84 PHYSICAL SCIENCE 



structure it possessed at that temperature is stereo- 

 typed almost perfectly by the process of sudden 

 chilling, and can be examined at leisure in the 

 cold metal by polishing and etching it with acid 

 in the usual manner. 



In this way equilibrium curves lying below 

 the solidus were detected and traced. Such 

 curves represent changes of structure which occur 

 in a mass completely solid, and quite explain the 

 changes in physical properties caused by anneal- 

 ing or chilling. Take as an example the two 

 curves /x and e'x, which cut each other in the 

 point X, and recall in their general form and 

 relations the simple curves of equilibrium between 

 liquid and solid for alloys of silver and copper 

 already described and illustrated in Fig. 6 (p. 75). 

 The analogy is more than one of mere form. Just 

 as crystals of silver or copper separate out of the 

 homogeneous liquid of Fig. 6, so crystals of new 

 substances separate out of the homogeneous solid 

 solution which exists within the triangular space 

 Ixflm Fig. 10; and, as the crystals of silver or 

 copper are in equilibrium with the liquid alloy in 

 states represented by points on the freezing-point 

 curves of Fig. 6, so the new crystalline structures 

 are in equilibrium with the homogeneous mother 

 substance lying within our present triangle. 



The positions of these curves of equilibrium 

 between solid phases are investigated chiefly by 

 the microscopic examination of ingots of metal, 

 which are fused, allowed to cool very slowly to 

 the temperature to be investigated, in order that, 

 as far as possible, equilibrium may be reached, 

 and then suddenly chilled by immersion in cold 

 water. A section of the ingot is polished, and 



