12 THE PRODUCTION AND USE OF LIGHT 



I. The rate of evaporation from any given amount of liquid 

 increases when the exposed surface is increased. 



II. Increasing the temperature of a liquid increases the rate 

 of its evaporation. 



III. The rate of evaporation is increased by the continual 

 removal of vapor above the surface of the liquid. 



IV. Some liquids evaporate much more rapidly than do others. 



V. Evaporation of a liquid always produces a cooling effect 

 upon surrounding bodies. 



TEMPERATURE AND THE THERMOMETER 



14. Temperature. We are all familiar with the common 

 use of the term TEMPERATURE. By it we mean the hotness or 

 the coldness of a body. On a cold day we say that the tempera- 

 ture is low; on a warm day we say it is high. We can gen- 

 erally tell when one body is warmer than another if we feel 

 of the two bodies at the same time. We cannot, however, be 

 certain of our judgment. Different substances feel to be of 

 different temperature when they are in fact of the same tem- 

 perature. The bare-footed boy knows this to be true. A 

 piece of iron and a piece of wood lying side by side on a hot day 

 will be of the same temperature. Will they feel so? Which 

 feels the warmer? What would be the case on a cold day? 

 Which would then feel the colder? 



If at the close of a long ride on a very cold, windy day we 

 were to step into an unheated room we would at once say that 

 the room was warm. If we were to remove our wraps and sit 

 down we should soon find, however, that the room was really 

 cold. 



The truth is we cannot depend upon the appearance of 

 objects, or upon our sensations of heat and cold, to tell us the 

 temperature of surrounding bodies. In all our work we shall 

 be obliged to use an instrument especially constructed for this 

 purpose, the THERMOMETER. 



15. The Principle of the Thermometer. 



