60 HEAT 



by saying that the particles are crowded closer together. 

 Scientists believe that matter really consists of such 

 particles, and call them molecules, or " little masses." 

 The molecules are believed to be in motion. Each 

 molecule needs a portion of space in which to move 

 about; hence the volume of a body is the sum of the 

 spaces needed by all the molecules, in addition to the vol- 

 umes of the molecules themselves. 



The energy of the molecules is heat. When we add heat to a body, 

 each molecule moves more rapidly, and pushes its neighbors farther 

 away. Because of this the distance between molecules becomes 

 greater, and the volume of the body increases. We assume that 

 when bodies expand and contract the volumes of the molecules remain 

 unchanged. 



The molecules are very small. Scientists estimate the number of 

 molecules in 1 c.c. of a gas as about 27 million million millions (27,000,- 

 000,000,000,000,000). 



62. Thermometers. We make use of the expansion 

 and contraction of matter in the thermometer, the instru- 

 ment with which we measure temperature. The common 

 thermometer consists of a glass tube having a bulb at one 

 end. The bulb and part of the tube contain mercury. 

 The inside of the tube is of a very small bore, so that if 

 the mercury expands only a very little in the bulb, it will 

 make a great difference in the length of the mercury 

 "thread" inside the tube. Glass expands when heated, 

 as well as mercury, but only V 7 as much; hence the ex- 

 pansion we see in a thermometer is the difference between 

 the expansion of mercury and that of glass. 



When a thermometer is made, the end of the tube is open, and there 

 is enough mercury to fill the bulb and a little of the tube. The instru- 



