32 ADAMS AND WILLIAMSON : PHYSICAL CONSTANTS OF MUSTARD GAS 



pheric pressure. The values of Av as a function of P may be 

 represented by a power series yielding the equation 



A^i'o = 4.24 X 10-5 (P - Po) - 6.3 X 10-^ (P - PoY 

 however, the results are expressed equally well by the exponential 

 equation 



A / or — 0.364.10-' (P — Po)l /■ \ 



— MyVo = o.ii8[i— £? " j (i) 



which gives a more reasonable course to the compressibility 

 curve and hence is to be preferred for extrapolating to zero 

 pressure. In the third column of table i are shown the values 

 of —div/vo calculated from equation (i). 

 By differentiation this equation becomes 



-dv/dP = 49 . 5 e-'-'''''"^' ^ (2) 



from which we find the compressibility ( —dv/dP) at P = o to 

 be 49.5 X 10 ~^ per megabar, while at 1000 and 2000 megabars, 

 respectively, the compressibility is 34.4 X 10 ~^ and 23.9 X 



10 



-6 



TABLE I 

 Decrease in Volume op Mustard "Gas" under Pressxhie 



After the conclusion of the measurements of compressibility 

 the freezing pressure and resultant change of volume at a few 

 temperatures were determined. This could be done without 

 removing the material from the apparatus. By referring to 

 figure I it may be seen how the desired quantities may be ob- 

 tained from a series of readings, at constant temperature, of 

 pressure P, and piston-displacement R. When freezing or 

 melting of the substance in the capsule takes place, P remains 

 constant while R increases or decreases and the resulting discon- 

 tinuity at once locates the freezing pressure for the given tem- 

 perature.^ Moreover, the change in volume on melting may be 



' Conf. Bridgman, Proc. Amer. Acad. 47: 415. 191 1. 



