FUSION AND SOLIDIFICATION 87 



based are illustrated In Fig. 15. Here a new 

 or secondary crop of a crystals has begun to 

 grow. This ingot was chilled at 558 , and there 

 is no doubt that the new growth of a took 

 place in a mass which had solidified completely 

 long before. 



The further growth of the new a is seen in 

 Fig. 16, which represents an alloy of slightly 

 higher content of tin (14 atomic per cents.) chilled 

 from a temperature of 530 . As the alloy in 

 cooling passes the temperature of 500', the whole 

 of the /3 substance is transformed into a complex 

 consisting of a crystals intimately mixed with a 

 new solid solution called S. This complex is 

 shown in Fig. 17 as a light background; while, 

 in contrast with it, the a crystals come out dark 

 after the treatment adopted. 



These changesagain occur inamassthoroughly 

 solid throughout, and explain in a most striking 

 manner the effect of such processes as annealing 

 and tempering, in which the properties of a metal 

 are altered by heating it to a temperature well 

 below its fusion-point and then cooling it either 

 slowly or rapidly. 



Heycock and Neville's investigation of the 

 bronzes was a very laborious undertaking. One 

 hundred micro-photographs were published, and 

 these represent only a selection of those taken ; 

 many observations of freezing-points were also 

 made. But the labour of the work is well repaid 

 by the magnificent results finally obtained. 



Iron and steel, as used in the arts and in- 

 dustries, consist of pure iron alloyed with various 

 substances, chiefiy carbon. Solid solutions, similar 



