WATER, AND ITS USES 87 



contained in the water may convert the whole liquid mass 

 instantly into steam. 



By use of " vacuum pans" having air-tight covers, and 

 with a pressure upon the liquids contained in them kept less 

 than one atmosphere by use of an exhaust (air) pump, the 

 boiling may go on rapidly at temperatures much below 100 C. 

 By reason of a lessened atmospheric pressure due to increase 

 in altitude the boiling point of water is lowered about one 

 degree in temperature for approximately 1000 feet elevation. 

 At Denver one mile above sea level the boiling point is 

 approximately 95 C. 



In the use of rubber bags containing hot water as a source 

 of warmth through a period of several hours, we have an 

 illustration of another important property of water. Differ- 

 ent bodies having the same weight and same temperature 

 give off widely different amounts of heat in cooling through 

 the same number of degrees. This is another way of saying 

 that the same amounts of heat taken up by equal masses of 

 different substances having the same initial temperature 

 give widely different final temperatures to these masses. 



Water has the highest specific heat of the various liquids 

 and solids. Where equal weights of mercury and water are 

 heated equally, the rise in temperature of the mercury is 

 about thirty times greater 1 . Dry land surfaces (soils) heat 

 much faster and get much hotter under a summer sun than 

 bodies of water, cooling off during the night much more 

 rapidly and to a much lower temperature. 



A cubic foot of water weighs about sixty-two and one-half 



!The specific heat of any substance may be defined as the number of cal- 

 ories of heat necessary to raise the temperature of one gram of the substance 

 one degree centigrade. The specific heat of water is i.oo (see page 147), and 

 the specific heat values of the following substances are given merely for 

 illustration: 



Mercury (solid) 0.033 Aluminum 0.214 



Iron 0,113 Ice 0.550 



