40 
MESSRS. C. T. HEYCOCK AND F. H. NEVILLE ON 
shown striation in the /3. The ingot appears to have been chilled almost exactly on 
the curve IC. 
Sn 14. Chill at 600°. V.s.c. chill (fig. 30). 
We give a photograph of this ingot (fig. 30). The new a is seen as a pure white 
crystallisation, larger than the striation pattern and quite different in character from 
it In this ingot we are evidently below the IC line, but not far below it, as one 
end of tlie ingot, the larger half, is quite free from a. This portion is not shown in 
the photograph. The photograph brings out a very important point, namely, that in 
the immediate neighbourhood of an a crystal the /3 is dark and unstriated. We shall 
return to this point. 
The chills at 675° and 600°, together with the chill of Sn 15 at 600°, determine a 
point on the IC line with considerable accuracy, for this line separates the region in 
which the /3 is free from a from that containing the new a which has crystallised out 
of the solid fi. 
Sn 14. Th<: chill at 530°, v.s.c. chill (fig. 31). 
Here we have, as might have been expected, a much more abundant and large a 
crystallisation. Moreover, the yS, except in one spot far removed from a, has not 
broken up into the striation. This phenomenon, seen also in the preceding chill, is 
one reason, perhaps the most conclusive, for the truth of our view that the striation 
did not exist in the /3 at temperatures above 500°, but is a change produced during 
and sometimes after the chill, and due to the fact that the ^ of Sn 14 is so rich in 
copper that at low temperatures it is in a very unstable state and readily breaks up 
into a copper-rich and a tin-rich phase, thereby producing the striation. It is clear 
that yS slowly cooled below the IC line, out of which a has already crystallised, will 
be less supersaturated and therefore more likely to bear a low temperature unchanged 
than the y8 chilled above the IC line. For the same reason Sn 9 chilled at 546° 
contains a mother-substance of y8 unstriated. We shall see that the ^ of Sn 16 and 
17, necessarily poorer in copper, is much more stable and rarely striates. We are not 
disposed to think that the two phases forming the striated yS are identical with the a 
and 8 of an unchilled alloy. 
Sn 14. The chill at 470°, v.s.c. chill (fig. 32). 
Here, as usual, the residual y8 has broken up into the C' complex of a and 8; there 
is no yS left in the ingot, consequently the a is darkened and the 8 remains a pure 
white : a good example of the complete reversal in the etching that occurs when we 
pass below 500°. A close inspection of this eutectic shows that it difters very much 
from st]iated y8, and could hardly have arisen out of it. This is another argument 
for the view that striation did not exist in the y8 at temperatures above 500°. 
