THE CONSTITUTION OF THE COPPER-TIN SERIES OP ALLOYS. 
S.'S 
detected, except, perhaps, by an increasing angularity in the margins of the combs. 
Moreover, the action of ferric chloride on all these alloys is the same, the a remains 
bright and the yd is turned dark, but seems to remain homogeneous. But at 500° 
the yd breaks up into the C' complex of a 8, and the action of the ferric chloride is 
reversed, as it now darkens the a and leaves the 8 a pure wliite. 
Sn 9. V.s.c. chill at 470° (iigs. 17 and 19). 
We here see the transformation, the a has become dark and the yd has broken up 
into a pearlite or solid eutectic. As this transformation is common to all alloys 
containing yd vBen they cool slowly through the O' temperature of 500°, we give in 
fig. 18 a more highly magnified patch of yd from an ingot of Sn 9, very slowly cooled 
to 546° and then chilled. It will be seen that although there is a narrow line of 
white, probably 8, separating the a and the enclosed patch of yet the latter is 
patternless and, as usual with a ferric chloride etch, darker than the a. surrounding it. 
This should be contrasted wfith the patch of the C' complex taken from the alloy 
chilled at 470° and also etched with ferric chloride, fig. 19. Here we see that the 
border of 8 has become very wide and that the interior of what was the patch of yd 
has broken up into a complex of 8 and a copper-rich body which so closely resembles 
a that we give it the same name. These two photographs were both taken at a 
magnification of 280 diameters. 
This transformation yd = a + 8 is an exothermic one, and gives rise to the halt in 
the cooling curve that we first see in our cooling curve of Sn 10 at 500°, though, as 
we have shown (fig. 13), the same transformation can be detected by the microscope 
even in Sn 6. In fact, this change of solid yd into a complex occurs at or a little below 
500° in all alloys from Sn 6 to Sn 20. It is marked in our diagram (Plate 11) by the 
broken line h'O'^KD^. It must be borne in mind that at this temperature the alloys 
are rigid solids. 
Sn 12. 20'3^er cent, hy weight of tin. 
The solidification of this alloy begins, as in the cases already discussed, by the 
crystallisation of a combs, but when these cease to form, on account of the residual 
liquid having reached the C composition, there is much more of the liquid than in the 
case of Sn 9, and hence the resolution of the « during the transformation at the C 
temperature is more marked. This is well seen in the cooling curve of Sn 12, in 
which the C halt is a prolonged one, but it is made evident in another way by an 
examination of the ferric chloride etch of the v.s.c. chills at 805° and 775° (figs. 20 
and 21). The first of these shows us the maximum possible amount for this alloy, of 
primary a, and also a minute dark chill primary which is probably yd. The lower 
chill, made after a very slow cooling of several hours through the intervening 30°, 
shows much less a, and what is left has a disjointed look, as if the combs had been 
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