THE CONSTITUTION OF THE COPPEE-TIN SEEIES OF ALLOYS. 
43 
Sn 15‘5. 2'he chill at 452° (figs. 36 and 37). 
Polish alone leaves this a perfect mirror with no pattern, but ignition or a slight 
etch with either ammonia or very dilute FeClg brings out a fine mother-of-pearl 
effect, the surface being now full of iridescent reflections, especially under oblique 
illumination (fig. 36). These are in patches oriented in many different directions, so 
that the effect varies with tilt and rotation. It requires a power of 500 to resolve 
these striations, when we see that the material consists of two substances, one darkened 
by the etch, the other a pure white (fig. 37). These sometimes lie side by side in 
groups of very narrow lines, an arrangement which accounts for the iridescent effect. 
In other places the darker copper-rich a (as we think it) is in dots surrounded by the 
white 8. This is the solid eutectic of all the alloys up to Sn 20, but in Sii 15-5 alone 
it occurs free from large crystals either of a or of the white 8; in Sn 16 we shall find 
such large crystals. 
The CD Alloys .—^All these, when chilled l)etween the liquidus and solidus, or more 
exactly when chilled above IcmD, contain large copper-rich combs of /3. These combs 
invariably etch out deeply in the process of polishing, and are then visible as a pale 
copper colour, while the mother-substance surrounding them is pure white. They 
etch with either ammonia or ferric chloride to a dark copper or black, and do not 
show any tendency to break iq? into the striation pattern. At the cniJ) transition 
temperature there is some difference between Sn 16 and the alloys containing more 
tin. but below the solidus all the alloys of the group, if they have been slowly cooled 
down to the moment of chilling, appear to be uniform solid solutions, a condition 
which persists with little or no change until the line C^XD^ is reached, when a 
crystallisation of 8 out of the solid solution commences. 
Sn 16. 
The upper chills of this alloy throw a very clear liglit on the process of solidifi¬ 
cation. The ingots are all coated externally with a raised pattern of rectangular 
combs. 
Sn 16. Chill at 773°. 
When the ingot is cut and polished one finds that the polishing has etched out 
large combs exactly similar to those on the outside. A gentle ignition oxidises 
these combs more rapidly than the mother-substance, but on further heating the 
lobes of the combs develop margins, and changes evidently take place in them; the 
combs are (2, very rich in copper, and therefore somewliat unstable. 
Sn 16. Chill at 752°, v.s.c. chill (fig. 38). 
The /3 combs are large and rounded, filling by far the larger part of the area, but 
G 2 
