ADAMS AND WILI^IAMSON : PHYSICAL CONSTANTS OF MUSTARD GAS 33 



obtained by multiplying the cross-section of the piston by the 

 quantity i?2 — ^i which is obtained graphically (see Fig. i ) . 

 The necessary readings must be taken with decreasing pressure 

 since, on account of the propensity of liquids for under-cooUng 

 to a temperature several degrees below their melting point, 

 they will generally support a pressure far beyond the true freezing 

 pressure before soUdification takes place. ^ Dichloroethylsulfide 

 requires at ordinary temperatures about looo megabars super- 

 pressure to start it freezing. This corresponds to an under- 

 cooling of 14°. ,— 



PRESSURE 



Fig. I. Diagram to illustrate how the freezing pressure and consequent change 

 of volume may be obtained by plotting pressure against piston-displacement. 

 When the pressure on the liquid is raised, freezing does not take place promptly 

 and the liquid thus passes into the undercooled or metastable region, as shown by 

 the dotted line. 



The results for the freezing-points at several pressures are 

 shown in figure 2 and in table 2, which also gives the change of 

 volume V — V^ in cm^ per gram. From these results it may 



' A corresponding superheating of the solid has been observed only in rare in- 

 stances. 



