■64 BRIDGMAN. 



cussion I shall make the suggestion that the velocities as ordinarily 

 measured are the velocities of a surface of separation, whereas the 

 minute velocities which are just detectible in some cases are due to 

 the transition running at corners or edges separating one phase from 

 the other. For our present purposes we shall neglect these very small 

 velocities, and discuss without reserve the "region of indifference" as 

 if the reaction velocity were mathematically zero within this region. 

 The width of this region is evidently another datum of significance to 

 be obtained from the velocity curves. It varies greatly from substance 

 to substance, and also varies greatly with pressure and temperature 

 along the transition curve of the same substance. By collecting into 

 a single diagram the widths of the indifferent region for the same sub- 

 stance, we obtain curves showing the width of the region as a function 

 of pressure or temperature along a transition line. In the following 

 these curves are gi\Tn. 



The data for individual substances follow. These comprise curves for 

 the limiting acceleration from above and below, and width of the band 

 of indifference, plotted against equilibrium pressures on the transition 

 line, together with such comment on individual peculiarities as may be 

 necessary. In a number of cases, fragmentary data have been collected, 

 not sufficient to collect into curves. These isolated values are also 

 ^iven. 



Data for Individual Substances. 



Phosphorus. — The transition curves for the two varieties of white 

 phosphorus were given in an earlier paper (C) of this series. The 

 transition velocitv data were determined at that time, but were not 

 published. Figure 6 shows the transition accelerations both from 

 above and below, as a function of pressure on the equilibrium line 

 (the temperature range is from 0° to 68°), and in Figure 7 the width 

 of the band of indifference is shown. The observed reaction velocities 

 cover a range of 150 fold. It is to be noticed that the acceleration is 

 greater with falling than with rising pressure, that it is greater at the 

 lower pressures and becomes nearly constant at the higher pressures, 

 that the band of indifference passes tlii'ough a minimum and becomes 

 rapidly greater at the higher pressures, and that it does not run paral- 

 lel to the velocity curve. 



Ammonium Sulfocyanide.^ — The limiting accelerations from above 

 and below are given in Figure 8. The results are somewhat irregular; 



3 E, p. 72. 



