Many substances are capable of existing in all three states. Iron, for instance, may be solid as we ordinarily see it, or liquid as it comes from a blast furnace, or a gas, as it exists in the tremendously hot atmosphere of the sun. Substances usually expand in volume as they change from the solid to the liquid state and they always do as they change from the liquid to the gaseous. Ice is a notable exception to the general rule, since when water freezes its volume increases. If it were not for this, ice would not float. Metals that are suitable for casting must have the property of expanding when cooling or at least of shrinking but a trifling amount. This is a most valuable property of type metal and cast iron.
27.The Transference of Heat.
Experiment 23. -Cut off 15 cm. of No. 10 copper and No. 10 iron wire and the same length of glass rod of about the same diameter. Holding each of these by one end place the opposite end in the flame of a Bunsen burner. Which of the three conducts the heat to the hand first?
Experiment 24. -Fill a test tube about 3/4 full of cold water.
Holding the tube by the bottom carefully heat the top part of the water until it boils. Be sure that the flame doesnot strike the tube above the water, else the tube will break. A little piece of ice in the bottom of the test tube makes the action more apparent. A bit of wire gauze ora wire stuffed into the test tube will prevent the ice fromFig. 27.
coming to the surface. Water conducts heat poorly. The hot water does not sink. It must be lighter than the colder water.
Without the heat of the sun there would be no life upon the earth, no flowing streams, no changing winds, none of the restless energy which makes the world as we know it. It is therefore essential to understand how heat is transferred from one place to another.
Through solid substances, such as metals, heat travels quite readily, through others such as glass, less rapidly. In Experiment 23, we found that heat traveled along some rods faster than it did along others. In no case, however, was there any indication that there was a transference of the particles composing the rods. In the boiling of the water at the top of the test tube, there was no indication that the water particles moved to the bottom of the tube. In these cases, the heat is simply transferred from molecule to molecule.
This kind of heat transference is called conduction. Conductorsmay be good or bad, as was shown by the different materials used in the experiments. We use iron for our radiators so that the heat of the steam may readily be given out to the room, and we cover our steam pipes with asbestos when we wish to retain the heat, because asbestos is a poor conductor and will keep the heat in the pipes.
Experiment 25. -Hold a piece of burning paper under a bell jar held mouth downward. Notice the air currents as indicated by the smoke. Paper soaked in a moderately strong solution of saltpeter and dried, burns with a very smoky flame.
Experiment 26. -Fill a 500 cc. round-bottomed flask half full of water and place on a ring stand above a Bunsen burner. Stir in a little sawdust. Some of itFig. 28.
should fall to the bottom of the flask. Gently heat the bottom of the flask.
Notice the currents.
When the water was heated at the bottom of the flask and when the burning paper was held under the bell glass, currents were seen to be developed. The heated and expanded water and air rose. Here the heat was transferred by the upward movement of the heated water and air. This method of heat transference is known as convection. The efficiency of the hot water and hot air furnaces which heat our houses is due to the convectional transference of heat. We shall find later that if it were not for convection there would be no winds or ocean currents.
Fig. 29.
If an incandescent electric lamp is turned on and the hand held immediately below the lamp, it will be warmed, although the glass bulb itself, a poor conductor of heat, remains cool. The white-hot filament is surrounded by an almost perfect vacuum. It can set up no convection currents, neither does the cool glass. The sensation of heat cannot be due to conduction because the air which surrounds the bulb is not in contact with the filament. It is also a poorer conductor than glass and the glass itself does not become hot for some little time.
There must therefore be another mode of transferring heat beside conduction and convection. It also appears that in this method ofHOT WATER FURNACE.
The hot water rises from the top, passes through the radiator and returns as colder water to the bottom.
transferring, no material substance is necessary, as the hot filament is surrounded by an almost perfect vacuum. Now astronomers tell us that there is no material medium between our atmosphere and thesun and that the heat of the sun travels to us with the tremendous speed of light, 186,000 miles per second, and does not warm the intervening space. The convection and conduction processes are, when compared to this, very slow. Radiation is the name given to this method of heat transference. If heat did not travel in this way the earth would be uninhabitable.
If a body, heated to ordinary temperatures, is surrounded by substances which do notreadily permit of conductional or convectional heat transference, the heat is retained within the body. Application of this is made in the fireless cooker and the thermos bottle (Fig. 30). In one, the hot substance is surrounded by felt, wood fiber, asbestos or similar nonconducting substances, and in the other by glass and a space from which the air has been nearly exhausted.
Both of these arrangements prevent the transference of heat from the hot body. The cooking thereforeFig. 30.
continues in the fireless cooker and the liquid in the thermos bottle remains warm for a long time or, if cold when put into the bottle, it remains cold, as the heat from the outside cannot reach it. Clothing is placed upon the body in order to prevent the body heat from being conducted to the surrounding air.
