34 
MESSRS. C. T. HEYCOCK AND F. H. NEVILLE ON 
polish was found with difficulty. A section etched with FeClg is reproduced. The 
magnification is too high to show the large « combs well, but individual lobes are 
seen lighter than the ground. The section contains rounded a combs filling rather 
less than half the area, and immersed in the mother-substance are the minute combs, 
also of a, that were formed during the chill. On heat-oxidation the large combs 
turn red more rapidly than the minute ones, presumably because the large ones are 
richer in copper. It can be seen that the minute combs sometimes grow out from 
the margin of the large ones. We have here a very good example of the result of 
chilling a semi-liquid alloy. 
Sn 9. Sloiv-cooled chill at 805° (not reproduced). 
This chill shows very large and symmetrical primary combs of a, but their area is 
much less than in the corresponding chill of Sn 6. A ferric chloride etch leaves the 
a combs bright and darkens the mother-substance, hut in the heart of each patch of 
mother-substance there is often an irregular line of ]3ure white, which a high power 
shows to be a final tin-rich residue, the last matter to solidify. The alloy gives one 
the impression of having been chilled rather below the intended temperature. The 
rounded character of the lobes or teeth of the combs is noticeable. 
Sn 9. Slow-cooled chill at 775° (not reproduced). 
In the interval of several hours that elapsed between this chill and the preceding 
the re-action a liquid = ^ has been going on, the original a combs have been 
much modified and somewhat diminished in amount, and the surrounding j3 shows 
the characteristic striation pattern. We do not give photographs of these two ingots, 
because the important change produced in the alloys by the C re-action is better 
seen in such an alloy as Sn 12. 
We give, however, in fig. 15, a ferric chloride etch of an alloy not quite so slowly 
cooled and chilled at 777°. This is a typical example of a ferric chloride etch of an a 
and /3 comj^ilex, the a combs being light and the /3 dark. In the heart of the S one 
can see threads of a white tin-rich material that must have been liquid at the 
moment of chilling. The presence even of tliis minute amount of liquid at 777° is a 
result of a somewhat too rapid passage through the C temperature. Polishing this 
and similar alloys without etching leaves the a a bright copper colour and the a 
pure tin white, and a strong ammonia etch has the same efiect, but for photographic 
purposes we prefer the efiect of ferric chloride. The ^ in this ingot does not show 
striation, it is apparently uniform. 
Sn 9. V.s.c. chill at 546° (figs. 16 and 18). 
All the chills down to above 500°, for example that at 546° (fig. 16), are like the 
above, and if there is any growth of a in the solid alloys, it is not enough to be 
