EFFECTS OF PRESSURE ON NITRATES. 585 



the large probable error because of the great width of the region of 



dr 

 indifference at the lower end of the curve, no values of AH or -t~ are 



dp 



tabulated for this part of the curve. 



The feature of particular interest for AgNOs is the remarkably 

 sudden increase of curvature of the transition line beyond 6000 kgm., 

 a feature unique to this substance. In fact, the increase of curvature 

 is so great that when I first found the transition at 20°, it seemed as 

 if there must be a third modification, and that the phase diagram would 

 look like that of Agl. The whole of the field was very carefully 

 explored for other modifications; the strongest motive for repeating 

 the measurements was that the discordant At; points lent color to the 

 suspicion that there might have been an undetected third modifica- 

 tion near the bend with a small A?;. No other form could be found, 

 however, to 12000 kgm. at room temperature, or at 200°, and on the 

 repetition of the experiment none could be found in the neighborhood 

 of the bend. 



It is possible to pass over the curve at any point without the reac- 

 tion starting. The width of the region of indifference within which 

 the reaction does not run after it has once started changes in a curious 

 way with the pressure. The detailed data for this effect are to be 

 given in the succeeding paper. 



There are several measurements of the transition data at atmos- 

 pheric pressure. Schwarz ^ gives 159.2° to 159.7° by an optical 

 method for the transition temperature, Hissink ^ finds 159.8° with a 

 dilatometer, and Guinchant * gives 159°. The value found by extra- 

 polation of the above data of my own was 159.4°. The change of 

 linear dimensions during the transition has been measured by Guin- 

 chant,* who found an increase of from 0.22 to 0.25% on cooling, and a 

 decrease of 0.17% on heating. Assuming 4.3 for the density, this is 

 equivalent to about 0.00047 cm^ per gm., against 0.00250 found 

 above by direct methods. This shows that the change of volume 

 during the transition does not take place equally in all directions, an 

 effect shown by other substances also. Guinchant ^ has also found 

 the latent heat of the transition to be 4.9 cal. against 3.4 found above. 

 The agreement should be closer. 



The difference of compressibility of the two modifications was 



2 W. Schwarz, Diss. Gott. 1892; Beibl. 17, 629 (189:3). 



3 D. J. Hissink, ZS. phys. Chem. 32, 537-563 (1900). 



4 J. Guinchant, C. R. 149, 569-571 (1909). 



5 J. Guinchant, C. R. 145, 320-322 (1907). 



