THE CONSTITUTION OF THE COPPER-TIN SERIES OF ALLOYS. 
25 
and 31 (Plates 2 and 3), But there are cases, especially where /3 and a occur together, 
in which the has undergone, during and after the chill, a change into an apparently 
fibrous condition which gives it a structure like that of asbestos, except that in the ^ 
there are several intersecting systems of fibres. Tliis is well seen in fig. 21. When 
this change has become very pronounced in the /3, ferric chloride does not produce so 
marked a contrast between the shade of the two materials, though, as the a is never 
striated, we have in this peculiarity another distinction between the two. In the 
chills of Sn 16 and Sn 17, chilled between the liquidus and the solidus, in which case 
a is absent and the combs are embedded in a tin-rich mother-substance, the ^ is 
darkened by etching or ignition, and is rarely striated, while the tin-rich mother- 
substance remains a pure white. 
The y Mixed Crystals .—These occur as the primary crystallisation in all the DEFG 
alloys, provided they are chilled above the line cdefG. The y combs are distinguished 
from the preceding by a well marked obliquity ; instead of gridirons, they resemble 
sprays of foliage with rounded leaves. Heat-oxidation fails in this region, but HCl, 
with or without an oxidising agent, darkens the ground and leaves the y combs pure 
ivory white (see Sn 38, fig. 79). The y material is never found in unchilled alloys, 
for in cooling below the G temperature it breaks up into y and liquid. 
The S Material .—This substance, which we are strongly disposed to regard as the 
compound Cu^Sn, occurs as the white tin-rich ingredient of the eutectic in all the 
BLCD alloys, provided they are unchilled or chilled below 500° (fig. 19). It also 
occurs in larger masses as a rosette or fern-leaf and in Ijars in the Cl) alloys, provided 
they were not chilled above the line (^S* 49 a). In all these alloys heat-oxidation 
leaves the substance 8 white when the copper-rich a present has turned to a deep 
brown, but a better contrast is obtained by etching with HCl, when the a. is blackened 
and eaten away and the 8 left untouched in the purest white. Ammonia has no 
action on this substance. 
The 7 ) Material .—This occurs mixed with 8 in the DE alloys when they have been 
chilled below tlie line D'E'. We believe it to be in general the compound CuySn. 
When in presence of 8 the y can be distinguished by heat-oxidation, under which 
treatment the invariable rule holds good that the copper-rich 8 oxidises and darkens 
faster than the tm-rich y, we thus get a pattern in which the y is white and the 8 
dark brown. But for photography a sharper result is obtained by etching with a 
mixture of HCl and FeClg, when the y is blackened and the 8 remains white (fig. 56). 
In alloys containing more than 25 atomic per cents, of tin, at all events those 
chilled above 400°, the y is unattacked liy etching re-agents, while the more tin-rich 
matter surrounding the y is eaten away and darkened. The y rarely forms crystal 
skeletons, but occurs in jolates which are crystals bounded by plane faces; this is 
particularly well seen in the GH alloys, as in these the y has crystallised out of a 
liquid. If, however, these alloys are unchilled, or chilled below 400°, the y crystals 
are coated with H, as a result of the imperfectly accomplished transformation 
VOL. CCII.-A. 
E 
