Determination of Chemical Constants. 7 



Haber and Kerschbaum (Zeit. Elektrochem. xx. 1914) 

 have determined the vapour pressure of mercury at 20 D C. 

 by the quartz-fibre manometer and obtain a value 0*00126. 

 At this temperature, Knudsen's formula would give the 

 vapour pressure as 0-00119. As Haber's figure is the 

 weighted mean of only two series of determinations (with 

 values 0-00115 and 0-00130 respectively) it can hardly 

 be asserted that Knudsen's values are too low, but it 

 perhaps may be said that the true values are not lower 

 than those given by the Knudsen formula. 



As x T = HT d ^fi, 



ax. 



and above 250° O. : 



\ogp = 9-9073136 -0-6519904 log T- 3 ^ 7 ^ 28 , 

 and below 150° CI : 



log^ =10-5724-0-847 logT-^i^ 6, 

 differentiating and multiplying by RT 2 , 



above 250° 0. : 



X T - 14976-35-1-2924 T, 

 and below 150 3 O. : 



\t = 15277-46- 1-6812 T. 



Now, X T = A + (C/ — c. p ) T, if Cp and 0/ are constant. 



The following graph gives the variation of c p with tempe- 

 rature as determined by Naccari (J.de Pltys. (2) viii. p. 612, 

 1889), by Barnes and Cooke (Phys. Rev. xvi. p. 65, 1903), 

 and others. 



The mean value at the lower temperatures is about 0'0331, 

 so that A T = A — (6'64-4-96)T = X -l-68T, which agrees 

 with the constant derived from the Knudsen vapour-pressure 

 formula. Although the value of. c l p decreases (to 140° C. 

 according to Barnes), it does not do so sufficiently rapidly to 

 account for the value 1*2942 in the equation for X T obtained 

 from the Smith and Menzies formula, and it seems pro- 

 bable therefore that c l can no longer be considered nearly 

 constant. 



