Back Aa

Nature's Miracles: Familiar Talks on Science, Vol. 1.

Close

<h2>CHAPTER XXII.</h2> <h3>WATER AND ICE.</h3> We now have entered upon a subject that is of intense interest, studied from the standpoint of facts as they exist to-day and of history as we read it in the rocks and bowlders that we find distributed over the face of the earth. The whole northern part of the United States extending to a point south of Cincinnati was at one time covered with a great ice-sheet, traces of which are plainly visible to anyone who has made anything of a study of this subject. The glaciers now to be seen in British Columbia and Alaska, great as they seem to one viewing them to-day, are by comparison with what once existed simply microscopic specks of ice. Glaciers, like rivers, flow by gravity, following the lowest bed and lines of least resistance; the difference being that in the one case the flow is rapid, while in the other it is scarcely visible, except by measurement from day to day. Before entering upon a description of the law that governs the flow of glaciers, let us stop and give a little study to the phenomena of water as exhibited when it is at the freezing point. Water is such a large factor in the make-up of our globe and the air that surrounds it that it becomes a very interesting and important study to anyone who wishes to understand the phenomena of nature that are closely related to it. As all know, pure water is a compound of two gases, oxygen and hydrogen, combined in the proportion of two atoms of hydrogen and one of oxygen. Let us now study this fluid in its relation to heat. The reader is referred to the chapters on heat in Vol. II., where it is stated that heat is a mode of motion. It is also stated that heat is a form of energy, and that energy is indestructible, that an unvarying amount of it exists in some form or another throughout the universe. It is not always manifested as heat or electricity, although both of these are always in evidence as active agents of force. Much of the energy is simply stored—all the time possessing the ability to do work or to be converted into any of its known forms, such as heat, light, electricity, or mechanical motion. A weight that is wound up has required a certain amount of energy to elevate it to the position that it occupies. While in its elevated position it possesses energy, although not active. Energy in this form is called potential (possible) energy, and has the power to do work if released. Active energy is called kinetic (moving) energy, and the sum of these two energies is a constant quantity. We will now study energy as it is related to water in the form of heat. There is a kind of heat called "latent heat," which is not heat at all, but stored energy, waiting to be turned into heat, or light, or some other active form. Properly speaking, heat is a movement of the atoms of matter, the intensity of which is measurable in degrees, and called its temperature. To use the term latent heat as meaning concealed heat, which must reappear as heat, is a misnomer and is very misleading. If it is proper to call a wound-up spring or weight latent heat then its present use is a correct one. What was formerly termed latent heat is simply a form of potential energy. When sensible heat that is measurable, as temperature, disappears in the performance of some sort of work, especially in connection with certain phenomena relating to water, we call it—or rather miscall it—latent heat: but the phrase would better be "stored energy." The action of water under heat is very peculiar, and in order to get a correct understanding of the phenomena exhibited in glacial action we also need to understand the phenomena of water at the freezing point. As is well known, fresh water freezes at 32 degrees Fahrenheit, and at the moment of freezing there is a sudden expansion to such an extent that a cubic foot of ice will occupy a much larger space than it will in the form of water; and because it occupies so much larger space it is lighter than the same bulk of water would be, and therefore it floats in water. At the point of freezing, the thermometer if placed on the ice will register 32 degrees. If the ice is allowed to melt, the water at the moment of liquefaction would be found to register the same degree of temperature as the ice when first frozen. And yet there has been a vast expenditure of energy between the points of liquefaction and congelation, notwithstanding the temperature of ice may be lowered, after it is formed, many degrees, which is measurable by the thermometer. Suppose we take a piece of ice which is 10 degrees below the freezing point and insert in it a thermometer. If now we apply heat to this ice the thermometer will gradually rise until it reaches the melting point at 32 degrees Fahrenheit, where it will stand until all the ice is melted. The application of heat is going on steadily, but there are no indications of movement in the mercury until the last trace of ice with which it is in contact has been liquefied. After the ice is all melted, if the application of heat to the body of liquefied ice be continued, the column of mercury will resume its movement upward until it reaches the boiling point, where it is again arrested. And no matter how much heat is applied to the boiling water, if in an open vessel, the thermometer remains the same until all the water is evaporated. Here are two curious facts, and they are facts that, if we can master them, will serve as a key to the understanding of much that is mysterious in nature. 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. <hr style="width: 65%;" /> <div style="break-after:column;"></div>

1 of 2

2 of 2