THE CONSTITUTION OF THE COPPER-TIN SERIES OF ALLOYS. 
13 
that, in spite of the approximate uniformity of the alloy above the line C'XD', there 
must be round each ^ grain an envelope, somewhat richer in tin than the grain itself, 
produced by the collection between the grains of the last traces of mother-liquid. 
These intergranular spaces would, therefore, as the temperature fell, be the first parts 
to become saturated with tin, and the tin-rich S, in consequence, began to crystallise 
in these spaces. The substance between the crystals of S is unifoiin until the 
temperature X is reached, when the residual solid solution breaks up into the 
O' eutectic complex of a and 8. A careful examination of this complex witli a high 
power proves it to be essentially the same in all alloys from Sn 6 to Sn 20. The 
photographs we give will, we think, justify this statement. The fact that the eutectic 
point C' of the LO alloys is a little higher than the eutectic point X of the CD alloys 
is evident in the pyrometric curves of Roberts-Austen and Stansfield, and we have 
also verified it, as may be seen in our cooling curves. But Professor Boozeboom 
suggests that the true eutectic angle for all alloys from I to is at C', and that the 
apparent depression of X is a retardation due to the difiiculty experienced by tlie a 
in crystallising without a nucleus of its own kind. Thus in the region X D^D tlie 
alloys are a complex of ^ (or y) and 8, while below XDg they form a complex of 
a. and 8. 
The curves I C'XD' record the equilibrium between the solids a and 8 and the solid 
solution out of which they crystallise, and tlie method of examining cliilled ingots has 
enabled us to follow the whole process in a very satisfactory manner. The a must be 
itself a solid solution, but we are strongly disposed to think that 8 is the compound 
Cu^Sn. The alloy Sn 20, although the cooling curve indicates that it undergoes a 
well-marked exothermic transformation at the D' temperature, remains substantially 
uniform after the transformation. The fact that it has recrystallised is, however, 
shown in the ingots chilled below T)', by minute traces of the O' eutectic visible 
between the large crystals of 8 tliat almost entirely fill the ingot. It may be that 
the chemical compound Cu^^Sn does not exist above tlie temperature D'. 
(5.) The DE alloys, containing from 20 to 25 atomic per cents, of tin, that is from 
31‘8 to 38’4 per cent, by weight. When chilled between the liquidus and the solidus, 
these alloys are found to contain primary combs of y. On the solidus, these combs 
fill the alloy, and just below it they form a uniform solid solution, but it is very 
difficult in this region to prevent, by chilling, a commencement ot the transformation 
proper to the D'E'' curve. However, our chilled ingots afford a considerable amount 
of evidence that the condition of the alloys at temperatures between de and D^E' is 
that of a uniform solid solution. When the temperature falls below the curve D^E', 
long, straight, very uniform bars separate out of the solid solution. These are richer 
in tin than the solid out of which they crystallise. They are dark in the photographs. 
Near D' these bars are usually very slender and scanty, but they increase as we 
approach E', and at that point they fill the whole alloy. These bars appear, from 
what one occasionally sees in some ingots, to be really plates seen edgeways, and their 
