42 
MESSRS. C. T. HEYCOCK AYD F. H. XEAMLLE OX 
at 500°, or thereabouts, the cooling curve has a flat corresponding to the 
formation of the solid eutectic Ct Thus the cooling curve has only two singularities. 
Sn 15’5 chilled at 767°, 760°, 759° (not reproduced; see fig. 34). 
These, when etched with FeClg, show a pattern of /S combs on a lighter tin-rich 
ground, the pattern resembling that of Sn 17 chilled at 731°, though not cjuite so 
regular. The yS combs in shape and arrangement closely resemble a combs, indeed, 
we think the two bodies are isomorphous. But these combs of yS are distinguished 
from a in two ways, they etch out in polishing much more readily and deeply than do 
a combs, and in Sn 15'5 they can be made to show the striation pattern, though 
vdieu originally examined they did not show it. After the first examination they 
were ignited, a process which brings out the combs dark brown on a white ground. 
A repolish and an etch with FeClg now shows that many of the lobes have develojDed 
a large striation pattern identical with that in the yd previously discussed. We give 
a photograph (fig. 34) of this effect at 750°, because it links up the y8 previously 
described to the unstriated jd combs we shall find in the succeeding alloys. The 
photograph also shows chill primary of ^ immersed in the white tin-rich mother- 
substance. In the chill at 750°, which was not specially slowly cooled, the combs fill 
about three-fourths of the area. Had the coolino- been slower, we think the alloy 
would have been entirely solid at this temperature. 
Sn 15’5. Chill at 705° (not reproduced). 
Tliis, unlike the previous chills, does not develop a pattern by polishing, for the 
large junnaries have disapj^eared from the interior, though they are still visible on 
the outside. Ignition to orange brings out a few large softly rounded patches, 
differing a little in tint, l^ut nothing that could be called a, pattern. Chemical 
etching also falls to develop anything except a few large polygons. The alloy is a 
very uniform f)ody with no striation or other pattern. We infer that, before the 
chilling, the pilmaries had grown, assimilating the mother-substance as they grew, 
until their margins met. Thus at the moment of complete solidification the alloy 
consisted of crystal grains of the solid solution yd, each grain having its own 
orientation no doubt, hut being, from a chemical point of view, approximately nniform 
and identical with the other grains around it. 
8n 15'5. Chills at 600° and at 552° (fig. 35). 
These resemble the preceding. They show a few polygonal divisions, but otherAvise 
are uniform; they show no striation and no pearlite effect. We give a photograph 
of the chill at 552° (fig. 35), taken Avith oblique illumination, for comparison Avith the 
next alloy. 
