THE CONSTITUTION OF THE COPPER-TIN SERIES OF ALLOYS. 
55 
a copper-ricli and a tin-rich phase. The lace bars are probably, as we said in 
discussing Sn 21 , due to the similar breaking up of 77 bars that were too rich in 
copper for the final equilibidum. As in the ingot of Sn 21 , chilled at 580°, the paler 
bars were probably intermediate phases. 
A chill at 560° is very similar to that at 565°, but shows better the separate 
crystalline bars of the copper-rich phase. 
Chills at lower temperatures show no further change, as can be seen from a not 
specially slowly cooled chill at 495° (fig. 61), so that the unchilled alloy must be a 
complex of two phases, which may be Cu^Sn and CugSn, but are more probably 
conjugate solid solutions composed of these two bodies, and best described as S and r^. 
An ingot that took several days to cool to ordinary temperature without a chill 
shows all detail on a very large scale. There is not the lace ground-work, though 
there are some traces of lace in the bars. Instead of the lace, the very slow cooling 
has allowed the white 8 phase to crystallise in definite needles round the margins of 
the dark rj bars, and sometimes in the heart of these bars."^' 
Sn 23. 35‘8 per cent, of tin, and Sn 24, 37'1 per cent, of tin. 
These alloys form copper-rich primaries of y when chilled in the narrow region 
between the liquidus and the solidus, and at temperatures between the solidus and 
the transformation curve we believe that they form uniform solid solutions of y, but 
we have never been able by chilling to obtain perfectly uniform ingots of the solid 
solution in this region. 
Below the transformation curve a large scale crystallisation of p in bars and 
ribands take place. No change can be detected at the D' temperature, so that 
unchilled alloys at ordinary temperatures must be regarded as consisting of two 
phases, a copper-rich one of the 8 type and a tin-rich one of the p type, the two 
being, as in Sn 22 , either the two compounds themselves, or solid solutions of them. 
Sn 23. S.c. chill at 710° (not reproduced). 
The ingot was very liquid at the moment of chilling, and, consequently, was badly 
granulated and blown into holes, but one can very well see a few broken sju’ays of y 
primary on a darker ground. The y is sometimes twinned, part of a spray being light 
and the rest dark, and it is evidently very unstable, dark p bars being visible, running 
across the lobes of primary. There is a great deal of chill primary, indeed the true 
mother-substance is shown by a high power to be a thin network of grey, 
* The flatness of the liquidus between D and E caused us to suspect at one time that the alloy, while 
wholly liquid, might have separated into two liquid phases; the above-mentioned extremely slow cooling 
was arranged in order that by difference of density two layers might be produced; there was no trace of 
such a separation, even the plates of were uniformly distributed throughout the whole depth of the ingot. 
