Science for the School and Family, Part I. Natural Philosophy

by Worthington Hooker

323. Relation of Latent Heat to the Forms of Substances.—Whether a substance shall be in the form of a solid, liquid, or gas depends upon the amount of heat which is latent in it. If you take a piece of ice and melt it in a vessel, the ice and the water in the vessel that comes from the melting of the ice are both at 32° until the ice is all melted. But all this time heat is being communicated to the ice and water. What becomes of it? It is all taken up by the ice as it changes from its solid to its fluid state, and becomes latent in it. In fact every particle of ice must have just so much of latent heat in order to become fluid. So, also, if water be heated to the boiling point, 212°, and be kept boiling, the water will remain at that point till it is all vaporized. All this time the water is receiving heat, which, instead of raising its temperature, is becoming latent in the particles as they change from their liquid to their vaporous state. As I said of the change from the solid to the liquid state, so here, every particle of the liquid must have just so much of latent heat in order to become aeriform. Whenever therefore any solid substance becomes liquid, or liquid becomes aeriform, heat is absorbed and becomes latent. So, on the other hand, whenever any aeriform substance becomes liquid, or liquid becomes solid, latent heat is given out, and becomes free and sensible. The freezing of water, then, is a source of warmth to the air in its neighborhood—a fact which is practically made use of when tubs or pails of water are placed in conservatories to keep plants from freezing; and the thawing of snow and ice is a source of cold, as is exemplified by the chilliness of the air occasioned by this process.

324. Clouds and Latent Heat.—The water of which clouds are composed is heavier than air. Why, then, does it remain suspended? Why is it necessary that it should be collected into drops in order to have it fall? This question can be answered by looking at the manner in which clouds are formed. A cloud, I have stated in § 288, is made up of minute vesicles or bubbles containing air. Now the air in these bubbles is lighter than the air that is around the cloud, because it is warmer. But how does it get its heat? In order to understand this observe what the bubble is made from. It is made from the water which was in the air in a state of vapor, or in its aeriform state, for this is the state of water that is evaporated and dissolved in the atmosphere. But when it forms the vesicle it goes out of this state and becomes a liquid, for the wall of the vesicle is a liquid wall, just as the wall of a soap-bubble is. Now in passing out of the aeriform into the liquid state some latent heat must be made sensible. What becomes of this sensible heat? It just heats the air in the vesicle, and so makes it like a heated air-balloon. So all clouds are collections of innumerable heated air-balloons, and the reason that some clouds are higher up than others perhaps is that their balloons have warmer and therefore lighter air in them.

325. Freezing Mixtures.—The intense cold produced by these mixtures is the result of the change of free or sensible heat into latent. For example, when salt and snow are mingled together a melting of the two is quickly produced. In this sudden change of a solid into a fluid a great quantity of heat must be rendered latent, and therefore there will be a great loss of sensible heat by whatever the freezing mixture comes in contact with. The process here, you see, is the opposite of solidification in relation to latent heat. A portion of the snow, after melting with the salt, becomes solid ice. Why is this? It is because it gives up its sensible or free heat to portions of the snow that are in the process of melting and are therefore making heat latent.

326. Cold from Evaporation.—If you pour a little ether into the palm of your hand it will rapidly disappear in vapor, producing a very cold sensation. This sensation occurs because, in the change of the liquid into the vaporous or aeriform state, some of the sensible heat of your hand is abstracted to become latent in the vapor. The evaporation of water also produces cold, though not as decidedly as ether, because its change into vapor is not so rapid at ordinary temperatures. We make a practical use of the evaporation of water in many different ways. Thus we sprinkle water in a hot day upon the floors of piazzas, steps, etc., that the evaporation may make much of the sensible heat about our houses latent. For the same purpose, in hot climates, apartments are often separated from each other by mere curtains, which are occasionally sprinkled with water. So the inhabitants of such climates often cool their beverages by keeping a wet cloth for some time wrapped around the vessels that contain them. Evaporation is an important remedy for many cases of disease. For example, if the head be hot, a steady application of a wet cloth to the forehead, though a simple remedy, is generally effectual, and sometimes is very important. Most people make the application in a wrong manner. They put on several thicknesses of cloth, when a single thickness is the best, because it will best secure the evaporation, which is the cause of the relief afforded.

Fig. 221.

327. Freezing in the Midst of Boiling.—It is from the quantity of heat rendered latent by evaporation that water can be frozen in the midst of boiling ether; and, paradoxical as it may seem, the boiling of the ether is the cause of the freezing. The experiment is performed in this way: Place a test-tube or a little thin vial with water in it in the midst of some ether in a shallow vessel under the receiver of an air-pump. On exhausting the air the ether will boil, evaporation taking place rapidly because the pressure of the air is taken off from the ether. Now as the ether passes into vapor it extracts so much free heat from the vial of water that the water is cooled down to the freezing point, and so becomes solid. Water can be frozen even by its own evaporation. It is done in this way: Let a shallow vessel, b, Fig. 221, contain a little water, and the vessel c oil of vitriol or sulphuric acid. When the air is exhausted, the pressure of air being taken off from the water, vapor rises from it freely. As the sulphuric acid has a great attraction for water it absorbs this vapor, and so vapor continually rises from the water the more rapidly because what is formed is absorbed, instead of remaining to make pressure on the water. The result is that this rapid formation of vapor, requiring that a great quantity of heat should be made latent, at length abstracts so much heat from the water that remains that this becomes solid.

328. Degree of Heat Endurable by Man.—It was formerly believed that the human body could not endure with impunity, even for a short time, a much higher degree of temperature than that which is met with in hot climates. But in the year 1760 it was accidentally discovered that a much higher temperature than this could be endured. An insect was destroying at that time the grain gathered in some parts of France, and it was found that if the grain was subjected to a certain high degree of temperature the insect was killed, and yet the grain was not injured. In trying some experiments in regard to this matter the experimenters wished to know the point at which the thermometer stood in a large oven. A girl attending on the oven offered to go in and mark the thermometer. She did so, remaining two or three minutes, and the thermometer was at 260°, that is, 48° above the boiling point of water. As she experienced no great inconvenience from the heat she remained ten minutes longer, when the thermometer rose to 76° above that point. These facts were published, and prompted scientific men to try other experiments. In England, Dr. Fordyce, Sir Charles Blagden, and others, went into rooms heated even to 240° and 260°, and remained long enough to cook eggs and steaks, and yet themselves suffered little inconvenience. The pulse was quickened, the perspiration was very profuse, but the heat of the body, as ascertained by putting the thermometer under the tongue the moment they came out, was scarcely raised at all. The air in which they were roasted eggs quite hard in twenty minutes, and when it was applied by a pair of bellows to a steak it cooked it in thirteen minutes. The question arises, how is it that this high degree of heat did not produce more effect upon the body? One reason is that the heat of the air in the immediate neighborhood of the body was continually reduced by the evaporation of the free perspiration, sensible heat being thus converted into latent. Another reason is that the air is not a good conductor, and therefore did not communicate its heat readily to the body. Dr. Fordyce and his friends found that they could not touch with impunity any good conductor, as the metals, and they were obliged to wear upon their feet some non-conducting substance.

329. Formation of Ice.—Before dismissing the subject of heat I must notice the grand exception which we have to some of the operations of heat in the formation of ice. Heat generally produces expansion. But in the case of water this law of expansion is set aside, and the reverse is established. This is done, however, only within a small range of temperature, viz., from the freezing point up the scale about seven degrees. In all degrees above that the usual expansion by heat takes place. The exception occurs at this part of the scale for a special purpose, viz., that water, in distinction from other substances, shall become more bulky, and therefore lighter, as it takes the solid form.