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100° as the zero mark is below 100°, the new mark will be 200°. In this way we can mark mercury thermometers up to the temperature at which mercury boils and down to that at which it freezes. Why would not water be as good as mercury for a thermometer?

200

C.

F.

100

212

90

190

80

180

170

70 160
E150
60 E 140
130

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50 120

110

40

-90

30 E

20

10

10

20

30

289

FIG. 42. In the Centigrade thermometer the

freezing point is

called 0° C. and

the boiling point

100° C.

66. Exercises. 1. Which is really the colder, a metal door knob or the wood of the door? Which feels colder? Why? Do your woolen mittens, or your rubbers, feel the colder, if you have left both out of doors? Why?

2. Why can ice cream be carried in a paper box through heated air and yet melt very little? Would a tin box keep it better? Why?

3. Why are houses built with double walls having an air space between the walls?

4. Why is the knob on the cover of a teakettle made of wood?

5. Why does ice keep through the summer when packed in sawdust?

6. If you put the bowl of a silver spoon into a cup of hot water, the handle becomes hot; explain. Would the handle of a wooden spoon become hot too? Put an iron (“tin ") spoon and a solid silver spoon into a cup of hot water at the same time. Which handle becomes hot the sooner? Explain.

7. What method of heating is used in your house and school? Is anything done to keep the air moist? 8. How could you make an air thermometer? 9. Should steam pipes be put near the floor, or near the ceiling, in order to heat the room? Why? Where should the cold-brine pipes be put in a cold-storage room in order to cool the room? Why?

10. What are some of the advantages and disadvantages of the different heating systems?

CHAPTER IX

MORE ABOUT HEAT

67. Are Heat and Temperature the Same? - What is the temperature of any object? We have learned that it is the hotness or coldness, or degree of heat, of the object. Boiling water has a temperature of 100° C.; lukewarm water has a temperature of about 40° C.; cold water has a temperature of 0° to 10° C. It makes no difference, when you are speaking of the temperature of water, how much water you have; a cupful of boiling water has exactly the same temperature, or degree of heat, as a pailful has. But has it the same amount of heat? If you heat water over a gas burner, which requires longer heating, the cupful, or the pailful? Of course, the pailful takes the longer heating. If you used the water to warm yourself, which would give you the more heat? Of course the pailful would give you the more. In a house heated by hot water, we know that a large room requires a greater amount of hot water, that is, a larger radiator, than a small room does. From all these cases, we say that the amount of heat in a body depends not only upon the temperature of the body, but also upon the amount, or weight, of matter in the body.

68. Can Heat Be Measured? If you want to warm some cold water, you can heat it over a fire, or you can put some hot water with it. Is there any way of know

ing just how much hot water to use in order to heat the cold water to any given temperature? If you mix a pound of water at 100° C. with a pound of water at 0° C., what will the result be? You will have two pounds at 50° C. If you mix 1 pound at 100° C. with 2 pounds at 0° C., you will get 3 pounds at 333° C. One pound at 100° C. with 3 pounds at 0° C. will give 4 pounds at 25° C., and so on. The cold water gains the heat which the hot water gives up.

There is a great need for a unit of heat, so that we can speak of the quantity, or amount, of heat, just as we speak of the amount of sugar or iron. The amount of heat that is needed to heat a pound of water through one degree, Fahrenheit, as from 32° F. to 33° F., is called a British Thermal Unit (B. T. U.). The amount of heat required to heat 1 gram of water through one Centigrade degree, as from 15° C. to 16° C., is called a calorie. If you put a piece of hot iron into 1 gram of water and heat the water from 0° to 10° C., the iron adds 10 calories of heat to the water. If 100 grams of water at 10° C. are put into an ice box and cooled to 0° C., the water gives off 1000 calories of heat to the ice box. Since food is used to heat the body and to enable it to do work, we measure the value of food in calories.

What becomes of

69. Is Heat Needed to Melt Ice? the ice in an ice box? It melts. What causes it to melt? Of course the cause is the heat that is given to the ice by the box itself, by the air in the box, and by the food that is put into the box. As these give their heat to the ice, they themselves are cooled.

The amount of heat needed to melt 1 gram of ice is able to heat 80 grams of water from 0° C. to 1° C.; that is, 80 calories. It is because ice takes up so much heat in being changed to water that it cools the air in the box (cf. Fig. 43). People

often speak as though we added "cold" to the food. Does ice conduct or radiate "cold"? When we think of the matter, we see that cooling anything is not adding "cold" to it, but taking heat from it. The food is cooled because it gives heat to the ice. By losing heat the food becomes cold. If you put enough chipped ice into water, the water is cooled to

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70. How Does the Body Keep Its Heat? - First let us ask how the body gets its heat? We learned in § 46 that the heat comes from the oxidation of our food (cf. Fig. 44). The excess of heat produced in some organs is given to the blood. The blood, by circulating through the body, makes the temperature of the body nearly the same everywhere.

Does clothing make us warm or cool? You will see that when you put on clothing in a cold room, the cloth

But it keeps of the body

ing has the same temperature as the room. your body warm by preventing the heat from escaping. If you wished to keep a piece of ice from melting, would you wrap it in woolen cloth, or in cotton cloth? Tramps put newspapers inside their coats, instead of putting on overcoats. Why? Clothing for preventing the escape of heat from the body should be of a material which is a nonconductor of heat.

To heat the

FIG. 44.

body.

about

Respiration. 1/60

Heart, 16

A
Day's
Work. 3/16

What becomes of the heat we obtain from

our food?

Wool is the best of our common materials for warm clothing. Woolen goods are made from the natural covering of sheep, animals which live through very cold winters in the open. The feathers of birds keep the birds warm, not because the feathers are nonconductors of heat, but because they are loose. Did you ever By doing so they imprison

see

birds fluff out their feathers? a great deal of air in the meshes of the feathers. It is this air that is the real nonconductor and keeps the bird warm. Linen and cotton do not hold so much air as wool and are therefore better heat-conductors than wool. They make better clothing for summer than for winter.

71. Why Does Perspiration Cool the Body? - Perform this experiment : Wet your hands with water having the temperature of the room, and wave them rapidly to and fro, until the water has evaporated. Do

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