TEMPERA TURE. 



307 



points of water ; or, more accurately, the temperature of 

 melting ice and the temper- 

 ature of steam as it escapes 

 from water boiling under 

 a pressure of one atmos- 

 phere. 



478. Determination 

 of the Freezing Point. 



Ice in contact with water cannot 

 be raised above a certain tem- 

 perature ; water in contact with 

 ice cannot be reduced below the 

 same temperature. Here, then, 

 is a temperature fixed and easily 

 produced. The thermometer is 

 placed in melting ice or snow 

 contained in a perforated vessel. 



FIG. 237. 



When the mercury column has come to rest, a mark is made on the 

 glass tube at the level of the mercury. This point is, for the sake 

 of brevity, called the freezing point. 



479. Determination of the Boiling Point. The 



temperature of steam issuing from water boiling under any given 

 pressure is invariable. Fig. 238 represents a metal vessel in which 

 water is made to boil briskly. The thermom- 

 eter being supported as represented is sur- 

 rounded by the steam but does not touch the 

 water. That the steam may not cool before 

 it comes into contact with the thermometer, 

 the sides of the vessel are surrounded by what 

 is called a "steam-jacket." A bent tube open 

 at both ends and containing mercury in the 

 bend is sometimes added. When the mercury 

 stands at the same level in both arms, the 

 pressure upon the surface of the boiling liquid 

 is just equal to the external atmospheric pres- 

 sure, which should be 760 mm. When the 

 mercury column has come to rest, a mark is 

 made on the glass tube at the level of the 

 mercury. This point is, for the sake of 

 FIG. 238. brevity, called the boiling point. 



