AT HIGH PRESSURES BY OPTICAL METHODS. 
143 
however, lie only a couple of atmospheres apart. The pressures at which the 
velocity of crystallization attains noticeable values are given in Table V. as Pi, and 
the pressures at which rapid melting takes place are given there as Pg. 
( 2 ) Determinations of the Melting-point of the Unstable Modification .—The melting 
and crystallization of the unstable modification at difierent pressures occurs in very 
much the same manner as in the case of the stable modification. The only difference 
is that the velocity of crystallization is greater. Working isothermically, it has been 
possible to determine the pressures at which growth-structures are suddenly formed. 
These pressures lie about 350 kg./cm.^ above the melting-point pressures, which 
corresponds to approximately 6°‘5 C. All that has been said with regard to the 
superheating of the stable modification applies also to this case, but the slowness 
with which the melting takes place is here still more marked. The point where a 
rapid melting suddenly begins is not reached before at about 150 kg./cm.^ lower 
pressure than the melting-point pressure. This corresponds to a superheating of 
about 2 °'5 C. 
In consequence of the extreme slowness with which the melting and the crystalliza¬ 
tion takes place in the vicinity of the equilibrium-curve crystal—liquid we would, 
when working isothermically and increasing or decreasing pressure at a moderate 
speed, only be able to observe a somewhat sudden volume change at the pressures at 
which rapid melting and rapid crystalline-growth takes place. We would thus obtain 
pressure values as much as 500 kg./cm.^ apart, corresponding to 9° C., and if we 
assume—as is generally done—that the true equilibrium pressure lies halfway between 
the obtained limiting pressures, we would be about 100 kg./cm.^, corresponding to 
1°‘8 C. in error. This is, of course, an extreme case, but it serves well to indicate the 
uncertainty of the volumetric methods, both with regard to the determination of 
melting-point curves and the determination of transition-point curves between different 
crystalline modifications in such cases where transition takes place very slowly. 
Table VI. 
Temperatures 
(corrected). 
Pressures, P 3 
(corrected). 
°C. 
(51 
1 kg./cm.b 
52-60 
215 „ 
55-52 
300 
60-56 
535 
65-39 
775 „ 
70-64 
1,045 „ 
75-40 
1,325 „ 
The pressures at which crystallization begins to take place at noticeable speed, 
determined on a very carefully purified sample, are given as P 3 in Table VI., and 
