EVAPORATION BOILING, TEMPERATURE, ETC. 13 



Exercise 11. How Heat Affects the Thermometer 



Fill a small flask with water. Fit a glass tube, 12 or 15 in. long, 

 into a one-hole rubber stopper. Press the stopper down tightly into 

 the flask. This should force the water well up into the tube. 



Light an alcohol lamp or Bunsen burner. Hold the flask for about 

 one second in the flame. Notice carefully the first movement of 

 the surface of the water. Does it rise or fall at first? What does it 

 do later? Repeat the experiment till you feel certain that you see 

 that the second motion is opposite to the first motion. 



The first motion is due to the fact that the heat first strikes 

 the glass and expands it. This makes the flask larger in- 

 creases its volume. If the volume of the flask grows suddenly 

 larger, what would you expect to see happening to the sur- 

 face of the water? The second movement is due to the fact 

 that, when heated, the water increases in volume much more 

 rapidly than the glass vessel does. 



The common thermometer consists of a very small glass 

 tube ending in a glass bulb at its lower end. This bulb and 

 the tube are partly filled with mercury or alcohol. The air 

 is nearly all removed from the upper portion of the tube. 

 The tube is then closed by heating the glass till it softens and 

 closes together. The principle of the thermometer may be 

 summed up thus: When heatedj the glass in the thermometer 

 expands, increasing the capacity of the bulb and tube, but the 

 heat soon penetrates to the liquid which then expands to a 

 still greater extent. Alcohol expands more than 40 times as 

 much as does the glass vessel, and mercury expands about 7 

 times as much as does the glass vessel for a given change in 

 temperature. 



16. The Fixed Points of Temperature in Nature. The ther. 

 mometer just described is still without any scale. In placing 

 a scale upon it, the position of the surface of mercury or alcohol 

 is marked when the thermometer is cooled or heated to some 

 certain FIXED TEMPERATURES IN NATURE. The distance be- 

 tween the lower and higher temperature marks is then divided 

 into a certain number of equal parts called DEGREES. 



1. It has been long known that water always freezes and 



