3(5 MESSRS. C. T. HEYCOCK AND F. H. NEVILLE ON 
subjected to the action of a solvent (fig. 21). Moreover, the ground has entirely- 
changed, being now full of the striation characteristic of We may regard the 
s.c. chill at 775° as giving the minimum proportion of a to he found in this alloy, for 
as the temperature falls from C to C', that is, from 790° to 500°, the a grows in 
amount at the expense of the solid The alloy chilled at 550 and etched with 
ferric chloride, fig. 22, illustrates this growth of the a out of a solid solution. This 
particular ingot was not cooled before the chill so slowly as were the preceding, and 
therefore the detail is somewhat smaller, hut the ratio of the wdiite a to the darker ^ 
has evidently increased enormously, being greater than in the chill at 805 . The 
peculiarly pointed and flamhoyant character of the a which has grown out of the 
solid ^ should he compared with the rounded lobes of tlie a combs in the chill at 805°. 
The general character of tlie jiattern in the chill at 550° resembles that in an 
unchilled alloy, except that the unchilled alloy, or one chilled below 500°, would, with 
the same etch, have the a dark and the ground a brilliant wdrite. The ground, 
moreover, in the chill at 550° is uniform jB, but in the unchilled alloy it would be the 
C' complex, 
Sn 13'5. 22’5 'per cent, by iveight. 
This alloy is important, because it fixes the point I at which the amount of a formed 
above the C temperature is just enough to re-act with the whole of the residual C 
liquid in order to form ; in fact, the re-action a* -f liquidc = A can complete itself 
in this alloy. 
Sn 13‘5. S.c. chill at 805°. 
This has less a than the chill at the same temperature of Sn 12, hut otherwise it 
closely resembles it; hence we do not give a photograph. 
Sn 13-5. Chills at 775° 
If an ingot of Sn 13-5 be cooled very slowly through the C temperature, so as to 
be maintained for several hours at or very near 790°, the a is entirely dissolved and 
we obtain an ingot composed wholly of y8. In the photograph (fig. 23) one can see 
a single minute spot of a that has survived, and is paler than the But this fi, 
after chilling and etching with ferric chloride, has the well marked striation pattern. 
Considerable light is thrown on the transformation of the a into ^ by the examination 
of some ingots which were cooled somewhat too rapidly through the transformation 
point to allow of a complete solution of the a. A chill at 765°, etched with ferric 
chloride (fig. 24), shows the effect well—there is still a little a in slendei combs and 
crosses, and these, though isolated from each other, are so symmetrically arranged as 
