100 



Introduction to the Study of Science 



Water has a property which makes it an excellent medium 

 for heat transfer besides its differences in density due to unequal 

 heating. Every one knows that a hot-water bag is superior 

 to a hot flatiron as a foot warmer. Why this is so is not dif- 

 ficult to demonstrate. 



WATER SUPPLY 



Exercise : Relative heat capacity. Place upon the top of a range 

 where the heat is uniform equal weights of water, iron, and any other 



material that you may have con- 

 venient, as lead, aluminum, stone, 

 or brick. It is soon apparent from 

 the thermometer that the temper- 

 atures of the several substances 

 change very unequally and at 

 different rates of speed. The lead 

 \ ^ becomes hot very promptly, the 

 iron more slowly, and water most 

 slowly. The fact is that the 

 amount of heat necessary to raise 

 the temperature of a given weight 

 of water (one kilogram or one 

 pound) through one degree Centi- 

 grade or Fahrenheit will raise the 

 temperature of an equal weight 

 of aluminum about five degrees, 

 of iron about ten degrees, of silver 

 about twenty degrees, and of lead 

 thirty degrees. Which of these 

 substances takes and holds the 

 largest amount of heat? Which 



FIG. 33. Trace the course of cold 

 and hot water in the system. 



has the lowest capacity for heat ? 



We must consider not only the capacity of a substance for 

 heat, but also the way it gives up heat. A good medium for 

 heat transfer should hold a great deal of heat and at tne same 

 time be capable of surrendering it freely to surrounding objects 

 which are colder. Try this experiment to show the relation 

 between heat capacity and heat delivery and the rank of water 

 among other substances in this respect. 



