30 
MP]S.SRS. C. T. HEYCOCK AND F. H. NEVILLE ON 
the chills at 800° and those below 770°, as the characters of a uniform solid solution 
are more conclusively exhibited by Sn 4. 
The unchilled very slowly cooled ingot (not reproduced). 
This ingot w^as examined to ascertain if Sn 2 underwent any changes in the solid 
wdien cooled below the hQ line. The polished surface, after etching, consists of several 
irregular rounded polygons, dovetailing into each other without a trace of mother- 
substance. These show the rotation effect. There is little or no indication of cores. 
Sn 4. 7‘2^)er cent, of tin hy weight. 
This alloy, containing about 7 per cent, of tin, affords a more striking example of 
solid solution than the preceding. 
Sn 4. Chilled at 990° (not reproduced). 
The ingot was granulated by the operation of chilling, but it wms possible to polish 
a good face on it. It contained the same pattern as the highest chill of Sn 2, that is, 
large primary combs of a, filling perhaps a quarter of the area, a good deal of chill 
primary, and a small amount of white mother-substance. It is clear that the large 
a combs were the only parts solid before the chill. As this closely resembles the 
u})per chill of Sn 2, we do not reproduce it. 
Very slow-cooled chills were made at the following temperatures :—900°, 835°, 
775°, 700°, and one ingot was sloAvly cooled down to the melting-point of tin. All 
these proved to be compact ingots, and w^ere completely covered with a raised net¬ 
work of rectangular a. combs. Besides these, other ingots were chilled after a some¬ 
what more rapid, hut still a slow cooling. 
Sn. 4. The chill at 900° (fig. 6). 
This contained large almost coreless a combs, filling about 90 per cent, of the area; 
in the interstices between the large combs there is a very beautiful crop of chill 
primary embedded in a little of a tm-rich mother-substance, but a higher magnifi¬ 
cation is needed to show the chill primary. At the time the photograph Avas taken 
the a, though much more oxidised than the ground, Avas blue and hence actinicall}^ 
active. Consequently the contrast betAveen the tAvo materials is Aveak and reA'ersed 
in the photograph. 
Sn. 4. Chdl at 835° (not reproduced). 
This contains very little, not more than 1 per cent., of mother-substance, and the 
a combs are so fused together that their margins cannot in general be distinguished. 
