14 ELEMENTS OF GENERAL SCIENCE 



many ways. For instance, an empty bottle may be inverted 

 and its mouth placed under water, and the bottle warmed 

 by holding it between the hands (fig. 13) or by using a 

 flame. The air will soon be warmed, and if it expands, bub- 

 bles of air will overflow from the mouth of the bottle and 

 rise to the surface of the water. What other ways can you 

 devise to test the notion that air ex- 

 pands when heated ? 



13. Thermometers. Air expands when 

 heated and contracts when cooled. It 

 is possible to arrange apparatus in such 

 a way that this expansion may be ob- 

 served and measured. An instrument 

 by means of which such measurements 

 could be made would give us some 

 idea of the degree of heat. It would, 

 in fact, be an air thermometer. Such 

 thermometers have been constructed 

 and used, but only for special pur- 

 poses. For ordinary purposes much 

 more satisfactory thermometers are 

 made from other substances. 



Thermometers such as are in com- 

 mon use depend upon the expansion 

 and contraction of a liquid. The liquid 

 most commonly used is mercury, though 

 alcohol which has been colored red or 



blue is sometimes used. Each thermometer is furnished with 

 a scale of degrees by means of which the temperature may 

 be read. There are two sorts of scales in use in this country 

 (fig. 14). The one in most common use is called the Fahren- 

 heit scale, after the man who designed it. On this scale the 

 freezing point of water is 32 and the boiling point is 212. 

 The centigrade scale, which is the one used in all scientific 

 work the world over, places the zero mark at the freezing 



FIG. 13. Expansion of air 



When heated by the hands 

 the air in the flask expands, 

 and bubbles of air appear 

 at the open end of the tube 

 under the water 



