POLYMORPHISM AT HIGH PRESSURES. 165 



later withdrew this. I could find none to 12000. KXO2 shows a 

 transition line of rather large and unusual curvature; it is unfortunate 

 that the substance was so impure. 



The reason for trying the six substances from KNO2 to Potassium 

 Acid Tartrate was merely that polymorphism seemed to he rather 

 more common among the compounds of Potassium than of other ele- 

 ments, and it seemed worth while to try a number of examples. If 

 this surmise is correct, these six substances were not such as to bear 

 it out. K4P2O7 was tried only at 20°, because it had some water in it. 

 K4S2O7 was tried at 20° and 200° as usual. Both wei'e without result. 

 Several runs were made with K0CO3. The first showed a small tran- 

 sition near 6500 kgm. at 200° and at a somewhat higher pressure at 

 185°. This was found unmistakably with two different fillings of the 

 apparatus. But after standing in the apparatus for three days, the 

 transition at 185° had entirely disappeared, and that at 200° had a 

 smaller change of volume. An attempt to repeat the measurements 

 after sLx months showed no trace of the transition. The result is hard 

 to explain. I am inclined to think that the transition may be a genu- 

 ine one, but that 200° is in the very viscous region, so that sometimes 

 the transition will run, and sometimes not. The effect may, however, 

 be due to moisture; the first specimen may possibly have been a 

 trifle moist, but the second was carefully dried in vacuum. 



Methyl Oxalate was tried because Tammann ^^ has announced two 

 modifications. I could find no other form; the matter has been dis- 

 cussed at considerable length in the paper on melting. 



There was no particular reason for trying Urethane, except possibly 

 its rather interesting method of decomposition on heating, and its poly- 

 morphism seems to have no suggestive connection with that of other 

 substances. Naphthaline and Sugar offered no special promise of poly- 

 morphism; they are simply substances readily available for miscel- 

 laneous exploration. 



Discussion. 



A compact summary of the nature of the effects for all the poly- 

 morphic transitions investigated to date is given in Figures 30 and 

 31. These diagrams show the location of the transition lines between 

 the several phases, which are indicated by Roman numerals or by L 

 for the liquid; the arrows on the lines show the direction in which Af 



39 G. Tammann, Kristallisieren unci Schmelzen, p. 205. 



