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remains behind. In actual practice a certain amount of steam passes over with the alcohol vapor, and the alcohol vapor and steam pass through another vessel surrounded with boiling water, thus

FIG. 5.

condensing out the steam, while the alcohol which does not condense at so high a temperature passes on.

This process is called fractional distillation.

In order to ob

tain alcohol which is very nearly pure, or absolute alcohol as it is called, the distillation is repeated several times.

CONDUCTION.

If we hold one end of a bar of iron in the fire the end which we hold soon becomes hotter than we can bear. This is because it has been heated by conduction, the molecules first heated giving some of their heat to those adjacent and thus passing it on to those beyond, i. e., there is a transfer of motion from molecule to molecule. This process by which heat passes from the hotter to the colder parts of a body is called conduction of heat. It is gradual and does not depend upon the shape of the bar, whether it is crooked or straight.

If we use a rod of glass, the end in the fire would melt but the other end would not become very warm. If we use a wooden stick, the end in the fire would burn and but very little heat would be transferred to the end held. Thus we see that some substances are good conductors of heat, while others are not.

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If a bar of iron and one of copper of equal lengths be placed end to end and heated equally at the point of junction, the farther end of the copper bar will become hot before the farther end of the iron bar. The following table gives the relative thermal conductivity of a few metals.

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NOTE. The above metals would be arranged in the same order according to the conduction of the electric current, and thus a relation is suggested between heat and electricity.

Liquids and aeriform bodies are poor conductors of heat. Mercury which is a metal is the only fluid which may be readily heated throughout.

CONVECTION.

Water should be heated from below; the heated molecules expand and rise, the cooler ones descend to take their place nearest the source of heat. In this way currents are produced in the water.

Suppose we have a tube whose sides form a rectangle, and a portion of the water in it is colored by some coloring matter. Then it we heat one side of the rectangle the water on that side begins to rise, while the water from the side of the rectangle away from the heat descends and flows across to take the place of the water which has risen. The colored part of the water shows the movement or the current. The same thing takes place in any vessel containing water, the water rising in one part (usually the center) and descending in other parts (usually those parts next to the sides of the vessel). In this way the water becomes heated throughout. This method of diffusing heat by actual motion of heated fluid masses is called convection. It is the result of expansion by heat and the action of the force of gravity.

Convection takes place also in gases; it takes place in the air thereby giving rise to many phenomena, such as wind, rain, etc.

We have seen that in

RADIATION.

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.

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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,—

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Hence the heat received at a distance of 12 feet is 21 units.

EXAMPLES FOR PRACTICE.

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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

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