper tint of blue, indicating where from local causes greater or more persistent pressure has cleared it of bubbles of air. Glacier, and probably other ice, under similar conditions of pressure, becomes laminated, or develops planes of cleavage, resembling those of slate-rock in the quarry; and this structure is shown in the decay of the ice, as its prismatic structure is shown in the decay of that on lakes and rivers.
By the physical properties we have noticed, ice becomes a dynamic agent of tremendous power. The play of forces and plasticity of structure which make it a toy in the laboratory, have changed the aspects of Nature, and modified the surface, as they have the distribution of life, upon a large portion of the globe. Rocks are broken by its expansive energy, but they are also crushed by its weight, and ground to dust in its irresistible motion. A sheet of ice a mile in thickness rests upon the bed, over which it moves with a pressure of more than 260,000 pounds for each square foot of surface, and many such as this covered the old glacier-regions through an unmeasured period of time. In their movement lakes were excavated, water-courses changed, and landscapes cast into innumerable forms of beauty. We utilize the results. Our forests and our harvests grow upon soil ground in the glacial mill, and we build our cities on mounds of glacial rubbish. Nor can we fail to realize that here, as elsewhere in the phenomena of Nature, there is a ministration to our conscious life, as there is an appeal to our sense of beauty, and that, whatever form it may assume, whether of the feathery spangle which rocks upon the waves of air, or the profound glacier that buries a continent, ice is Winter's benefaction.