d. Iron tires are heated, slipped on to wagon wheels and then cooled,

the contraction on cooling setting them tightly in place.

e. Metallic thermometers depend upon the movement due to the expansion of a coiled strip of metal which turns a pointer on the dial of the instrument. (See Fig. 129.)

f. The wires that are fused into glass in incandescent light bulbs must have the same coefficient of expansion as the glass. Platinum has therefore been used for this purpose. (See table above.)

Important Topics

1. Expansion of Liquids; peculiarities. Anomalous expansion of water and its results.

2. Expansion of solids; peculiarities, applications.

3. Coefficient of linear expansion.

4. Coefficient of cubical expansion.

Exercises

1. The gas within a partly inflated balloon has a volume of 1000 cu. ft. at a pressure of 74 cm., and a temperature of 15°C. What will be the volume of the gas when its pressure is 37 cm. and the temperature is -17°C.?

2. A man taking a full breath on the top of a mountain fourteen thousand feet high inhales 4 liters of air, the pressure being 40 cm. What volume would this same mass of air have in a place 600 ft. above sea-level when the barometer reads 75 cm. and the temperature is the same as on the mountain top?

3. If the coefficient of linear expansion of iron is 0.000012 per degree C., how much will an iron bridge 1000 ft. long change in length in warming from -20°C. on a winter day to 30°C. upon a summer day.

4. What are some of the results that would follow in freezing weather if water continually contracted on being cooled to zero instead of beginning to expand when cooled below 4°C.?

5. Mention two instances that you have noticed of expansion occurring when a body is heated?

6. Compare the density of air at 30°C. with that at 10°C. at the same pressure. If both are present in a room, where will each be found? Why?

7. Compare the density of water at 40°C. with that at 10°C. If water at the two temperatures are in a tank, where will each be found? Why?

8. If water at 0°C. and at 4°C. are both in a tank, where will each be found? Why?

9. How much heat will be required to raise the temperature of a cubic foot of water 10°F.?

10. How much heat will be required to raise the temperature of 4 liters of water 25°C.?

11. How much longer would the cables of the Brooklyn suspension bridge be on a summer's day when the temperature is 30°C. than in winter at -20°C., the length of cable between the supports being about 1600 ft.

12. If 25 liters of air at -23°C. is warmed to 77°C. under constant pressure, what will be the resulting volume of air? Explain.

13. White pig iron melts at about 2000°F. Express this temperature upon the centigrade and absolute scales.

14. If 200 ccm. of air at 76 cm. pressure and 27°C. temperature be heated to 127°C. at a pressure of 38 cm. what will be the resulting volume?

15. A balloon contains 10,000 cu. ft. of gas at 75.2 cm. pressure and 24°C. It ascends until the pressure is 18 cm. and the temperature is -10°C. What is the volume of gas it then contains.

16. A gas holder contains 50 "cu. ft." of gas at a pressure of one atmosphere and 62°F. How much gas will it hold at 10 atmospheres and 32°F.

17. One thousand "cubic feet" of illuminating gas has what volume with 75 lbs. pressure and temperature of 10°C.

18. Define a "cubic foot" of illuminating gas.

=150. Methods of Transmitting Heat.=--One of the most practical benefits of the study of heat is clearer understanding of the different methods by which heat is transferred from one place to another and an intelligent idea of the means employed to prevent the transfer of heat.

It should be definitely understood at the beginning that _cold signifies the absence of heat_, just as darkness implies the absence of light, so when one speaks of cold getting into a house what is really meant is either the entrance of cold air by some opening or else the escape of the heat.

There are three distinct methods by which heat energy is transferred from one place to another, depending upon the medium or substance that transfers the heat.

a. A solid transmits heat by the method called _conduction_.

b. A fluid, either a liquid or a gas, transmits heat mainly by the method called _convection_.