LESSONS IN PHYSICS. 29 



mercury thaD in the water, while that devoted to expansion is 

 less. We find that the temperature of mercury rises much faster 

 than that of water. It takes thirty times as long to raise water 

 to a given temperature as it does mercury, which means that 

 there must be thirty times as much heat in the water as in the 

 mercury when the same temperature is reached by both. We see, 

 then, that at the same temperature different substances may 

 have different quantities of heat in them. The relative quanti- 

 ties of heat in different bodies at the same temperature is called 

 specific heat. Water is the standard of specific heat. At a given 

 temperature water contains more heat than any other known 

 substance, its specific heat being 1, when the specific heat of 

 mercury is .03. By this is meant that when equal weights of 

 water and mercury are at the same temperature mercury will 

 contain only .03 as much heat as the water. 



The expansion of a solid body continues nearly uniform until 

 its temperature has reached the melting point; the temperature 

 then stops rising while the expansion increases, and continues 

 until the solid is melted, the liquid resulting having the same 

 temperature as the melting point of the solid; that is, after the 

 body reaches the melting point all the heat applied is used in ex- 

 pansion, and none is used to increase temperature. 



The melting points of different substances is uot the same. 

 Ice melts at 32 F., mercury at -39, iron at about 3,000, plat- 

 inum at about 5,000. Melting ice contracts while most other 

 bodies expand in melting. The expansion of a. liquid continues 

 gradually until the boiling point is reached ; at the boiling point 

 the temperature stops rising, while the expansion increases and 

 continues until the liquid is vaporized or becomes a gas. The 

 temperature at which a liquid begins to boil is called its boiling- 

 point. Liquids change to vapor slowly at all temperatures, the 

 change being called evaporation, but at the boiling point the 

 body changes rapidly into vapor, and the change is called vapor- 

 ization. The temperature of the boiling point depends upon the 

 purity of the liquid ; the presence of some impurities raises the 

 boiling point, while the presence of others lowers it. Salt water, 



