HEAT 



155 



out of the end of the delivery tube, a trap such as is shown in 

 Fig. 70 should be used. 



There is an absorption of heat which has no effect on the 

 thermometer whenever a liquid is converted into a vapour. 



We have taken the case of water as an example, but the same 

 general phenomenon can be noticed in every case. 



EXPT. 156. Pour a few drops of any very volatile liquid, 

 such as ether or carbon bisulphide, upon the hand. It soon 

 disappears, and as a result of its disappearance the hand feels 

 very cold. The heat necessary to effect the change of state 

 has been taken from the hand. 



EXPT. 157. Place a few drops of water between the bottom 

 of a small beaker and a block of wood. Put some ether into 

 the beaker and blow over it with a pair of bellows having a 

 tube fastened to the nozzle. The water will freeze. 



When the weather is hot it is a common plan to wrap wet 

 cloths round bottles of wine to keep the beverage cool. In this 

 case the evaporation of the water on the cloth is possible only 

 by the absorption of a large amount of heat, which is extracted 

 from the bottle. For precisely the same reason, water is kept in 

 porous vessels in hot countries. 



CHIEF POINTS OF CHAPTER X. 



Change of Size. 



Substances expand 

 when heated and con- 

 tract when cooled, 

 the amount of change 

 varying with differ- 

 ent materials. 



Effects of Heat. 



! 



Change of Tem- 

 perature. 



When a body is 

 gaining heat it rises 

 in temperature, and 

 when it is losing heat 

 it falls in tempera- 

 ture. 



Change of State. 



By the addition of 

 heat a solid can be 

 melted into a liquid, 

 and then vaporised ; 

 the changes happen 

 in the reverse order 

 if sufficient heat is 

 withdrawn. 



A Thermometer is an instrument for measuring temperature. Its 

 action usually depends upon the fact that substances expand when 

 heated and contract when cooled. 



Why Mercury is generally used in Thermometers. The following 

 are the chief reasons why mercury is the best liquid to use in ther- 

 mometers under ordinary circumstances : 



