THE CONSTITUTION OF THE COPPEE-TIN SERIES OF ALLOYS. 45 
very slender white lines. The grain of the material of the polygons is very uniform ; 
it is presumahly the solid solution ^^e therefore place the solidus at 750 , ^We 
do not give photographs of these, as, beyond fixing the position of the solidus more 
exactly, they do not add to the information supplied by the preceding photographs. 
Sn. 16. Chill at 725°. S.c.c. (not reproduced). 
This had all the slow cool of the alloy chilled at 738°, and, in addition, was allowed 
an hour and a-half to cool from 738° to 725°. There is now on the outside only a 
trace of a very coarse, rounded network. The cut and polished section is very uniform, 
with no polygonal boundaries—probably because the ingot has become all one crystal— 
no striation, indeed no pattern of any kind. The slow-cooled chill at 650° is similar, 
there is no sign of the solid solution breaking up. The chills at 546° and 500° were 
not slow-cooled, and therefore the number of polygonal divisions is larger, but the 
dividing lines are very fine, and the ground of the polgyons is uniform. The sections 
do not shov/ the pearlite effect when the polished and etched surface is obhc|uely 
illuminated. 
Sn. 16. Chill at 477°. 
This chill, which must be very close to, if not below, the curve /C'X, does not yet 
show the typical pattern of an unchilled alloy. There is no iridescence, the ground 
not having broken up into the C' eutectic. But there are some broad irregular liands 
of a tin-rich substance having an asbestos-like growth. There are also stars of true S 
in the copper-rich ground. Ingots chilled a little lower, or unchilled alloys, aie veiy 
diflPerent. In these, after polishing and etching the surface, the pearlite effect is very 
well shown ; indeed, with oblique illumination, a lens, or even the unaided eye, shows 
the surface as made up of iridescent patches. The whole alloy has broken up into the 
C' eutectic, exactly like the photographs we give of the same phenomenon in the 
Sn 15‘5 ; but, in addition, there is in Sn 16 a new feature ; this consists in sprays of 8, 
which began to crystallise as soon as the temperature fell to a point on the curve 
C'XD'. We give a photograph (fig. 40) of an unchilled ingot showing this, because 
it proves that Sn 16 lies to the right of the sohd-eutectic point G , and consequently 
when the solid solution begins to break up, it is a tin-rich and not, as with Sn 14, 
a copper-rich phase which crystallises. 
The Transformation at the Point D. 
Sn 16, as we have seen, appears to solidify homogeneously to a uniform mass of /3 
combs, but the alloys of the group with a larger percentage of tin do not do so. 
Although they all contain yd combs when partially solid and chilled in the area ADC, 
yet at the temperature of the cD line (740°) the yS becomes unstable, and at this and 
