RADIATION. We have seen that in a heated body the molecules vibrate with an excess of rapidity. The motion of these vibrating molecules is communicated to the ether, and transmitted by it with great velocity in the form of waves. If near a fire we feel the heat from it and heat thus propagated by the ether, instead of by material means, is called radiant heat, while the process is called radiation. In this way the sun transmits heat to the earth. That radiant heat traverses a vacuum may be shown experimentally. Radiant heat travels in straight lines through a uniform medium and the radiation is equal in all directions. Thermometers placed at equal distances from a source of heat show the same temperature. The amount of radiation depends upon the temperature of the source and is inversely proportional to the square of the distance. Hence a differential thermometer at a distance of 2 feet would only show the radiation that it would at 1 foot. If heat rays strike a surface they may be transmitted, absorbed or reflected. If they are transmitted they may be refracted, or bent, as light is by means of a lens, Fig. 6. Rock salt crystals FIG. 6. transmit nearly all the rays, absorb very little and reflect very little. Polished silver reflects nearly all, absorbs a little and transmits none. Charcoal absorbs nearly all, and hence increases in temperature quite rapidly when under the influence of heat rays. Bodies that transmit radiant heat freely are called diathermanous, and those that do not are called athermanous. These terms are to heat what transparent and opaque are to light. Heat may also be reflected, the angle of incidence equaling the angle of reflection, or in other words, if a perpendicular be drawn to the surface at the point where the heat ray strikes, this heat ray will make the same angle with the perpendicular before reflection that it will after reflection. If parallel rays, such as those of the sun, strike upon a spherical or parabolic mirror they are reflected to a focus. Suppose that the mirror at the right, Fig. 7, has a source of heat at its focus. Then it will be found that the ball at the focus of the left hand mirror will be heated by reflected rays. H FIG. 7. Suppose a body receives 10 units of heat and is at a distance of 6 feet from the source. How may we find the amount of heat received at a distance of 12 feet? Since the heat radiated is inversely proportional to the square of the distance, the amount of heat received at a distance of 12 feet must be that received at a distance of 6 feet. Expressed as an equation we have,— Hence the heat received at a distance of 12 feet is 21 units. EXAMPLES FOR PRACTICE. 1. If a body receives 8 heat units at a distance of 6 yards from the source, how many heat units will it receive at a distance of 24 yards? Ans. heat unit. 2. A body shows a temperature of 80° C. when at a distance of 2 feet from a source of heat; what will be its temperature dependent upon radiant heat received at a distance of 4 feet from the source? Ans. 20° C. LATENT HEAT. The units used in measuring heat are called thermal units. The one most used in this country is called the British thermal unit. It is the amount of heat necessary to raise 1 pound of water at maximum density (39.2° F.), one degree F. in temperature. In the French system we have the gram-calorie which is the amount of heat necessary to raise 1 gram of water from 4° to 5° C. and the kilogram-calorie, which is 1000 times as great, is the amount of heat necessary to raise a kilogram of water from 4° to 5° C. It does not make any practical difference what unit is used; it is a matter of convenience, but the unit must not be changed in the course of any calculation. Suppose, now, that we take a block of ice at -10° C. (14° F.) and heat it. A thermometer placed in it rises to 0° C. The ice then begins to melt, but the thermometer does not indicate any further rise in temperature. It remains at 0° C. until the ice is melted and then begins to rise again. If we continue to apply heat, the water rises in temperature until it reaches 100° C. (212° F.) when the water begins to change into steam and the thermometer shows no further rise. By this we see that we have applied a very considerable amount of heat without producing any rise in temperature. This heat which does not produce any rise in temperature is called latent (the word meaning hidden) heat. Latent heat may be defined as the heat that must be added to a body in a given state to change it into another state without altering its temperature. The latent heat which is required to melt ice, or any other solid, is called the latent heat of fusion. In the case of ice we may represent the action of latent heat by the following equation. Water at 0° C. ice at 0° C. + latent heat of water. If we dissolve a solid, i. e., put it into solution, there is a diminution of temperature and this diminution depends upon the fact that in the process of solution as well as in fusion, heat is rendered latent. This latent heat of solution is the basis of the action of freezing mixtures. Thus when ice is melted by salt the water formed dissolves the salt itself and this double liquefaction, or the melting of the ice and the dissolving of the salt, abstracts a certain amount of heat from whatever may be in its proximity. In the ice-cream freezer the heat is taken from the cream, thus lowering its temperature and causing it to freeze. A freezing mixture of 1 part salt and 2 parts snow by weight gives a temperature of about 18° C. and furnishes the zero adopted by Fahrenheit. Solidification takes place when a liquid changes to the solid condition. During solidification we have an increase in heat. Sensible heat which disappeared as latent heat during liquefaction is no longer employed in maintaining a liquid state, and is therefore re-converted and is immediately employed in increasing the molecular vibrations; in other words the molecular potential energy is transformed into molecular kinetic energy. The melting point and the solidifying point of the solid substance are identical, but it is possible to keep substances in a liquid condition at somewhat lower temperatures. If water is perfectly quiet it sometimes cools several degrees below the melting point without freezing, but the slightest agitation, such as tapping on the vessel in which it is contained, causes solidification to take place immediately. When this takes place the temperature rises instantly to the freezing point. Most substances shrink when they solidify but a few, among which are ice and cast iron, expand upon solidification. Antimony and bismuth expand, and for this reason antimony is used as one of the constituents of type metal. Gold coins have to be stamped, and clear cut castings cannot be obtained from lead because these metals contract when they solidify. Water expands considerably when changing to ice. This is apparent from the fact that vessels filled with water will be broken if exposed to a low temperature. When a liquid is vaporized there is, as we know, a disappearance of a large quantity of heat, and frequently a diminution in temperature. There is also a change of sensible into latent heat; of kinetic into potential energy. We may represent a simple case by the following equation. Steam at 100° C. water at 100° C. + latent heat of steam. = Let us now see how great an amount of heat is latent in the cases of fusior. and vaporization of water. If we mix a pound of water at 0°C. with a pound of water at 80° C. we have as a result 2 pounds of water at 40° C., but if we mix a pound of ice at 0° C. with a pound of water at 80° C. we have 2 pounds of water at 0° C. Hence, the heat which before was used to raise the temperature of the water has been used in melting the ice. The amount of heat required to melt a quantity of ice without changing its temperature is 80 times as great as the heat required to warm the same quantity of water 1° C. In the case of vaporization of water, the amount of heat necessary to evaporate a weight unit of water is the amount sufficient to raise the temperature of 537 weight units 1° C. In other words, the amount of heat necessary to evaporate a certain quantity of water without changing its temperature is 537 times as great as the heat required to raise the same quantity of water 1° C. If instead of the above method we use the English system, then the amount of heat necessary to convert a pound of water at 212° into steam at 212° is 966 British thermal units, or sufficient heat to raise 966 pounds of water 1° F. (the water being at maximum density and the pressure one atmosphere). The total heat of evaporation for a given temperature is the amount of heat which is added to change water at freezing, 32° F., into steam at the given temperature. It is the sum of the sensible and latent heats at that temperature. Let us see what the total heat of evaporation is for 1 pound of water at freezing, to steam at boiling temperature and atmospheric pressure. The sensible heat is 212 32 180 British thermal units, and the amount of latent heat to change the water at 212° into steam at atmospheric pressure is 966 British thermal units. Hence, the total heat applied is 180 +966 = 1146 British thermal units, which is the total heat of evaporation in the above case. The following table gives the sensible, latent and total heats of evaporation for 1 pound of water at various pressures. Temperature Pressure in Sensible Heat Latent Heat Total Heat We see that in the last column the figures do not vary to any |