FUSION AND SOLIDIFICATION 85 



etched with acid or other suitable Hquid, in order 

 to bring out the structure-pattern. Each pure 

 metal, compound, or solid solution, crystallising 

 from the mother liquid, possesses a characteristic 

 appearance, which can readily be recognised 

 after some practice In interpretation of the 

 micro-photographs. Such photographs enable 

 us to trace the formation, development, and 

 decay of new crystal-species in a liquid or in a 

 solid matrix. 



The effect on the microscopic structure of 

 differences in the rate of cooling is well shown 

 in Figs. II, 12, and 13. The same alloy Is 

 represented in all these photographs, and was, 

 in each case, chilled from about the same tempera- 

 ture. The differences In structure depend solely 

 on the differences in the rate of cooling from a 

 liquid condition to the temperature at which the 

 ingot was chilled in cold water. 



The alloy contained 13.5 atomic percentages 

 of tin, and is represented by the vertical dotted 

 line in Fig. 10. When this alloy in cooling 

 passes the liquidus abc, crystal skeletons of 

 a solid solution called a appear mixed with 

 the mother liquid. These skeletons somewhat 

 resemble the larger fern-like structures of Fig. 2 

 on p. 72, which, however, chosen chiefly for its 

 beauty, was taken from a bronze of another 

 composition. 



When the alloy we are now considering 

 passes the line /c (Fig. 10), a new kind of 

 crystalline solid solution, called /3, begins to 

 form ; and, if time is given it by keeping the 

 ingot hot, the /^ substance gradually eats up 

 the existing crystals of a. This process is 



