POLYMORPHISM AT HIGH PRESSURES. 97 



The difference of compressibility, thermal expansion, and specific 

 heats between the several phases may be found in the usual way.^^ 

 Direct experimental values were found for the difference of thermal 

 expansion of the liquid and I at 78 kgm., and for the difference of 

 compressibility of II and I. For the difference of expansion (A/3) I 

 found 0.00050, which is in rather good agreement with the value of 

 Block, ^^ 0.00048. It is somewhat of a puzzle that Block seems to be 

 able to get good values for A/3, whereas his values for Av are often 

 widely in error. Using the experimental value of A/3 and the values 



for -J— and —; — that may be computed from the table, we find Aa = 



O.O464 and ACp = -0.016 cal. The negative value for ACp is im- 

 probable in spite of the experimental statement of this by Guillot.^^ 

 If we assume that ACp = 0, the approximation usually made, the 

 value of Aa becomes O.O465 and ths value of A/3 O.OsoG. This shows 

 again the insensitiveness of Aa to the value of ACp. The fact that 

 A/3 calculated with this assumption differs no more from the ex- 

 perimental value makes it probable that ACp is really very small 

 at atmospheric pressure. 



The values of Aa and A/3 calculated at other points on the L-I and 

 the L-II curves are shown in Table II, making the usual assumption 

 that ACp = 0. This is probably a bad assumption on the L-I curve 

 at 2000 kgm., because the value for A/3 becomes too small, and also 

 because Aa does not check with the values for L-II and I-II. Also 

 error is probably introduced by a too rapid increase of AH on the 

 upper end of the L-II curve. If the calculations are carried through 

 at 10000, A/3 is negative. On the I-II curve the experimental data 

 showed that the difference of compressibility of I and II is not more 

 than O.O5I. This gives us a maximum value of AjS O.O52. As men- 

 tioned above, the values given for Aa (L-I and L-II) do not check 

 at 2000 with Aa (I-II); probably Aa (L-I) is too large. 



In an unsuccessful search for other modifications pressure was 

 carried to 12500 kgm. at 30° and 170°. It should be remembered that 

 Tammann found some evidence for another form below 0°; I did not 

 investigate this. 



AcETAMiDE. — This substaucc has a second modification of the 

 solid, not known before, the transition point being at 6000 kgm. at 

 room temperature, and the transition line running nearly vertically. 

 The material was obtained from Eimer and Amend. Two series of 



16 D, p. 100. 



