47 6 PHYSIOLOGICAL PHYSICS. [Chap, xxxvin. 



layer of steam between the drop of liquid and the hot 

 metal does not permit sufficient heat to radiate to the 

 drop to boil it. As soon as the metal cools so that it 

 is no longer able to produce steam of sufficient ten- 

 sion, the drop comes into contact with the metal, and 

 at once hisses and boils away. 



All volatile liquids show this property. It is in 

 virtue of it that the hand, moistened with water, can 

 be momentarily plunged into a mass of red-hot metal 

 without being burnt. A layer of vapour is formed 

 between the moist tissue and the hot metal, which 

 acts as a protective. This experiment must be per- 

 formed with rapidity. 



CHAPTER XXXVIII. 



SPECIFIC HEAT : CALORIMETRY. 



Specific heat. It takes a much larger quan- 

 tity of heat to raise the temperature of 1 pound of 

 water by 1 than to raise the temperature of the 

 same weight of lead by 1. This is expressed by 

 saying that water has a greater capacity for heat than 

 lead, or that the thermal capacity of water is greater 

 than that of lead. The thermal capacity of a body is 

 denned as " the number of units of heat required to 

 raise that body 1 of temperature." If water be 

 taken as a standard, then the thermal capacity of one 

 body may be estimated in reference to it. If the 

 quantity of heat necessary to raise 1 pound of water 

 1 of temperature be called 1, the quantity necessary 

 to raise 1 pound of lead 1 of temperature will be a 

 fraction of 1. The number that expresses this ratio 

 between the quantity of heat necessary to increase 



