270 
MESSRS. C. T. HEYCOCK AND F. H. NEVILLE 
primary, crystals are alpha, if to the right of L they are D. In this way the 
microscope shows that L lies between A1 19 and A1 19 ‘5. 
Cooling Curves. 
We have determined the cooling curves of a number of alloys ; we will discuss that 
of A1 20, of which we give a reproduction. This curve, fig. 2, shows a period of heat 
evolution beginning at the B temperature and continuing for some time with a 
slightly falling temperature ; this is due to the solidification of the solid solution 
beta, and is very characteristic of the solidification of solid solutions. The next heat 
evolution, near 450° C., is probably due to the 
crystallisation of D as the solid solution cools 
across and below the cL line. The formation 
of the L eutectoid is indicated by a well marked 
flat at 424° C., the invariable temperature at 
which this phenomenon occurs. But the most 
striking feature of the curve is the recalescence 
that began at 388° C. and, almost instantly, 
raised the temperature to 428° C. 
A similar recalescence can be detected in all 
alloys from A1 10 to A1 27, though in those 
remote from A1 20 it is slight, while in those 
near A1 20 the effect is almost as marked as in 
that alloy. Both the initial temperature and 
the rise in temperature are very variable, the 
latter being sometimes as much as 70° C. This 
is essentially an explosion phenomenon, that is, 
a change commencing below the temperature at which it ought to take place, and the 
temperature to which the recalescence carries the ingot may not reach that at which 
the new system would be in equilibrium with the old one. We have also traced the 
heating curves of A1 20 and neighbouring alloys. In these heating curves the first 
marked absorption of heat, causing a flat, is seen to begin at 520° C., and we think 
that it indicates the equilibrium temperature of the recalescence reaction. A strong 
reason for this opinion is that if we take several pieces of an ingot that has under¬ 
gone recalescence and heat them, annealing and chilling at successively higher 
temperatures, we find, on examination with the microscope, that the pattern produced 
by the recalescence persists up to 520° C., but that if the alloy be ever so little 
further heated this pattern disappears and is replaced by that characteristic of the 
pre-recalescence state. 
By introducing a thermocouple into a molten ingot, tracing the cooling curve, 
remelting, cooling to selected points on the curve and chilling at these points, we can 
by microscopic examination correlate the change in structure with the thermal 
