80 
MR. S. W. J. SMITH ON THE THERMOMAGNETIC ANALYSIS OF 
cooling below d 2 is interrupted and that the temperature of the mixture of solution 
and crystals is raised. The crystals will not begin to go into solution again at once, 
for the solution surrounding them is already supersaturated. They will not begin to 
re-dissolve until the temperature is attained at which the solution is saturated, and 
this temperature will be higher in proportion as the temperature from which the 
reheating begins is nearer 0 2 . (It is, of course, supposed throughout that the salt is 
one of which the solubility increases with the temperature.) Hence, if the tempe¬ 
rature is subject to variation between that at which the cooling was interrupted and 
that at which the solution ceases to be supersaturated, there will be no solution of the 
crystals formed during the first cooling from above 
On the other hand, if the temperature is kept constant at any value between the 
limits in question, there will be a slow growth of crystals around those already 
formed and the concentration of the solution will approach slowly, from near the 
crystals outwards, to that corresponding with saturation at the temperature 
considered. 
Further, when the reasons why the rate of growth round the nuclei is slow are 
considered, it is seen that the alternation of temperature between limits 6' 2 and 0\ 
(where 0' 2 > 0 2 and 0\ < ff) will cause the growth to occur more rapidly than if the 
temperature were kept constant at 0' 2 . The salt crystallises round the nuclei because 
at the temperature 0 2 the solution in contact with them is supersaturated. By 
removal of the salt this solution approaches the normal saturation strength at 6' 2 , 
and further crystallisation does not occur until the concentration loss is made good by 
the diffusion of salt inwards from the more concentrated layers of solution further 
away from the crystals. Increase of temperature will increase this rate of diffusion 
and hence, also, the rate of crystal growth. The effect of increase of temperature 
will be most pronounced at first because the concentration gradient in the solution 
will have then its maximum value—the crystals being then surrounded by an 
extremely thin layer of solution saturated at 0' 2 , immediately beyond which is the 
supersaturated solution. At each alternation of temperature the distance over which 
the concentration varies between saturation at 0' 2 and that of the main bulk of super¬ 
saturated solution will become larger and the concentration gradient will consequently 
become smaller. Hence each successive heating to 6\ will be accompanied by the 
approach of a smaller amount of crystal lisa ble material, to the crystallising nucleus, 
than the last, assuming each reheating to occupy about the same length of time as 
the one preceding it. Effects of the kind here contemplated will be most pronounced 
in solutions of which the viscosity is not only large, but subject to rapid decrease 
with rise of temperature. They may be represented qualitatively by a diagram, such 
as that given below (fig. 26, I.), in which the ordinates represent the amount of crystals 
present in contact with the solution and the abscissae are temperatures. 
It is assumed that an amount of crystals, represented by OA, is present originally 
in contact with a solution saturated at 0„, and that when the solution and crystals 
