ye^ PHYSICAL SCIENCE 



manner we have traced for solutions in water.^ 

 The point of intersection of the curves corre- 

 sponds with the point of saturation both of silver 

 with copper and of copper with silver. When 

 the fused alloy has this proportion, crystals of 

 silver and copper freeze out together, just as 

 crystals of salt and water freeze out together 

 when the composition of the solution is that of 

 the cryohydrate. The point we are considering, 

 then, corresponds with the cryohydric point for 

 salt and water. The composition of the solid is 

 here the same as that of the liquid, and therefore, 

 as the process of solidification goes on, the 

 residual liquid always has a constant concentra- 

 tion. Thus the freezing-point remains constant 

 throughout the operation, and is identical with 

 the melting-point at which liquid first appears 

 when the solid alloy is heated. Similar phe- 

 nomena constantly appear in the study of other 

 metals ; and if an alloy of this composition is 

 polished, etched with acid, and examined under a 

 microscope, it will be seen to consist of a uniform 

 conglomerate of the two kinds of crystals. An 

 alloy of any other proportion exhibits larger 

 primary crystals of that metal which is present 

 in excess, and was frozen out first, connected by 

 regions filled with the conglomerate referred to 

 above. On account of its more uniform texture, 

 this conglomerate, which, as we have seen, corre- 

 sponds with a so-called cryohydrate, is named 

 the eutectic alloy. Fig. 4, on the plate facing 



^ Osmond thinks that, in this particular case, the primary 

 crystals are not perfectly pure. He adduces evidence to show- 

 that a slight trace of copper is dissolved in the solid crystals of 

 silver. Any such effect, however, is hardly appreciable. 



