24 
MESSES. C. T. HEYCOCK AND F. H. NEVILLE ON 
such re-agents as ammonia. Hence they are evidently richer in copper than the outer 
jDarts of the crystals. The most striking and certain way of developing cores is to 
take a polished surface of an ingot of Sn 5 that has not been chilled nor very slowly 
cooled, and to heat-oxidise it. Oxidation through orange, red, blue to the white 
stage may not develoj^ much pattern, the a combs, which up to Sn 6 almost entirely 
fill the field, remaining of a flat tint; but when the white stage is reached, the 
outlines of the combs may he here and there marked by a trace of mother-substance 
which has remained an indigo blue. If, however, we proceed to heat further, a 
pattern of deep red or blue cores wiU develop itself, while the surrounding a, probably 
three-fourths of the area , remains white. This colour combination does not photograph 
well, but we sometimes get the core pattern vdth the first orange, in which case it 
can be photographed. We show the effect in the photograph of the unchilled Sn 1 
(%• !)•. 
As visilde in unchilled alloys, these cores have been described by Stead and bv 
Chaepy. The cores are not divided from the substance outside them by a sharp 
line, l3ut by a ])lurred or graded zone, so that under a high power there seems to be 
no Ijoundary at all. Herein the cores differ from the a combs, the margin of which, 
whenever visible, is a sharp line even under the highest powers. The v.s.c. chill 
of Sn 4 at 775°, fig. 8, shows the character of the cores. They are only seen 
in the ABL alloys, and in the AB group they can, by slow cooling, be made very 
faint, or by rapid cooling very well marked A decidedly slowly cooled ingot of Sn 4, 
chilled at 800° and etched with HCl, fig. 5a, shows the complete a combs with sharp 
margins, but the darker cores, due to the earlier a, are also evident. These 
developed in the interval of time between etching and photographing, and are due to 
the more rapid air-oxidation of the copper-rich parts. This ingot was not cooled with 
the extreme slowness of that of hg. 7, hence a few dark spaces can be found that may 
have been due to a trace of mother-substance liquid at the moment of chilling. 
When the cores haAm l^een developed by etching, their intensity depends very much 
on the nature of the etch and on the time that has elapsed betAveen the etch and the 
examination ; for example, the s.c. chill of Sn 6 at 805° can be so treated as to show 
well-marked cores or none. We must conclude, therefore, that though the presence 
of cores indicates some difference of composition, yet the intensity of the Ausible core 
cannot be taken as a measure of the difference in composition existing betAveen it and 
the matter outside it. 
The /3 Material. — This is alAAmys considerably richer in tin than the a, hence when 
l)oth substances are present it is easy to distinguish them. This can be done by 
heat-oxidation AAdien the a oxidises first, by etching Avith strong ammonia Avhen the 
a is darkened and dissolAmd and the /3 left a pure iAmry Avhite, or by etching Avith 
acid ferric chloride. Ferric chloride, the re-agent Ave almost iiiAmriably emplo}" in 
preparing the surfaces for photography, acts on a mixture of a and /3 in such a Avay 
as to darken the /I and to leave the a bright; this effect is Avell seen in figs. 15, 16, 
