THE CONSTITUTION OF THE COPPER-TIN SERIES OF ALLOYS. 61 
or, more concisely, yog =0-5 CugSn + 0*5 73 ^, where the subindices give the atomic 
percentage of tin in the particular kind of y. 
The change that occurs abruptly in passing through the /G temperature is 
represented by the equation :—- 
0’5 Cu-gSiioj 4" 0'5 Cu-gSn^- = 0'941 Cu^gSiiog + 0'059 Cu 5 gSn 4 . 2 , 
or, more concisely, tlie final state is 0’941 CugSn -j- 0‘059 liq^. 
Thus, assuming that the unit of volume of each substance contains 100 atomic 
per cents, of copper and tin jointly, we may say that in this alloy the G reaction 
produces about 6 per cent, of liquid. Considering Sn 27 in the same way, we find 
for the final state, just below the /G line, about 12 per cent, of liquid. This liquid 
is represented in the photographs of Sn 26 and Sn 27 by the lines of dark and the 
rows of black dots in the bars of 7 ;. The regression of the solidus from/to records 
this partial liquefaction ; although not unprecedented, it is not a very common 
phenomenon. The chills at 600^ show the effect even better than that at 625 , and 
a chill at 565°, which was not cooled with extreme slowness before the chill, shows 
the rows of dots along the axis of each plate still better. We give a photograph of 
this (fig. 72). 
The EG Alloys. -Sn 28. 42 per cent, of tin. S.c. chills at 650° and 625° (figs. 73 
and 74). 
In the upper of these (fig. 73 ), the rounded y combs are quite uniform and free from 
inclusions, l)ut the chill primary, of which there is a considerable amount, consists of 
bars of CugSn. In the lower chill (fig. 74), the rounded outlines of the y combs are 
still visible, as they by no means fill the field, but are frequently separated by patches 
of chill primary. The G transformation has, however, taken place, for the combs aie 
broken up into a delicate pattern of bars, and these have the rows of inclusions in 
them. It is clear that the y combs do not, at this percentage, ever quite fill the alloy, 
so that the transition point / must lie to the left of Sn z8. 
Sn 33 . 47’9 per cent, of tin. 
Tlie highest chill we have of this alloy is at 676°, not more than 20 ” below the 
liquidus; it affords a good example of primary combs of characteristic y (fig- f5), 
surrounded by a large excess of matter that was liquid at the moment of chilling; 
most of this is shown liy a higher power to be chill primary, not unlike the large 
combs. 
The chills of Sn 33 at 644° and 628° (figs. 76, 77, and 78) illustrate, admirably the 
transformation at the G temperature. The combs of y in the upper chill are rounded, 
quite smooth and free from the inclusions of liquid, while in the lower chill we see 
