72 FIRST YEAR COURSE IN GENERAL SCIENCE 



water, the sensation is different in the two hands. The 

 water feels cold to the hand which has been in hot water and 

 hot to the one which has been in cold water. This shows 

 that the body sense is not an accurate test of temperature. 

 An instrument for measuring the temperature of a sub- 

 stance is a thermometer. It 

 consists of a glass tube with a 

 bulb or enlargement at one end. 

 To make a thermometer, the 

 bulb and part of the tube are 

 filled with mercury (sometimes 

 called quicksilver), and then 

 the air is removed from the 

 upper part of the tube and the 

 tube is sealed. The sealed tube 

 and bulb are placed in melting 

 ice and the point at which the 

 mercury then stands is marked 

 freezing point. On the Fahren- 

 heit thermometer, commonly 

 used in this country, the freezing 

 point is at 32. 



If heat is then applied to the 

 water around the thermometer, 

 the mercury begins to rise in 

 the tube as soon as all the ice 

 is melted. The temperature 

 does not change until the ice 

 is melted, because the heat 

 energy is used up in changing 

 the solid to a liquid. The heat 

 expands the mercury in the 

 bulb and it must therefore rise in the tube. It continues 

 to rise until the water boils and then the mercury remains 

 stationary. The temperature will remain constant until 



FIG. 29. HEATING WOOD IN 

 A TEST TUBE 



Some dry pieces of wood 

 are put into a test tube and 

 heated until the tube is red hot. 

 1. What kind of change has oc- 

 curred in the tube itself? 2. The 

 wood does not melt; it does not 

 burn because heat expanded the 

 air so that nearly all of it left the 

 tube; gas and smoke come from 

 the wood, which grows black. 

 What kind of change has taken 

 place there? 3. Why do you 

 think it was such? 



