FUSION AND SOLIDIFICATION 



75 



ture of a mass of ice and water remains constant 

 till the whole is frozen. Thus, by watching the 

 thermometer, the temperature at which solid 

 begins to form can be estimated. 



The melting-point of silver is 960" C. and the 

 addition of copper lowers it just as the addition 

 of salt lowers the freezing-point of water. This 

 is best shown by plotting the observations on a 

 diagram, as in Fig. 6, in which the horizontal axis 



Silver 



Copp 



er 



Fig. 6. 



denotes the composition of the mixture expressed 

 in percentage numbers of atomic equivalents of 

 silver and copper, and the vertical axis the tem- 

 peratures. On the other hand, pure copper melts 

 at 1081°, and the admixture of silver lowers its 

 freezing-point. The two curves in the diagram 

 cut each other at a point which corresponds with 

 a temperature of 777', and a composition of 

 40 atomic percentages of silver and 60 of copper. 

 At other points on the curves, the process of 

 freezing consists in the separation of primary 

 crystals of one or other of the pure metals in the 



