MEASUREMENT OF TEMPERATURE 49 



Every kind of matter is now believed to consist of 

 little particles, or molecules, which are constantly moving 

 about hitting and bumping against each other in the 

 spaces which exist between them. The fact that minute 

 invisible particles may be given off by a substance is 

 readily shown by opening a bottle of ammonia or expos- 

 ing a piece of musk in a room. Soon in every part of the 

 room the presence of these substances can be recognized 

 by the odor. Yet nothing can in any possible way be 

 seen to have been added to the air. 



The molecules are too small to be seen by the most 

 powerful microscope. There are millions of them in a 

 particle of matter as big as the head of a pin. When a 

 substance is heated the molecules move more rapidly and 

 strike each other harder. This causes the substance to 

 expand. Heat is defined as being the motion of these 

 molecules. If a condition could be reached where there 

 was no molecular motion, there would be no heat. The 

 effect of heat in causing expansion of gases, liquids and 

 solids has been shown in the preceding experiments. 



25. Measurement of Temperature. From the experi- 

 ments it has been seen that gases, liquids and solids ex- 

 pand when heated and contract when cooled. It has 

 been found that most substances expand uniformly 

 through ordinary ranges of temperature, so 

 that if this expansion or contraction is meas- 

 ured, we are able to determine the change of 

 temperature. 



Experiment 21. Slightly warm the bulb of an air 

 thermometer tube and place the open end in a beaker 

 half filled with inky water. Allow the bulb to cool. 

 The tube will become partly filled with the water. When 

 the bulb has become entirely cooled mark the end of 

 the water column with a rubber band. Grasp the bulb with the 

 hand, thus warming the air in it. The water column will run partially 



