158 BRIDGMAN. 



The first specimen showed a small discontinuity like a transition in 

 the region of rounding just before melting, which did not appear with 

 the second specimen. Runs were made with this at 0°, 10°, and 21°, 

 pressure being transmitted by mercury. Only at 20° could the pres- 

 sure be raised to 12000, since at lower temperature the mercury freezes. 

 The approximate coordinates of the melting curve are: 7200 kgm. at 

 0°, 8100 at 10°, and 9100 at 21°. Two specimens of formamide were 

 also used, both impure. The maximum temperature of examination 

 was 26°. The impurity of the best specimen was considerable, be- 

 cause its melting point was below 0°, whereas that of the pure forma- 

 mide is listed as +3°. The formic acid used was also very impure, 

 but it showed nothing except a very rounded melting curve at 26° 

 and 50°. If purer materials were easily available, it might pay to try 

 these substances again. My specimens were made especially to order, 

 and are apparently of as great purity as can be produced in an ordi- 

 nary commercial laboratory. 



The oxamide was tried unsuccessfully at 20° and 200°. It decom- 

 posed slightly at 200°, as shown by a slight evolution of gas on taking 

 the apparatus apart. Pressure was transmitted directly to the oxa- 

 mide by kerosene; all the other substances of this group were sub- 

 merged beneath mercury. 



Compounds with two Benzol Rings. — Diphenylamine, mono- 

 clinic; Benzophenone, rhombic, monoclinic. 



Neither of these had new forms, although benzophenone has several 

 unstable forms. 



Substances with Suggestive Mixed Crystal Relations. — 

 KHSO4*, rhombic; NH4HSO4*, rhombic; MgS047H20, rhombic; 

 FeS047H20, monoclinic. 



The first two of these do not crystallize isomorphously, although 

 belonging to the rhombic system. They form a series of mixed 

 crystals with a gap, the third form being monoclinic. The relations 

 of the several forms have been fully treated in the detailed discussion, 

 and a possible mixed crystal diagram has been indicated. This 

 diagram would demand that the first new form of KHSO4 at high 

 temperatures be monoclinic, and the second new high temperature 

 form rhombic. 



The trichromates of K and NH4 have the same relations as the acid 

 sulfates, and should be tried. 



MgS047H02 and FeS047H20 do not crystallize in the same system, 

 but form a series of mixed crystals with a fairly wide gap. If 

 one extrapolates the density of the crystals with preponderating 



