Familiar Talks on Science-World-Building and Life. Earth Part 9

When water is under pressure the freezing point is reduced several degrees below 32 degrees Fahrenheit. This fact has been determined by confining water in a close vessel and putting it under pressure and subjecting it to a freezing mixture, and by this means determining the freezing point under such conditions. By putting a bullet or something of that nature into the water that is subjected to pressure one can tell by shaking it when the freezing point is reached. If water is put under pressure and cooled to a point below 32 degrees, and yet still remains in the liquid state, it may be suddenly congealed by taking off the pressure; this shows that the pressure helps to hold the molecules in the position necessary for the liquid state, and prevents the rearrangement of them that takes place at the moment of freezing. When the water molecules are arranged for the liquid condition they may be compared to a spring that is wound up and held in position by the heat energy that is stored in the water. And when this energy is given up to a certain degree the power that holds the spring wound up is suddenly released, when it unwinds and occupies a larger s.p.a.ce. There is a force that we may call polar force, which is constantly tending to push the molecules of water into an arrangement such as we see when crystallization takes place--as it always does in the act of freezing.

These polar forces cannot act so long as the energy in the form of heat is sufficient to hold the water in the fluid state. But the moment this energy, which tends to hold it in the fluid state, falls below that which tends to rearrange it into the crystalline form, it is overcome by the superior power of the latter force, and we have the phenomenon of solidified water.

A very interesting experiment may be performed with a block of ice by anyone when the ice is near the melting point. If a wire is put around the ice and a sufficient weight is suspended to it, the pressure of the wire on the ice will gradually liquefy that portion immediately under the wire, which allows it to sink into the ice slowly, and as this process goes on the ice freezes together again behind the wire, so that in time the wire will pa.s.s entirely through the block and leave it still a solid block, as it was before the experiment began.

This is an interesting fact which it will be well to remember when we come to explain glacial action, or rather the law that governs glacial action. If we take two pieces of melting ice and bring them together they immediately congeal at the point of contact. This phenomenon is called "regelation." Ice has some of the properties of a viscous substance. It will yield slowly to pressure, especially when near the melting point, but if put under a tensional strain it will break, as any brittle substance will, so that it has the properties of both viscosity and brittleness. Ordinarily we are in the habit of treating water as a fluid and ice as a solid, but from what has gone before the reader must understand that in a certain sense ice should be treated as having semi-fluidic properties.

CHAPTER XXIV.

WHY DOES ICE FLOAT?

Nature is full of surprises. By a long series of experimental investigations you think you have established a law that is as unalterable as those of the Medes and Persians. But once in a while you stumble upon phenomena that seem to contradict all that has gone before.

These, however, may be only the exceptions that prove the rule. It is recognized as a fundamental law that heat expands and cold contracts; that the atom when in a state of intense motion (which is the condition producing the effect that we call "heat") requires more room than when its motions are of a less amplitude. In other words, an increase in the amplitude of atomic motion is heating, while a decrease is cooling. It follows from the above statement that the colder a body becomes the smaller will be its dimensions. There are two or three, and perhaps more, exceptions to this rule, and the most notable one is that of water. Water follows the same law that all other substances do under the action of heat and cold, within certain limits only. If we take water, say, at 50 degrees Fahrenheit and subject it to cold it will gradually contract in bulk until it reaches 39 degrees Fahrenheit. At this point, very curiously, contraction ceases, and here we find the maximum density of water. If the temperature is still lowered we find the bulk is gradually increasing instead of diminis.h.i.+ng (as is the rule with other fluids), and when it reaches the freezing point there is a sudden and marked expansion, so much so that a cubic foot of ice, which is solidified water, will not weigh as much as a cubic foot of water before it freezes--hence it floats.

Let us try an experiment. Take a small gla.s.s flask, terminating in a long neck, say of four to six inches, and of small diameter. Suppose the water in the gla.s.s to be at 50 degrees Fahrenheit. Fill the flask with water until it stands halfway up the neck at 50 degrees temperature. Now immerse the flask gradually in hot water, and observe the effect. For a moment the water will lower in the neck of the tube, but this is due to the fact that the gla.s.s expands before the heat is communicated to the water and enlarges its capacity. But immediately the water will begin to rise as the heat is communicated to it, and will continue to expand up to the boiling point. Now take the flask out of the hot water and gradually introduce it into a freezing mixture made of broken ice and salt. Immediately the water will begin to fall in the tube, showing that it is contracting under the cold, and it will continue to contract until it reaches a temperature of 39 degrees Fahrenheit, when it will come to a standstill and then proceed to expand as the temperature of the water lowers. When it reaches the freezing point the fluid can no longer rise in the neck of the flask, which is broken by the sudden expansion that takes place at this point.

To show what an irresistible power resides in the atoms of which the body is made, let us take an iron flask with walls one-half inch or more in thickness; fill it with water and seal it up by s.c.r.e.w.i.n.g on the neck an iron cap; now plunge it into the freezing mixture, and the first effect will be to contract the water unless it is already below 39 degrees Fahrenheit, but when it reaches that point expansion sets in, which continues to the freezing point, when a greatly increased expansion takes place suddenly. The walls of the iron flask, although a half-inch in thickness, are no longer able to resist the combined efforts of the billions upon billions of the atoms of which the water is made up, in their individual clamor for more room, hence the flask is s.h.i.+vered into pieces.

There are one or two other substances which are exceptions to the general rule, but we will mention only one, which is the metal bis.m.u.th.

If we should melt a sufficient amount to fill an iron flask, such as we have described, and subject it to the same freezing process, the flask will be broken the same as in the experiment made with the water.

A query arises, Why this phenomenon? Why does water follow a different law in cooling from that of nearly all other substances?

This is a case where it is much easier to ask a question than to answer it. When water solidifies at the moment of freezing, crystallization sets in. But what is crystallization? Crystallization is a peculiar arrangement of the molecules of matter, which takes place in some substances when they pa.s.s from the liquid to the solid form. The molecules a.s.sume definite forms and shapes, according to the nature of the substance. When water a.s.sumes the solid form under the action of cold the molecules arrange themselves according to certain definite and fixed laws, the result of which is to increase the bulk to a considerable extent over that which the same number of molecules would occupy at a temperature of 39 degrees Fahrenheit. Hence, as has been heretofore stated, a given block of solidified water is lighter than the same bulk would be in the fluid state, and this is the reason why ice floats.

What would happen in case nature did not make this exception to the laws of expansion and contraction by heat and cold, in the case of water?

First, our lakes would freeze from the bottom upward; as soon as the surface became frozen, or even colder than the water underneath, it would drop to the bottom, the warmer water below coming up by a well-known law--that the warmer fluid rises and the colder falls. This circulation would continue until ice began to form, which would immediately drop to the bottom, and this process would go on until the whole ma.s.s were frozen solid. In the same way our rivers in the northern climates would freeze from the bottom, and in time our valleys would fill up with ice to a thickness that the summer's sun would never melt, and gradually all north of a certain zone would become a great glacier, rendering not only the lakes and rivers but also the surface of the earth unfitted for animal life.

Those who believe that the laws of nature are the creations of a beneficent and all-wise Intelligence will see in this exception to the general law in the case of freezing water a striking evidence of design.

But those who have no such belief will say it is a most fortunate though fortuitous circ.u.mstance (a saying they will have to make, regarding thousands of other things in nature), and go on floundering in the interminable sea of "I don't know."

The atom when it is acting under the direction of a fixed law is a giant in strength. And when its individual strength is multiplied by billions upon billions the combined energy exerted produces a power that is irresistible. Not only has nature endowed these atoms with this wonderful power, but she has also willed that they arrange themselves in lines of beauty. In confirmation of this we need only to study the work of the frost upon our window panes. As we lie in our beds on a cold night and exhale moisture from our lungs it settles upon the window panes of our bedrooms, where Nature--that wonderful artist--forms it into beautiful pictures that gladden our eyes when we awake:

Most beautiful things; there are flowers and trees, And bevies of birds, and swarms of bees, And cities, and temples, and towers, and these All pictured in silver sheen.

CHAPTER XXV.

GLACIERS.

Glaciers are rivers of ice, and, like other rivers, some of them are small and some very large. They flow down the gorges from high mountains, whose peaks are always covered with a blanket of eternal snow. Summer and winter the snow is precipitated upon these mountains, and from time to time the heat of the sun's rays softens the snow, when by its great weight it packs more closely together until it is, in many cases, formed into solid ice-cakes. If we take a quant.i.ty of snow or a quant.i.ty of granulated ice and put it under a sufficient pressure we can produce clear solid ice, and it is by this process that ice is formed out of the snow and hail that falls continually upon the tops of these glacial mountains. We have seen that ice possesses certain viscous or semi-fluidic properties and that it will yield to pressure, but if we put it under sufficient tensional strain it snaps like gla.s.s or any other brittle substance. As the snows upon these mountains pile up higher and higher the pressure becomes greater and greater until it reaches a point where the ma.s.s begins to move gradually down the mountain side, following the gulches and defiles that furnish a path of least resistance to its flow.

At the sides and bottom, where there is contact with the earth, the movement is slower than it is at the surface and in the middle of the ice stream. If there were no curves in the ravine or gulch through which it flows the point of greatest movement would be confined to the middle of its width. But in flowing through a winding gulch the most rapid flow follow the lines of greatest pressure, and this line is deflected from side to side, so that the line of greatest flow is more winding than is the bottom of the valley through which it flows. (The movement is called a "flow," but it is very sluggish, only a few inches in a day, as will appear later.)

If the bottom and sides of the valley were straight the surface of the ice would be comparatively even; I say comparatively, for as compared with a smooth surface it would be very rough; but there would be none of the great creva.s.ses or openings now to be found in the ice, which sometimes are very large and extend to a great depth. If in its downward course the bottom of the ravine suddenly becomes steeper, the top of the ice is put under a tensional strain which causes it to break, thus forming the creva.s.ses.

If at the bottom of the descent the valley curves upward or preserves the straight line for a considerable distance, these creva.s.ses will close at the top and perhaps open at the bottom, and the blocks of ice will freeze together to such an extent that the water caused by the melting ice will flow on top until it comes to another creva.s.se, where it runs through to the bottom or underflow, which is always an attendant of a glacier.

The glacier continues its flow down the mountain side till in some cases it reaches quite to the valley below, and in others it stops short, as the action of the sun is so great that it melts entirely away at this point as fast as it moves down. In the winter time, however, the glacier may flow far down into the valley and will acc.u.mulate greatly in bulk, owing to the fact that the ice forms from the precipitation of snow on top faster than it melts away underneath. If it were not for the fact that in summer the glaciers melt faster than they form, the whole valley would in time become a great river of ice. It is the case in Switzerland that some years the acc.u.mulation is greater from snowfall than diminution from melting. If this condition should continue it would become a serious matter.

In the downward flow of a glacier--slow as it is--there is an exhibition of wonderful power; great bowlders are torn from their beds and either ground to powder or carried down to the end of the glacier, to be dropped with the other debris that has been carried there by the same force, forming an acc.u.mulation that geologists call the "moraine." Of these moraines we will speak more fully later on.

It was the privilege of the writer some years since to visit the great glaciers of Switzerland and to some extent study their action. Some rivers have their origin chiefly in melting glaciers. They start as ice rivers and end in rivers of water. The effects during the great ice age of some of these glacial rivers, which are now extinct, are very remarkable; we shall have occasion to refer to them when we come to treat of the glacial period.

There is a glacial river flowing which is fed largely by the great Rhone glacier in Switzerland. The water from this river is almost as white as milk, which is occasioned by the grinding action of the great ice blocks on the rock as it flows down the sides of the mountain. These glacial rivers are much higher in summer, of course, than in winter, some of them having not only an annual fluctuation, but a diurnal one. The former is caused by the cold of winter, and the latter because it freezes to some extent at night and checks the flow of water. The difference between day and night in these high alt.i.tudes is very marked.

While it is extremely hot in the sun, it is cool the moment we step into the shade.

I remember walking across one of the glaciers in the Alps, called the Mer de Glace, one clear day in summer, when I suffered so much from the heat, although standing upon a sea of ice, that it was necessary to carry an umbrella. In fact, during my stay there was a case of sunstroke that occurred upon this same glacier. This intense heat during the day melts the surface of the ice, which forms streams that run along on the top of a glacier until they come to a creva.s.se or riffle in the ice river, where they plunge down and become a part of the glacial stream that is flowing underneath the ice.

The speed at which these ice streams flow varies greatly with the size of the glacier as to width and depth and the steepness of the grade, and many other conditions. In its movement the glacier is constantly bending and freezing and being torn asunder by tensional strain, yielding and liquefying at other points by pressure, only to freeze again when that pressure is removed. This, taken in connection with the friction of the great ice bowlders, produces a movement that is exceedingly complicated in its actions and interactions.

According to Professor Tyndall's investigations, the most rapid movement observed in the glaciers of Switzerland is thirty-seven inches per day at the point of greatest movement. From this point each way the motion gradually diminishes until it reaches the sides of the glacier, where the motion is not more than two or three inches.

The great North American glaciers move at a much higher rate of speed.

We are indebted to Dr. G. Frederick Wright, author of "The Ice Age in North America," who spent a month studying the Muir glacier in Alaska, for many details concerning that great ice river. This glacier empties into Muir Inlet, which is an offshoot of Glacier Bay. It is situated in lat.i.tude 58 degrees 50 minutes and longitude 136 degrees 40 minutes west of Greenwich. The bay into which this glacier empties is about thirty miles long and from eight to twelve miles wide. This bay, with its great glacier, has a setting of grand mountain peaks. I cannot do better than to quote the words of Dr. Wright when he describes the location of this glacier. Dr. Wright lived for a month in a tent on the edge of this bay, a short distance below the face of the great glacier, where the icebergs fell off every few minutes into the deep water.

He says: "To the south the calm surface of the bay opened outward into Cross Sound twenty-five miles away. The islands dotting the smooth surface of the waters below us seemed but specks, and the grand vista of snowclad mountains guarding either side of Chatham Strait seemed gradually to come to a point on the southern horizon. Westward toward the Pacific was the marvelous outline of the southern portion of the St.

Elias Alps. The lofty peaks of Crillon, 15,900 feet high, and Fair Weather, 15,500 feet high, about twenty-five miles away and about the same distance apart, stood as sentinels over the lesser peaks."

The Muir glacier might be likened to a great inland sea of ice fed by many tributaries or ice rivers. It narrows up at the point where it empties into Muir Inlet to 10,664 feet, or a little over two miles. An enormous pressure is exerted at this point, which causes the ice to flow in the central portion at the rate of about seventy feet per day. There is a continual booming, like the firing of a cannon, going on, caused by the bursting of some great iceberg either before it takes its final leap into the water or at the moment of its fall. At the point where these great icebergs drop off into the water they stand like a solid wall 300 feet above its surface. Dr. Wright says: "From this point there is a constant succession of falls of ice into the water, accompanied by loud reports. Scarcely ten minutes, either night or day, pa.s.sed during the whole month without our being startled with such reports; and frequently they were like thunder claps or the booming of cannon at the bombardment of a besieged city, and this though our camp was two and one-half miles below the ice front.... Repeatedly I have seen vast columns of ice extending up to the full height of the front topple over and fall into the water. How far these columns extended below the water could not be told accurately, but I have seen bergs floating away which were certainly 500 feet in length."

It is estimated that the cubical contents of some of these icebergs are equal to 40,000,000 feet. This great glacier is fed by the constant precipitation of snow upon the sides and peaks of the high mountains that surround its vast amphitheater, which is floored with icebergs.

Wonderful as this seems to us to-day, it is scarcely a microscopic speck of what existed during the ice age all over the northern part of North America.

There are many other great glaciers in the mountains of the Pacific coast. Some years ago I saw one of these immense glaciers in British Columbia, from a point called Glacier Station, in the Selkirk Mountains, on the Canadian Pacific Railroad. It was during the month of August, when all of the region was pervaded by a dense smoke occasioned by burning forests. This glacier is a very showy one, owing to the steepness of the side of the mountain and its great breadth. All the glaciers that exist to-day are gradually receding, and are destined eventually to entirely disappear, unless there is a change in meteorological conditions, which some scientists claim will be the case if we only wait long enough, when again all this northern country will be covered with a great ice sheet. There is no doubt in regard to the facts concerning a glacial period that must have existed in the ages past. To anyone who has made a study of the subject there is not wanting abundant evidence to prove that this northern country was at one time enveloped with a great ice sheet of enormous thickness. The conditions that existed to bring about such a state of things have been the subject of much speculation by philosophers, but no one, as yet, has arrived at any very satisfactory conclusion. Many theories have been advanced, some of them not worth considering, while others have many things that give them a show of plausibility. But all of them have what is said of the Darwinian theory, "a missing link." It will be interesting, however, and also instructive, to know what can be said in favor of a set of conditions that would produce such momentous results.

CHAPTER XXVI.

EVIDENCES AND THEORIES OF AN ICE AGE.

There is abundant and una.s.sailable evidence that at one time, ages ago, a vast ice sheet covered the whole of the northern part of North America, extending south in Illinois to a point between lat.i.tudes 37 and 38. This is the most southerly point to which the ice sheet reached.

From this point the line of extreme flow runs off in a northeasterly and northwesterly direction. The northeasterly line is through southeastern Ohio and Pennsylvania, striking the Atlantic Ocean about at New York, thence through Long Island and up the coast of Ma.s.sachusetts.

Northwesterly it follows the Mississippi River to its junction with the Missouri, which it crosses at a point some miles west of this junction, following the general course of this river a little south of it through the States of Missouri, Nebraska, Dakota, and Montana. The lines, especially the northeasterly one, are very irregular, shooting out into curves and then receding. This line of extreme ice flow is marked by glacial drift so prominently that no one who has studied glacial action can doubt for a moment what was the cause of these deposits. The line is called the "terminal moraine." By examining a map of North America and tracing the line of the moraine as we have described it, it will be seen that about two-thirds of North America was at one time covered with ice to a greater or less depth. How deep, is simply a matter of conjecture, but in the central portions of the great glacier, where was the bulk of snowfall, it must have reached a depth of several miles to account for the enormous pressure that would be required to carry the ice so far southward.

But let us go back and define what is meant by a moraine. A moraine is a name given to the deposits that are of stone, gravel, and earth that have been carried along by the movement of the glaciers and deposited at their margins, sometimes piled up to great depths. The composition of these moraines is determined of course by the nature of the country over which the stream of ice is flowing. Bowlders of enormous size have been carried for hundreds of miles, and the experienced geologist is able to examine any one of them and tell us where its home was before the glacial period. Moraines are divided into different cla.s.ses according to their position and const.i.tution. The moraine found at the extreme limit of ice-flow is called the "terminal" moraine, as before mentioned. Those that are found inside of this line and between two flows are called "medial" moraines. There is a subdivision called "kettle" or "gravel"

moraines, which are very prominent in northern Illinois and southern Wisconsin, and may be said to culminate in the vicinity of Madison. This moraine is a great deposit of gravelly soil. Where this moraine exists the face of the country is covered with "kettle holes" of all sizes and shapes, and in some of them there are small lakes, while others are dry.

The great chain of inland lakes that are found in southern Wisconsin and northern Illinois were formed by deposits of ice that had been covered by glacial drift, gravel and otherwise, brought down and deposited upon these ma.s.ses of ice which gradually melted away, leaving a depression at the points where they lay, while the drift that was piled around them loomed up and became the sh.o.r.es of the lake. This is substantially Dr.