62 
MESSRS. C. T. HEYCOCK AND F. H. NEVILLE ON 
instead an angular material, ruled Avith lines and containing, in A^diat Avere the lobes 
of y, many black dots due to the liquid liberated in the reaction. 
The only other example Av^e need give of this is in the case of Sn 38. Here, in the 
chill at 644° (fig. 79), the rounded combs of y, though large, are scanty, and they 
contrast AAmll AAuth the abundant chill primary Avhich approximates to the bar type. 
At 628° the y has been transformed, tliough the parts that Avere y combs can still be 
distinguished from the rest of the alloy, in spite of their noAv angular character and 
the bars that run through them (fig. 80). This ingot, hoAvever, Avas evidently chilled 
before the transformation had completed itself, and the orientation of the bars is Amry 
irregular. If Ave alloAv the change to complete itself during the sIoaa" cooling, Ave get 
the beautifully regular pattern of the chill at 606°, in A\diich the large and small 
crystals are of the same type (fig. 81). 
As far as Sn 42 (the G point), similar features are seen in the chilled alloys, but 
this point marks the extreme limit of the y type of primary. We shall refer to the 
loAver chills of the EG alloys in discussing the succeeding groups. 
Before leaving the region of chills in Avhich y is found, Ave Avish to draAA’ attention 
to the resemlilance betAveen the combs of y, from their fii'st appearance in Sn 21 to 
the latest we reproduce in Sn 38. It is Amry difficult to get good y combs in the 
upper chills of the DE alloys, because tlie material is so unstable that it is more or 
less transformed and turned into rj during the chill. But Avith normal combs of y, 
such as those of Sn 33 for comparison, it is easy to trace the y character in the 
primaries of Sn 21 (fig. 54), Sn 22 (fig. 58), and Sn 25 (fig. 65). Sn 27 forms 
normal y combs like those of Sn 33. The maximum amount of y found in each of the 
EG alloys occurs in the chill slightly above the G temperature, and this maximum 
decreases from the Avhole volume of the ingot in Sn 27 to a Amnishingly small 
fraction in Sn 42. 
llte GH Alloys .—These alloys, ranging in composition from Sn 42 to Sn 87, deposit, 
when they begin to solidify, crystalline plates of the y type. We haAm been aide to 
isolate these and analyse them. They appeal' to be the body CkigSn in an almost or 
quite pure state. We have, therefore, drawn the solidus as a vertical line, E^Eg at 
tlie atomic percentage Sn 25. For a particular percentage of tin the large primary 
plates of 7) (genei'ally seen in the figure as bars) increase in amount the loAA’er the 
chilling temperature, so long as this does not fall beloAv 400°. ' The chill primary is a 
iietAvork of slender bars or plates, tliat in the polished section may be described as 
needles. Polish alone shoAvs the y, both primary and chill primary, as a purplish- 
Avhite surrounded by a jiure AAdiite tin-rich substance. If the section is kept for some 
days the contrast becomes greater through the gradual oxidation of the y, the 
mother-substance remaining a very pure Avliite. For jdiotography a slight ferric 
chloride etch is preferable, as it leaAms the y unchanged and dissolves and blackens 
the ground. The chills of Sn 42 at 600°, and of Sn 45 at the same temperature, are 
of this type, but in the former (fig. 82) the large bars of y haA-e curA'ed lines of the 
