On the Constitution of Copper-Tin Alloys. 327 



and we have little doubt on the point, that the true eutectic angle for 

 all alloys from B to D is at C', and that the apparent depression of X 

 is a retardation due to the difficulty experienced by the a in crystalli- 

 sing from the solid solution without a nucleus of its own kind. Thus, 

 in the region XDoD' the alloys are a complex of ft and D', while 

 below XD2 they form a complex of a and D'. As we have said before, 

 we feel no doubt that this D' is the compound CojSn. 



The alloy Sn^o, although it undergoes a well-marked exothermic 

 transformation at the D' temperature, remains substantially uniform. 

 The fact that it has recrystallised is, however, shown by minute traces 

 of the C' eutectic, visible between the large crystals of CiitSn. It may 

 be that the chemical compound Cu 4 Sn does not exist above the 

 temperature D'. 



5. Ttie DE Alloys, containing from 20 to 25 atomic per cents, of Tin. 

 Between the liquidus and solidus these alloys 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 

 avoid a commencement of the transformation proper to the D'E' curve. 

 However, our chilled alloys afford abundant evidence that the normal 

 state of alloys between de and D'E' is that of a uniform solid solution. 

 When the temperature falls to a point on the curve D'E', long, straight, 

 very uniform tin-rich bars separate out of the solid solution. These 

 are very slender and scanty near D', but become massive and abundant 

 as we approach E', and at that point fill the whole alloy. These bars are 

 really plates of E', seen more or less edgeways, and their appearance of 

 greater or less breadth is partly due to -their inclination. These plates, 

 the first appearance of the E' phase, must be either pure Cu 3 Sn or mixed 

 crystals of Cu 3 Sn and Cu 4 Sn ; we are not at present able to decide 

 this point. Thus, in the area D'E"E 2 F'E', the alloys are a complex of 

 E' + y. But Roberts-Austen and Stansfield have proved that these 

 DE alloys show, when they fall to the temperature D', an evolution of 

 heat. This must be due to the conversion of the residual y into D', so 

 that below D'E" the alloys form the complex D' + E'. 



6. The EF Alloys, containing from 25 to about 27*5 atomic per cents, of 

 Tin. These go through the same stages of y + liq., then pure y, then 

 y + E', but at the temperature G the residual y breaks up into E' and 

 the G liquid. 



It may be noted here that the triangular area Ixf forms an island 

 of typical uniform solid solution, which could only have been discovered 

 by the examination of chilled alloys. 



7. The FG Alloys, containing from 27'5 to 42 atomic per cents, of Tin. 

 These alloys, like the preceding, begin by forming the complex y + 

 liquid ; their state, when the temperature G is reached, being y crystals 

 of the / percentage and liquid of the G percentage. The isothermal 

 transformation y crystals = E' + G liquid, now begins. 



