Fig. 222.
330. Description of the Process of Freezing.—In order to make the process of freezing clear to you I will describe it as it ordinarily occurs, that is, from the action of cold air upon the surface of water. The uppermost layer of the water imparts some of its heat to the air in contact with it. This air rises and colder air takes its place, which being warmed in its turn rises to make way for more of the cold air. You have therefore a constant current of warmed air upward from the water. In the mean time there is a current of a different character in the water—a downward one. As fast as the water at the surface parts with heat to the air it falls, other warm water taking its place, to cool in its turn and go down. This falling of the cooled water goes on regularly until a portion of water becomes cooled down to 39°, that is, 7° above the freezing point. This layer does not sink, but remains at the surface, for it is lighter than the warmer water below. This is because the law that heat expands matter is now reversed. Beyond this point of the thermometer the colder the water is the lighter it is. As the cooling now goes on from the air coming, as before, in successive layers to the water, the cooled water at the surface continually increases. At first it is a mere single layer of particles, but after a while it is quite a body of cold water lying on the warmer water below. At length some of it is cooled down to 32°, the freezing point, and a thin film of ice now forms. The state of things just at this stage of the process may be represented by a simple diagram, Fig. 222. Let the line a represent the film of ice. The space between a and b is the portion of water cooled down below 39°. The space below b is occupied by the water which is above this temperature. In the space between a and b the cooler the water is the nearer it is to the surface. That is, from the line b, where the water is exactly at 39°, as you go upward, the water lessens in temperature, it being successively 38°, 37°, 36°, etc., till, just in contact with the film of ice, a, it is at 32°. The ice goes on to thicken gradually by additions below. But it is to be remembered that ice is a good non-conductor, so that the very first layer of ice makes the cooling of the water proceed more slowly than before. And the thicker the ice becomes the slower is the cooling. This secures against too great a formation of ice.
331. Why the Above Exception to Expansion by Heat Exists.—That we may see the reasons in part for the grand exception to the general law of expansion by heat which I have illustrated, let us see what would be some of the results if the exception did not exist. In that case the process of freezing would be as follows: The water would communicate its heat from the surface to the air, as before described, and there would be a constant downward current of the cooled water. When any portion of the water became cooled by the air down to 32°, it would become ice, and would sink to the bottom. And after the process of freezing had once begun, there would be a continual accumulation of ice at the bottom so long as the air remained cold enough to cool the water with which it comes in contact down to 32°.
The result may be stated in the general thus: Freezing would not begin so quickly as it now does; but when once begun it would prove very destructive. It would not begin as soon, because the whole of any body of water must be cooled down to just this side of 32° before it could begin. This would not take long where the water is shallow, but it would where it is deep. All shallow bodies of water, then, would be frozen up quite early in the winter; and as water is a poor conductor, and thawing must go from above downward, some of them would not be thawed out again fully till quite into the next summer, if even then. And where the water is quite deep ice would at length begin to form, and when formed it would be exceedingly slow in thawing. In some cases it would never be thawed with such a body of non-conducting water to guard it against the warmth above. It is easy to see that the heat of spring and summer would not thaw out any thing like the quantity of ice that it now does. The reign of ice and snow on our earth would therefore be vastly more extensive than now, and what is worse, it would be extended more and more every year. Under such circumstances there would be great destruction of both animal and vegetable life. I will mention, however, but a single item, as it would occupy too much space to go into this subject with any fullness. In the water under the ice, which is always above 39°, except that which is close to the ice when freezing is going on, there is a vast amount of busy life which would be destroyed if ice were formed at the bottom, chilling all the water above.
332. Why the Freezing Point is at 32°.—If the freezing point of water were higher than 32°, freezing would occur so early in the autumn, and the ice and snow would last so late in the spring, that the season would be too short to raise our supplies of fruits and grains. If, on the other hand, it were at a lower point, the earth would not have the protection of its light coat of snow, but instead would be chilled by rains so cold that barrenness would be the result. The multitudes of animals, too, that now live so securely in the water, some of them even with the ice above them, would all perish with the cold.
333. Force of Expansion in Ice.—As ice occupies one seventh more room than the water from which it is formed, it exerts in its formation an expansive force which under various circumstances produces varied and often remarkable results. Of the many experiments which have been tried to show the force of this expansion I will mention but one. A bomb-shell was filled with water, at Montreal, and closed with an iron plug which was driven in with great force. The plug was thrown a distance of 400 feet by the expansion when the water froze. This expansion is sometimes an inconvenience to us, as in bursting water-pipes; but besides the great service which it does in the earth, already noticed, it is of service also in loosening the soil, and in supplying it with requisite ingredients from the rocks by breaking them up and pulverizing them in small quantities from year to year.