SCIENCE OF COMMON THINGS. 131 



When is a body cold f What is fire ? Effects of heat. 



itself among neighboring bodies until all have acquired 

 the same temperature, neat passes from the body of 

 higher temperature to the hand, and causes a peculiar 

 sensation, which we call warmth. 



SSO Under what circumstances do we call a body cold f 



When we touch a body having a temperature lower 

 than that of the hand, heat, in accordance with the above 

 law, passes out from the hand to the body touched, and 

 occasions the sensation which we call cold. 



831 What, then, really are the sensations of heat and cold f 



Merely degrees of temperature, contrasted by name 

 in reference to the peculiar temperature of the indivi- 

 dual speaking of them. 



833 When is a body said to be incandescent or ignited f 



When the body naturally incapable of emitting light 

 is heated to sufficient extent to become luminous. 



833 What is flame? 



Flame is ignited gas issuing from a burning body. 



834 What is fire f 



The appearance of heat and light in conjunction, pro- 

 duced by the combustion of inflammable substances. 



835 What character was attributed to fire by the ancient philosophers f 



They used the term fire as a characteristic of the 

 matter of heat, and regarded it as one of the four ele- 

 ments of nature. 



83B Enumerate the general physical properties of heat. 



It is invisible, without weight, with great tendency 

 to diffuse itself, and is absorbed by all bodies. 



837 What are the principal effects of heat 1 



Expansion, liquefaction, vaporization, and ignition. 



838 What do we understand by the term caloric f 



Caloric is a name often used to indicate the agent 

 which produces the sensation of warmth; since the term 

 " heat," as generally used, refers only to the sensation. 



839 Is caloric equally distributed over the globe f 



No ; at the equator the average temperature is 



