THE CONSTITUTION OF THE COPPEE-TIN SERIES OF ALLOYS 
O 
SECTION III. 
The Microscopic Study of the Alloys. 
Identijication of the Substances as seen in the Photogixtphs. 
The rjL Matericd. —This is the only substance present in unchilled alloys of the 
AB group. It is best seen in chilled ingots, appearing in the form of isolated combs 
or skeletons in the chills taken above the solidus, for example, in figs. 2 and 3 a, 
where it is the darker part, and in fig. 6, where it is the somewhat lighter part. 
The a also forms the primary crystallisation in the BL alloys, as in fig. 10, in which 
the a is dark, and fig. 11, in which it is the lighter part. 
The rectangular network of raised crystals that covers the outside of the ingots of 
all the alloys from A to L is entirely composed of a. This network is evidently the 
substance that solidified first, from which the mother-liquid retired in consequence of 
contraction due either to cooling or solidification. Both in the raised network, and in 
such sections as that of figs. 10 and 28, one sees the very rectangular character of 
the skeletons; wdiere the angle of intersection of two branches is not a right angle it 
is very frequently an angle of 60°, though for obvious reasons this cannot always be 
the case. It will be observed that in all ingots chilled above hG, or even a little 
lower, especially in the slowly cooled chills, the individual lobes are rounded; this is 
well seen in fig. 6. On the other hand, in the lower chills this roundness is absent, 
having been succeeded by an almost flamboyant angularity, well seen in fig. 22. 
The a is much richer in copper than any other material found in the alloys; it is, 
consequently, softer than the tin-rich material surrounding it, and polishing alone 
develops the pattern. In the jiattern thus obtained, the copper-rich a is red or 
yellow, and the tin-rich material is pure white. Prolonged polishing accentuates the 
pattern by oxidising the a to a deep red or purple, and in the method of developing 
the pattern by heat-oxidation, the a is always oxidised more rapidly than the 
surrounding material. Ammonia or hydrochloric acid attack, dissolve, and generally 
darken the a, leaving the tin-rich matter surrounding it a pure silvery white, as for 
example in fig. 3 a. The acid solution of ferric chloride that we have used in etching 
the surfaces for the photographs we I’eproduce has a similar action on the alloys 
chilled well above hG, but when, as in the case of alloys chilled below this line the a 
is in contact with /3, the a is paler than the /3, this substance being often left very 
dark, as in figs, 15 and 16. 
The a combs, at all events in ingots chilled below the solidus, often have another 
peculiarity; they are cored, that is to say, they contain an inner skeleton differing 
somewhat from the outer portions of the crystal. These cores oxidise more rapidly 
than the material outside them, and they are more rapidly attacked and darkened by 
