POLYMORPHISM AT HIGH PRESSURES. 157 



related chemically. They are built up by using the radicals NHo, 

 CH3, or OH in various combinations. It is possible, by combining 



any two of these radicals wath C = O to build up six compounds. 



Three of these have polymorphic forms, the fourth, acetone, is liquid 

 throughout most of the range so that new forms would not be expected, 



/0-H 

 the fifth is carbonic acid, C = O , which does not exist in a free state, 



\0-H 

 /NHo 

 and the sixth, C = O , does not exist. We therefore have poly- 



\0-H 

 morphism in all possible cases in this group. The phase diagrams, 

 near the triple point, are similar, in that the transition lines are steep, 

 and the latent heat of transition is small. In closer detail, however, 

 the transition lines are different; those of carbamide and acetic acid 

 are both straight with a positive slope, whereas that of acetamide is 

 distinctly curved and has a negative slope. The diagrams further 

 differ in that carbamide has three modifications and acetic acid and 

 acetamide have only two. We have seen, however, that it is not 

 impossible that acetamide has a third form at lower temperatures, 

 and Tammann ^^ says that acetic acid probably has a third form 

 considerably below zero. It may be that the apparent relationship 

 of the phase diagrams is only accidental; what we know about the 

 crystalline forms would suggest this. The system of acetic acid is 

 not known, carbamide is tetragonal, and acetamide is hexagonal. 

 It has turned out of late years, however, that nearly every so-called 

 hexagonal crystal has been found on careful examination to belong to 

 some other system, the apparent hexagonal form being due to the 

 particular manner of twinning. The evidence from the crystalline 

 forms is not conclusive, therefore. 



It is worthy of remark that the three radicals forming the compounds 

 just discussed are all of nearly the same molecular weight, OH =17, 

 NHo = 16, and CH3 = 15. The four other members of the group 

 listed above are formed by using radicals of quite different molecular 

 weight, and in no case were other forms found. Particularly careful 

 search was made in the case of formamide and propionic acid. Two 

 specimens of propionic acid were used, from different sources. Neither 

 was very pure, but the second was somewhat purer than the first. 



35 G. Tammann, Kristallisieren und Schmelzen, p. 277. 



