THE CONSTITUTION OF THE COPPEE-TIN SEEIES OF ALLOYS. 
49 
slowly cooled unchilled alloy may at ordinary temperatures consist of this compound. 
Occurring as it does at a singularity both in the liquidus and in the solid 
transformation curve, the alloy is a border-line one, separating two groups. Its 
cooling curve shows two well marked evolutions of heat, the first due to the latent 
heat of solidification, the second to the re-crystallisation that takes place in the solid 
at temperatures below the point D'. 
As the diagram indicates, solidification is complete at a temperature not more than 
30° below the freezing-point. Ingots chilled in the region of temperature between 
the liquidus and solidus contain copper-rich primary combs embedded in tin-rich 
mother-substance. These are well seen in a chill at a little below 740°, in which they 
are comparatively scanty, and in a chill at 726°, in which they fill more than half 
the ingot (fig. 51). These combs of primary are evidently ^ which has begun 
to disintegrate. The important point to notice is that this alloy, although it has a 
formula, does not solidify after the fashion of a pure body, but by the formation of 
crystals considerably richer in copper than the liquid. A little above 710°, however, 
these combs have disappeared, and from here down to 650° no pattern can by any 
means be developed in the chilled ingots, except sometimes the pattern of large 
polygons, or counties, so common in pure metals and solid solutions. The chill at 
605°, only just above the point I)', requires very prolonged exposure to strong ferric 
chloride to bring out anything exce]3t the polygons, but by this means one sees that 
the polygons are l)ordered by broad but vague white bands. A high power makes it 
evident that these bands are of the angular patchiness seen in some previous ingots. 
This is evidently a growth of tin-rich material in places where broad ribands of 
compact 8 would be seen in an unchilled ingot. It is quite doubtful whether these 
markings have any right to appear in this ingot, or whether they are not a result of 
imperfect chilling or a change that has taken place subsequently to the cliill. 
The chill at 558°, aliout 25° below D', is full of a splendid pattern of the 8 bands and 
rosettes. The oidy difierence l)etween this pattern and that of an unchilled alloy is 
that in the former there is rather more mother-siiljstance, and that this is uniform, 
instead of being, as in the unchilled ingots, a complex. The photograph of an unchilled 
ingot that we give (fig. 52) shows how very little of this mother-substance (dark in 
the photograph) is to be found in the alloy. Moreover, this mother-substance is itself 
mainly composed of the same material as that of the bands and rosettes, its dark 
colour after etching or ignition being due to a small admixture of spots and lines of a 
copper-rich material, which we think is a. We give a higher-power photograph 
(fig. 53a) of a patch of this eutectic, taken from an unchilled and somewhat slowly 
cooled ingot of Sn 20 ; the darker straggling patch in fig. 53a corresponds to one of 
the minute specks of black in the preceding figure. We also give in fig. 53 a high- 
power photograph of the eutectic in unchilled Sn 19. The resemblance to the 
eutectic, also formed at the same temperature, of Sn 14 is very great, and we have 
little doubt that this eutectic complex, formed in the solid alloys at a temperature of, 
yoL. ccii.—A, H 
