POLYMORPHISM AT HIGH PRESSURES. 163 



still less at higher pressures at room temperature. Therefore, if 

 arragonite had been formed at all in this experiment, the reverse 

 transition would not have run, and we infer that 12500 kgm. at 200°, 

 is not a high enough pressure to bring the calcite into a region where 

 the reaction to arragonite spontaneously runs. The specimen used 

 was not large enough to enable me to tell whether there was any 

 reversible change in the calcite itself up to 12000 at 200°. 



Substances with Reversible Transitions at higher Tempera- 

 tures. — K2Cr04, rhombic, triclinic; K2Cr07, two triclinic forms; 

 CU2I2*, cubic. 



These all have transitions near a red heat; it is not known to what 

 type they belong. If they were of the ice type, a high pressure might 

 bring the transition down to the range of this work. Reason has been 

 given for supposing that the transition found at 200° for CU2I2 is not 

 the same as the previously known high temperature transition. 



Organic Acids. — Carbolic*; Acetic*; Monochloracetic ; Stearic; 

 Tartaric, monoclinic; Benzoic, monoclinic; Oxalic, rhombic; Citric; 

 Propionic; Formic. 



Pressure was transmitted to all of these substances by mercury. 



I have already mentioned we would expect from Tammann's 

 theory of polymorphism that enantiotropic transitions would be 

 particularly common among the organic acids. Out of ten substances 

 tried, only two examples were found, and these were not new examples, 

 but were known before. If ten is a sufficient number of instances to 

 give the law of chances a fair test, this is not favorable to the theory. 



In the above list, monochloracetic acid, which has been previously 

 mentioned, has unstable forms, but this sort of transition is not con- 

 templated by Tammann's theory. Oxalic acid has also been men- 

 tioned; it was tried with and without the water of crystallization. 

 Citric acid is listed as crystallizing with water, but as I could produce 

 no change in its appearance by heating to 100° in vacuum for several 

 hours, I assumed that it had been supplied in the anhydrous condition. 



The tartaric acid was dried in vacuum at 100°. At 12000 kgm. it 

 showed no change on heating from room temperature to 200°, but on 

 releasing pressure at 200°, decomposition began at 5000 with an in- 

 crease of volume large enough to bring the pressure back to at least 

 7100. The product of decomposition was a sticky putty -like mass, 

 which swelled up and overflowed the mouth of the tube when the 

 apparatus was opened after cooling. The Nickel steel shell was split. 

 Professor Kohler was kind enough to examine the substance, and found 

 that it was one of the anhydrides of tartaric acid. 



