THE CONSTITUTION OF THE COPPER-TIN SERIES OF ALLOYS. 27 
retirement of the mother-liquid during the contraction accompanying solidification. 
The coreing is evidently due to imperfect adjustment of the a to the liquid, a result 
likely to occur during the solidification of a solid solution, and is a sti'ong confirmation 
of the view that the a crystals vary in composition. 
The chills of Sn 2. 3'67 per cent, by iveight of tin. 
Chills were taken at the following temperatures :—1025 , 982 , 957 , 932 , 907 , 
882°, 800°, 770°; all but the first of these were very slowly cooled before the chill. 
The upper chill was not cooled with any precautions to ensure slow cooling, being 
allowed to cool in the furnace after the gas had been extinguished, but the others 
were very slowly cooled from the freezing-point down to the moment of chilling. 
The total time required for an ingot to cool from the freezing-point down to 770 
was eight or ten hours, and for the upper chills the times were proportionately less. 
Sn 2. Chilled ctt 1025° (Plate 1, fig. 2). 
The ingot was compact, but showed some signs of granulation, on account of its 
semi-liquid state at the moment of chilling. It was cut in two, polished and etched 
with ferric chloride, but the pattern was well seen aftei- the polish. It consists of 
copper-rich a combs, dark in the figure. These sometimes stretch half across the 
face of the ingot and are well seen under a magnification of 5 diameters. They fill 
about one-third of the area, and their margins are sharply divided from the mother- 
substance round them. They are primary crystals, the matter which was already 
solid at the time of chilling. The material of these combs appears to be uniform 
throughout, it being impossible to develop cores in them. The magnification of the 
photograph is too high to show the great symmetry and length of the combs, but it 
makes it just possible to distinguish the network of much smaller combs in the 
ground. These are rectangular, and in all respects except size resemble the large 
combs. They are evidently chill primary, that is, a which crystallised during the 
rapid cooling of the chill. In the interstices between both sets of combs there is a 
small amount of a quite different material. This is the true mother-substance, a 
white tin-rich body that solidified last of all. 
Sn 2. Chill at 982° (not reproduced). 
This was cooled very slowly down to the moment of chilling. Like all which 
follow, it was a compact ingot completely covered externally with a rectangular 
network of a combs in relief. It was cut and polished. Prolonged polishing 
develops an oxidation pattern of large plump copper-rich a. combs, filling at least 
nine-tenths of the area. These show no cores. They are surrounded by a netwoik 
of a white tin-rich mother-substance, containing minute copper-rich chill primary. 
E 2 
