SOLIDIFICATION OF ALLOYS AND MAGMAS Sofa 
curve or changes in direction indicate an evolution or absorption of 
heat, and denote a change of state or a transition in the substance. 
In practice, the general method is to heat a metal or alloy of known 
composition to a desired temperature, introduce a pyrometer, and 
record the temperature at stated time-intervals during cooling. 
Such curves are shown in Fig. 1. The one on the left represents 
the cooling of pure platinum. ‘Temperatures are plotted on the ver- 
COOLING CURVES 
9 
---/77S PURE RON 
PLATINUM 
y 
& 
N ¢ 
g N 
a x 
KS x 
: : 
N N 
N 
TIME TIME 
PnGaer 
tical axis, and time on the horizontal. Starting at o, our metal cools 
gradually, until we reach 1,775°. Here we have a jog in the curve, 
an arrest in the temperature change due to the freezing of the metal 
and the evolution of the heat of solidification. Freezing completed, 
the material cools to room temperature without further break in the 
curve. 
On the right we have another cooling-curve, that of pure iron. 
Starting again at o, the metal cools to 1,505° when there is a break 
due to the solidification. On further cooling, we note a different 
condition from that observed in the case of platinum. ‘There are 
