110 BRIDGMAN. 



The superpressure required to start the reaction also varies greatly; 

 on one occasion 1750 kgm. beyond the equilibrium point was necessary 

 at 0°. 



Carbamide is an unusual organic compound with respect to its 

 melting as well as with respect to its solid transitions. At 150°, in 

 the search for other solid modifications, the melting data were approx- 

 imately determined. The melting pressure is shown by a cross in 

 Figure 8 and the approximate location of the melting curve is shown 

 by the dotted line. The change of volume at 150° is very low for a 

 melting, about 0.01 cm.Vgm. This, together with the unusual flat- 

 ness of the melting curve, a flatness that one associates with the melting 

 of a metal rather than of an organic compound, aroused the suspicion 

 that this point might really belong to a solid transition instead of to a 

 melting. To test this, another run was made at 124°, slightly below 

 the melting point at atmospheric pressure, and no transition of any 

 kind found. It was for this run that the carbamide was melted into 

 the form, as mentioned above. That the melting curve has approxi- 

 mately the location shown is also indicated by the fact that on one 

 occasion, on tr^ang to extend the I-II line to 170°, the transition 

 entirely disappeared, doubtless because of melting. It has already 

 been explained that the decomposition makes it useless to try for 

 accurate coordinates of the melting curve. 



The direct experimental measurement of the difference of the com- 

 pressibility of the several phases did not give results accurate enough 

 to justify an attempt to calculate AjS and ACp. It seems established, 

 however, that I is less compressible than II, and that the difference is 

 in the vicinity of the order of O.O54 cm.Vgm. per kgm., which is fairly 

 high. It is also probable that I is less compressible than III, but the 

 difference between I and III is considerably less than the difference 

 between I and II. The difference of compressibility between I and 

 II, large as it is, is not nearly large enough to account for the rapid 

 drop of Av between I and II with rising temperature. It is very 

 probable, therefore, that II is more expansible than I, and that the 

 difference is of the order of 0.0001 cm.Vgm. per degree. 



Camphor. — This material was obtained from Eimer and Amend, 

 "gum camphor, refined, powdered," and was used without further 

 purification. It was hammered cold into the inverted cup, and pres- 

 sure transmitted to it by mercury. The reason for trying this sub- 

 stance was that it is known to have an abnormally steep melting curve, 

 the rise of temperature being about 130° for 1000 kgm.^^ This sug- 



22 G. A. Hulett, ZS. phys. Chem., 28, 029 (1899). 



