VELOCITY OF POLYMORPHIC CHANGES BETWEEN SOLIDS. 81 



the case of CBr4, Wahl ^^ has observed optically that there is a region 

 within which the reaction could not be observed to progress. This 

 region was narrow, could be rather sharply observed, and was of 

 approximately the same width as I have found above. The pheno- 

 menon has also been observed by Tammann ^^ for the two varieties of 

 phenol at low temperatures. I am not aware, however, that anyone 

 has ever before actually measured the velocity at points outside the 

 region and shown by an extrapolation that there is really a region of 

 no appreciable velocity. I have been able to make such neasurements 

 only because of an apparatus absolutely without leak. 



The width of the region of indifference varies at different points on 

 the equilibrium curve. We should expect rising temperature to 

 decrease the width of the region, and rising pressure to increase it. 

 With NH4SCN the region increases in width with falling temperature 

 and rising pressure; the width is about twice as great at 0° as at 67°, 

 although the width is in either case comparatively slight. CBr4 

 shows a decreasing width with rising pressure and temperature; the 

 effect of rising temperature here overbalances the effect of rising pres- 

 sure. The band of indifference of Urethan decreases strikingly with 

 rising temperature on all three transition curves; this has been de- 

 scribed more fully in a preceding paper. ^^ Phosphorus shows on the 

 whole an increase of width with rising pressure and temperature, but 

 has a flat minimum which is of interest. This points to a preponderat- 

 ing effect of temperature at low pressures, but at the higher pressures 

 the retarding effect due to increased pressure overbalances the accel- 

 eration due to rising temperature. In a previous paper it was stated 

 that the width of the band for phosphorus was inappreciable,^* but 

 this is now seen not to be true. The previous statement was deduced 

 from a diagram like that of Figure 1 ; the advantage of the diagrams 

 used in this paper is obvious. Phosphorus shows that there is no 

 necessary connection between the width of the band and the accelera- 

 tion of transition. The band becomes wider with increasing pres- 

 sure and temperature, but the transition acceleration increases also. 

 CsNO.3 shows an effect similar to phosphorus; at 177° and 190° the 

 region of indifference has no appreciable width, but at 202° the width 

 is 100 kgm. This is doubtless the effect of increasing pressure, jsls 

 opposed to the effect of increasing temperature. AgNOs is interest- 



11 W. Wahl, Trans. Roy. Soc. (A) 212. 137 (1912). 



12 G. Tammann, Z8. phys. Chem. 75, 75-80 (1910-11). 



13 E, p. 122. 



14 B, p. 185. 



