ON DILUTE SOLUTIONS OF ALUMINIUM IN GOLD. 
271 
changes. The change in structure occurring at the recalescence is especially 
remarkable. For example, A1 14 chilled at any temperature above 420° C. is a 
uniform solid solution of closely fitting polygons showing no trace of a second 
material, but chilled at 400° C. it becomes filled with polygons, bars, and sprays of a 
second material, and from the fact that the new substance shows a tendency to 
margin the old polygonal divisions, it is probable that it is richer in aluminium than 
the original alpha (Plate 5, fig. 10). 
The same change occurs spontaneously in all alloys from A1 14 to A1 20 when they 
are cooled below 400° C. and either chilled or not. Ingots of A1 15 and of A1 18, 
chilled the first at 380° C. and the second at 410° C., show the striation in the alpha 
and the specks and crinkles in the beta, both alpha and beta having been uniform 
before the recalescence (figs. 11, 12, and 13). The change in the beta seems to be 
the most important and to contribute most of the heat evolved. 
The recalescence may occur in the beta itself before the temperature has fallen low 
enough to allow of the formation of the L eutectoid, but, as a rule, the eutectoid 
forms before the recalescence, and in such cases we can analyse the phenomenon more 
minutely. In the case of an ingot in which the eutectoid already exists, the chemical 
change causing the recalescence is, essentially, a reaction between the two phases 
alpha and D which make up the eutectoid complex; it results in the complete 
disappearance of all the D and of most of the alpha of the complex, a new body, 
which for the moment we will call Y, being formed. Hence, if an ingot, which has 
been successively through the eutectoid and the recalescence changes, be chilled 
immediately after the latter, we find, instead of the L complex, a very delicate 
reticulation of threads which etch bright, while in the cells of this network there is 
the Y body, etching a uniform brown. The primary, or pseudo-primary, whether it 
be of alpha or of D, is unaffected at this stage. Ingots of A1 23, chilled one at 
420° C. the other at 400° C., are excellent examples, the first of the eutectoid, the 
second of what it becomes after the recalescence (figs. 14 and 15. Compare also 
figs. 8 and 8a, 9 and 9 a). 
As the recalescence of the eutectoid always leaves a slight excess of alpha, it follows 
that the substance Y must contain rather less gold than the pure eutectoid of the L 
point; this points to the hypothesis that Y is the compound A1 Au 4 . 
Such an alloy as A1 23, chilled at 400° C. immediately after recalescence, contains 
three phases, alpha, Y, and D, while the Phase Rule only allows of two ; this anomaly 
is removed by annealing, when the D primaries slowly react with the residual alpha to 
form more Y, the process resulting in the disappearance of the D or of the alpha, 
according to whether the one or the other was in excess ; in A1 23 the alpha dis¬ 
appears, while in A1 20 a long anneal after spontaneous recalescence always leaves 
us with a trace of alpha (fig. 17). Two ingots of A1 20, the first (fig. 16) chilled 
at 440° C., before recalescence, and the second (fig. 17) chilled at 410° C., after 
spontaneous recalescence aiid a long anneal at that temperature, show the enormous 
