98 



MESSES. WALTER KOSENHAIN AND P. A. TUCKER, 



microscopic evidence shows that even the most delicate pyrometric methods which 

 have been applied in the present research are inadequate to detect the presence of the 

 first small proportions of eutectic ; in both series of cylinders (B and C) small traces 

 of eutectic could be found in the sections of alloys containing as little as 10 per cent, 

 of tin. These small traces of eutectic, however, can be ascribed to the fact that the 

 cooling of the alloys had not been slow enough to allow complete equilibrium to be 

 established. The effect can be readily explained from a consideration of the process 

 of solidification of a solid solution as indicated in the diagram fig. 4. In this diagram 



Fig. 4. 



the line adc represents the liquidus curve of a series of binary alloys of metals 

 A and B, while the line abc represents the solidus. The abscissa of the point b is thus 

 the limiting solubility of B in solid A, while be is the line of eutectic solidification 

 Now, consider the process of solidification of an alloy containing n per cent, of B, 

 where n is a little less than b. The cooling of the alloy will be represented on the 

 diagram by the line pqn. As the line reaches the point p solidification will begin and 

 a solid solution of B in A will be formed having a concentration represented by the 

 abscissa of the point r, where a horizontal line through p cuts the solidus curve ab. 

 Similarly, as has been indicated by RoofcEBOOM and others, the composition of the 

 solid formed at successively lower temperatures will correspond in composition to the 

 abscissas of successively lower points on the solidus ab. If sufficient time be allowed, 

 the composition of the whole of the solid present at any given temperature will 

 become equalised to that which is formed at that temperature ; this equalisation can, 



