Familiar Talks on Science: World-Building and Life; Earth, Air and Water.

It will be our endeavor to give the reader a mental picture of what is taking place during the time the ice is melting and the thermometer is stationary. Do not suppose that you can understand this, even so far as it is understandable, by a casual reading without thought. No man was ever yet able to present a picture to the mind of another, however clearly and simply it may be done, unless that other mind is receptive. When a photographer trains his camera upon an object, however intense the light may be and however clean-cut the picture that is thrown upon the plate in the camera, unless that plate is properly sensitized so that the picture may be impressed upon it, all of the other conditions are in vain. The reader is always a part of the book he is reading.

CHAPTER XXIII

STORED ENERGY IN WATER

In our last chapter we traced the upward movement in the mercury of the thermometer from 10 degrees below the freezing point up to the boiling point of water. We found that the thermometer was arrested at 32 degrees and remained stationary at that point until all the ice was melted, notwithstanding the fact that heat was being constantly applied. After the ice is all melted the mercury moves upward until it reaches the boiling point of water, where the movement is again arrested, and although the heat is being continuously applied, it remains stationary until all the water is evaporated. If we push the process still further, with a sufficient application of energy we can separate the vapor molecules into their original elements, oxygen and hydrogen.

Let us go back now to the freezing point of water and see what is becoming of the heat that is consumed in melting the cake of ice, and still does not produce any effect upon the mercury in the thermometer. Sensible heat, as before stated, is a movement of the atoms of matter, and temperature, as it affects the thermometer, is a measure of the intensity of motion exhibited by these atoms.

In the experiment of the block of ice that in the beginning is 10 degrees below the freezing point, as shown by the thermometer, the molecules have a definite intensity of motion. The intensity of this motion increases when heat is applied until it reaches 32 degrees, when it remains stationary until all of the ice is melted. At this point there is a rearrangement of the molecules of water as it assumes the liquid state. To perform this rearrangement requires a certain amount of work done, which is analogous to the winding up of a weight to a certain distance. There has been energy used in winding up the weight, but that energy now is not destroyed, nor still in the form of heat, but is in the potential state – ready to do some other kind of work. So, the heat that has been applied to the melting ice has been utilized during the process of its liquefaction in rearranging the water molecules and putting them in a state of strain, so to speak, like the weight that is wound up to a certain height. There is a certain amount of potential energy that is stored in the molecules of water that will be given up and become active energy in the form of heat, if the water is again frozen. To melt a cubic foot of ice requires as much heat as it would to raise a cubic foot of water 144 degrees Fahrenheit. But, as we have seen, while all of this energy is absorbed as heat, it is not lost as energy. It ceases to be kinetic or active and becomes potential energy. This (let us repeat) has been called latent heat. The term grew out of the old idea that heat was a fluid and that when it became latent it hid itself away somewhere in the interatomic spaces of matter and ceased to be longer sensible heat. It came into existence in the same manner and occupies the same place in the science of heat that the word "current" does in the science of electricity: both of them are misnomers.

When the ice is all melted potential energy is no longer stored, but is manifested in the sensible heating of water, the degree of which is measurable by the thermometer, until it reaches the boiling point, where it is again arrested. All of the surplus heat above that temperature is consumed in rending the liquid water into moisture globules that float away into the air, each one of them charged with a store of potential energy. Let us follow this vapor spherule as it floats into the upper regions of the atmosphere. Myriads of its fellows travel with it until it reaches a point where condensation takes place, when it collapses and unites with other vapor particles to form water again. In doing this the heat that was expended upon it to disengage it (whether the heat was artificial or that of the sun's rays) now reappears either as sensible heat or as electricity, or both. And this is what is meant in meteorology by latent heat becoming sensible heat at the time of condensation; in fact, it is stored or "potential" energy becoming active or kinetic, and assumes the form of heat or electricity, as before stated. We have thus reviewed the matter of the foregoing chapter in order to follow the course of the stored energy from the melting of the ice to the vapor, and back again to water: to doubly impress the fact that the energy used was not consumed, but still exists and is ready for further work.

During the progress of a hailstorm, it has been stated, one of the factors that is active to produce this phenomenon is the intense ascensional force that is given to the moisture-laden air, caused by intense heat at the surface of the earth. This condition forces the moisture vapor to higher regions of the atmosphere than is the case with the ordinary thunderstorm. Another factor that is undoubtedly active in producing hail under these circumstances is that when condensation takes place in the higher regions, and is therefore more energetic on account of the intenser cold, the potential energy that is set free by the moisture spherules takes, in a larger degree, the form of electricity rather than heat, as is the case under more ordinary circumstances. While in the end this electrical energy becomes active heat, it does not for the time being, and thus favors the ready congelation of the condensed moisture into hailstones. Hailstorms are always attended by incessant thunder and lightning, and this fact favors the theory advanced above.

It will be easily seen from a study of the foregoing what a wonderful factor evaporation (which is a product of the sun's rays) is, in the play of celestial dynamics. It ascends from the surface of the earth or ocean laden with a stored energy, the power of which no man can compute, and beside which gravitation is a mere point. In the upper regions of atmosphere this potential force under certain conditions is released and becomes an active factor, not only in the formation of cloud and the precipitation of rain, hail, and snow, but it disturbs the equilibrium of the air and sets that in motion.

Certain physicists deny that evaporation has anything to do with atmospheric electricity. They tell us that it is caused by the arrest of the energy of the sunbeam by the clouds and vapor in the upper atmosphere. We admit that a part of the energy is so arrested, and is stored, for the time, in moisture globules by a process of cloud evaporation to transparent vapor again. Yet this does not hinder the same process from going on at the surface of the earth wherever there is water or moisture. But they tell us that the electroscope does not show any signs of electrification in the evaporated moisture. Of course it does not. The electroscope is not made to detect the presence of energy except when set free as electricity.

A wound-up spring does not seem to be electrified, but if it is released the energy stored in it will be transformed into electricity if the conditions are right. Just so, the energy required to put the moisture spherule into a state of strain is latent until some power releases it, when it reappears as active energy of some form.

We have now followed the relation of heat to water from a point 10 degrees below freezing up to where it was forced into its original gases, oxygen and hydrogen. These gases have stored in them a wonderful amount of potential energy. When one pound of hydrogen and eight pounds of oxygen unite to form water the mechanical value of the energy given up at that time in the form of heat is represented by 47,000,000 pounds raised to one foot in height. And this is the measure of the energy that was put into nine pounds of water to force it from a state of vapor into its constituent gases. After the combination of the gases into a state of vapor the temperature sinks to that of boiling water. The amount of energy given up in condensing the nine pounds of vapor into nine pounds of water is equal to 6,720,000 foot-pounds. If this nine pounds of water is now cooled from the boiling point to 32 degrees Fahrenheit we come to the final fall, where the potential energy that is stored in the operation of melting ice is given up suddenly at the moment of freezing, which in nine pounds of water is 993,546 foot pounds.

Professor Tyndall, in speaking of the amount of energy that is given up between the points where the constituent gases unite to form nine pounds of water and the point where it congeals as ice, says: "Our nine pounds of water, at its origin and during its progress, falls down three precipices – the first fall is equivalent in energy to the descent of a ton weight down a precipice 22,320 feet high-over four miles; the second fall is equal to that of a ton down a precipice 2900 feet high, and the third is equal to a fall of a ton down a precipice 433 feet high. I have seen the wild stone avalanches of the Alps, which smoke and thunder down the declivities with a vehemence almost sufficient to stun the observer. I have also seen snowflakes descending so softly as not to hurt the fragile spangles of which they are composed. Yet to produce from aqueous vapor a quantity which a child could carry of that tender material demands an exertion of energy competent to gather up the shattered blocks of the largest stone avalanche I have ever seen and pitch them to twice the height from which they fell."

When we contemplate the foregoing facts as related to so small an amount of water as nine pounds, and multiply this result by the amount of snow- and rainfall each year and the amount of ice that is congealed and again liquefied by the power of the sun's rays, we are appalled, and shrink from the task of attempting to reduce the amount of energy expended in a single year to measurable units.

Having considered water in its relation to heat in the preceding chapters, we will now take up the subject of water in its relation to ice and snowfall and the phenomena exhibited in ice rivers, commonly called glaciers.

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 space. 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 pass 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 diminishing (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 glass flask, terminating in a long neck, say of four to six inches, and of small diameter. Suppose the water in the glass 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 glass 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 screwing 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 shivered 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 bismuth. 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 pass from the liquid to the solid form. The molecules assume definite forms and shapes, according to the nature of the substance. When water assumes 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 mass 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 circumstance (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 theseAll 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 quantity of snow or a quantity 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 glass 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 mass 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 crevasses 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 crevasses.

If at the bottom of the descent the valley curves upward or preserves the straight line for a considerable distance, these crevasses 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 crevasse, 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 accumulate 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 accumulation is greater from snowfall than diminution from melting. If this condition should continue it would become a serious matter.