METEORIC AND ARTIFICIAL NICKEL-IRON ALLOYS. 
83 
subsequent cooling to 0 O . The crystal growth during cooling, which can occur only 
round crystals already present, may be represented by the curve qq'. 
Next suppose that the heating is first interrupted at some temperature above that 
represented by a. In this case the strength of the solution is such that on cooling 
it will become labile before 0 O is reached. Let r be the point at which the heating is 
interrupted. Crystals will grow round those already present comparatively rapidly 
at first, on account of the relatively high concentration gradient and temperature, but 
the rate of growth will diminish subsequently, because the temperature effect and the 
effect of the gradient now conspire to retard it. When the temperature reaches the 
value corresponding to the ordinate r't the supersaturated solution at a distance from 
the crystals will pass into the labile state, and the formation of nuclei within it will 
begin. The solution in the immediate neighbourhood of the crystals, being more 
dilute, will still be metastable, and the last portion of it—that corresponding to 
saturation at the temperature represented by r '—will not become labile until the 
temperature (represented by t', fig. 26, II.) is reached. At 6 t all the solution will be in 
labile equilibrium, and the growth of crystals subsequently will be along CdTA'- 
(For the sake of simplicity it has been tacitly assumed that crystallisation round 
crystals is inappreciable below the point s.) 
It is easy to see that if the heating along ABC is interrupted at some point 
between q and r, such as u, the solution round the crystals will not become labile 
until some temperature lower than 0 O is reached. 
Finally, consider the case in which the cooling from the curve ABC is interrupted 
before any of the solution has reached the labile state and the system is reheated. 
Crystals will grow during the cooling round those present, as already described. 
When the reheating begins, assuming the solution round the crystals to be still 
supersaturated, crystals may continue to grow, but, obviously, before the outer curve 
ABC is reached the temperature will reach a value at which the solution round the 
crystals is not supersaturated. Beyond this temperature the crystals will begin to 
re-dissolve and the solution round the crystals will become saturated at the expense 
of the crystals which were deposited from it during cooling. When the temperature 
from which the cooling began is reached, practically all the solution will again be 
saturated at this temperature, and the result of further rise of temperature will be 
that the crystals will dissolve, following; the curve ABC. If the zone in which loss 
of salt by the solution takes place during cooling is of considerable extent, there will 
be a perceptible lag in the return of the state in which all the solution is saturated. 
In such a case the temperature at which the heating was first interrupted will be 
reached before the whole solution is saturated again at this temperature, and the 
amount of crystallisation may therefore, for some degrees above this temperature, 
exceed that corresponding to the saturation curve ABC. 
These conclusions rest upon the assumption, which is apparently justified by the 
experiments of Miers ( loc , cit.'j, that the temperature at which a supersaturated 
