144 
DE. WALTER WAHL: PHYSICO-CHEMICAL DETERMINATIONS 
plotted in the diagram, fig. 7, forming the curve P 3 . The values above 500 kg./cm.^ 
lie all on a straight line, which also passes through the melting-point at 1 kg./cm.^, 
51° C. At only a few degrees above the melting-point it is very difficult to judge 
exactly at what pressure the crystals really grow on account of the extreme slowness 
with which this takes place, and the pressure values obtained are much too large at 
temperatures just aloove 51° C.; but the difference between the obtained values and 
the values on the curve grows less with rising temperature and disappears at about 
58° C. That these high values, up to values 200 kg./cm.^ too high, are obtained at 
temperatures near the melting-point at 1 atmosphere depends upon the circumstance 
that the absolute value of the velocity of crystallization is so very small that the 
crystallization becomes noticeable first at pressures much higher and further apart 
from the true melting-point curve than at higher temperatures. With rising tem¬ 
perature the absolute value of the velocity of crystallization gradually increases, and 
the values obtained for the pressure at which the growth of the crystals becomes 
noticeable asymptotically approach the equilibrium curves until they, from about 
500 kg./cm.“, are found to lie practically on a straight line. 
The pressure values at which crystallization begins to take place in the case of a 
not sufficiently purified sample of the acid are given as P 5 in Table VII. In this 
