THE CONSTITUTION OF THE COPPER-TIN SERIES OF ALLOYS. 
33 
containing chill primary, the whole of this 10 per cent, having been liquid at the 
moment of the chill. This should be compared with the chill of Sn 4 at 800°, in 
which there is practically no mother-substance. The a in the chill of Sn 6 is 
remarkable for the deep cores that appear in the combs some time after etching, 
although in the figure we reproduce, which was put for a moment on the polishing 
wheel after etching for the purpose of simplifying the pattern, no cores are visible, 
and the combs are, on the whole, lighter than the mother-substance. 
Sn 6. S.c. chill at 775° (not reproduced). 
This shows about as much mother-substance as the preceding, but this mother- 
substance is now /3, which had solidified at the C temperature and is consequently 
darkened by the FeCl 3 etch. 
Sn 6. Unchilled, v.s.c. (fig. 12). 
This ingot affords the most striking evidence that Sn 6, when solid, is a complex. 
Here, in the slow cooling through the C' temperature of 500°, the small amount of yS 
between the a crystals has broken up into the C' complex of a -f- As the white 8 
is predominant in these patches, they appear after the ferric chloride etch as a very 
brilliant white, while the a is darkened. The plate shows two large crystals of a 
differently oriented, and, therefore, reflecting light differently. Inclosed in them, in 
the form of the usual very angular patches of a scanty matrix, are several per cent, 
of the white body. (The striations in the lighter grain of a are due to imperfect 
polishing.) 
In order to show the character of the O' complex, we give a more highly magnified 
patch of this mother-substance taken from the same ingot (fig. 13). Here it is 
evident that the patch is margined by a homogeneous band of the white 8, but that 
inside this Vjorder the 8 is mixed with lines and spots of a darker substance, which 
proves on examination with an immersion lens to be identical with the a surrounding 
the patch. As the sloiv-cooled Sn 4 contains only a, ivhile Sn 6 has several per cent, 
of this complex, we have placed the boundary between the two groups at Sn 5. 
Sn 6 may be regarded as the first of tlie BL group of alloys, the harder gun- 
metals, which when solid are a complex of two phases; but with increasing content 
of tin, as in Sn 9, the characteristic features of the group show themselves more 
plainly, the a decreasing in amount, and there being a corresponding growth of ^ or, 
in the lower chills, of the C' complex, and therefore of 8. 
Sn 9. 15‘& gwr cent, of tin. Chill at 880° (fig. 14). 
This was a very much granulated ingot, the result of throwing the semi-liquid 
alloy into water being to blow away the still liquid part and to leave the skeleton 
crystals of a as a mass of little fir trees. A portion solid enough to grind down and 
VOL. CCII.—A. 
F 
