POLYMORPHIC TRANSFORMATIONS OF SOLIDS. 123 



such as to permit the comtamination of the Hquid at high pressures. 

 Tammann finds for AV at atmospheric pressure 0.0573, and at 1455 

 kgm. 0.0378, against the corresponding values 0.0599 and 0.0322. 

 The agreement at atmospheric pressure is comparatively good, but it 

 is unusual that the discrepancy should be in the direction it is at high 

 pressures. Other values for comparison are 47° to 50° for the melting 

 point by McCreath,^^ and 48.5° by Block.^^ Block gives for the 

 change of volume 0.0576. For the latent heat there is 40.8 cal. by 

 a direct method and 41 by a cryoscopic method by Eykmann.^^ The 

 value found above, 40.8, is in unusually good agreement. 



Urethan has three triple points at each one of which the difference 

 of compressibility etc. between the phases can be calculated. The 

 data are not accurate enough, however, at the two triple points in- 

 volving the liquid to give accurate results. Thus a solution of the 

 equations at the triple point L-I-II demands that I be very much more 

 expansible than the liquid, which seems very unlikely. Probably a 

 large part of the error is occasioned by the abnormally rapid change of 

 slope of the AV curve for L-II. The only information of value is, 

 therefore, that given at the triple point I-II-III. We find on solving 

 the six equations at this point the following values, 



Aai2 = O.O5II 



Aa23 = O.O4II 



Aai3 = O.O4I2 



Ai8i2 = - O.O55 



• Ai323 = O.O3I35 



AjSis = O.O3I3O 



ACpi2 = - 0.127 > 



ACp23 = 0.43 r kgm. cm./gm. 



ACpi3= 0.30 ) 



There is doubtless a large amount of uncertainty in these numerical 

 values; probably Aoios is too large, and Aj8i2 too small. The signs of 

 the quantities are of interest. The compressibilities follow the order 

 of volumes, as we should expect, and the specific heats the order of 

 temperatures. Thus as we raise temperature at constant pressure we 

 may change III to I, and then I to II. On passing through each of 

 these transitions the specific heat increases. As regards the thermal 

 expansion, we do not know whether to expect the phase at the higher 

 temperature or that with the larger volume to have the greater expan- 



33 D. McCreath, Ber. D. Chem. Ges. 8, 383-384 (1875). 



