THE METALS: THE METALLIC OXIDES 59 



of the wax or ignition of the phosphorus, in times varying with the metals 

 employed. Silver and copper are good conductors, bismuth and antimony 

 bad conductors of heat. Good conductors of heat are also good con- 

 ductors of electricity. 



Metals expand when heated. In the construction of railways and iron 

 bridges this fact has to be taken into account. Timepieces lose time in 

 hot weather, owing to the expansion of balance wheels and pendulums. 

 In chronometers advantage is taken of the different expansibilities of 

 metals to provide for automatic regulation. 



Metals differ in heat capacity. The heat capacity of mercury being very 

 low (and its conductivity for heat being high), mercurial thermometers are 

 sensitive to rapid changes of temperature. 



Metals differ in their density. Sodium and potassium float upon water, 

 but all the other common metals sink. All the commoner metals, with the 

 exception of gold, float upon mercury, which is therefore one of the 

 heaviest metals known. 



Metals differ in their fusibility. Mercury is liquid at the ordinary 

 temperatures ; sodium and potassium both liquefy below the boiling point 

 of water ; tin and lead below a red heat ; zinc and aluminium at a low red 

 heat, and copper at a bright red heat ; while iron only liquefies at a white 

 heat. Alloying one metal with another often lowers its melting point ; 

 thus fine solder has a lower melting point than either the tin or lead of 

 which it is an alloy. 



Metals differ in their hardness. By comparing the power that solids 

 have of scratching each other, their hardness can be roughly ascertained. 

 Thus lead can be scratched by copper and copper by zinc. The presence 

 of an impurity even in small quantities often alters the hardness. Thus 

 steel, i.e. iron containing ^ to I per cent, of carbon, is much harder than 

 iron. The hardness of steel also depends upon the rate of cooling. Thus 

 steel which has been " tempered " by heating and slow cooling is much 

 less hard and elastic than steel which has been cooled quickly. 



The following table gives some of the physical constants for the principal 

 metals •— 



[Table. 



