144 BRIDGMAN. 



taken only because it happened to be on hand, and because it is known 

 to be polymorphic. Only a small quantity was available; I procured 

 all there was to be obtained in this country — 3 ounces from Merck 

 and 2 ounces from Eimer and Amend. This was only a quarter of the 

 amount required to fill the apparatus. 



The transition point at atmospheric pressure to the second modifica- 

 tion is known to be at about 225°, and the transition is of the ice type. 

 The effect of pressure has been investigated by Werner ^^ up to 3000 

 kgm. He found that the slope of the transition line is only —2° for 

 1000 kgm., and that the change of volume is excessively small, much 

 too small to allow accurate measurements in the present apparatus 

 with the small quantity of material available. It should, however, 

 be easy to detect the presence of the transition. The particular 

 interest of this present examination is in finding whether there is not a 

 third modification, giving a diagram like that of Agl. The change of 

 volume to this third modification would be larger than that between 

 the two known forms and should be readily detectible if present. The 

 principal result obtained by my work is that there is no third modifica- 

 tion up to 12000 at 200°. At higher temperatures the presence of 

 the transition line investigated by Werner could be detected, and his 

 conclusions verified within my wide limits of error. My data are not 

 inconsistent with his value for the slope of the curve, and my value 

 for the change of volume agrees with his at 2500 kgm. My data do 

 not indicate, however, the unusually large increase of Av found by 

 him toward the high pressure end of the line, but would indicate 

 approximate constancy of Av. The pressure limits of indifference to 

 the transition are wide, and become rapidly wider at the higher pres- 

 sures. W'erner's data indicate the same thing, although his measure- 

 ments were made with increasing temperature at nearly constant 

 pressure, whereas mine were made at constant temperature. 



Ammonium Potassium Phosphate [(NH4)2KP04]. — This substance 

 was obtained from Eimer and Amend, "tested purity," and showed 

 CI 0.0005%, Fe 0.0002%, with traces of SO3 and CaO. It crystallizes 

 with four molecules of water. For this experiment the Mater was 

 removed by heating to 100° in vacuum for two hours. The dried salt 

 was then hammered cold into an open steel shell, and pressure_trans- 

 mitted directly to it by kerosene. 



There is a transition to a new form at high pressures and at tempera- 

 tures in the neighborhood of 100°, but in this vicinity the substance^is 



28 M. Werner, ZS. anorg. Chem., 83, 275 (1913). 



