Determination of Chemical Constants. 5 



and if C be differentiated with respect to 1\ : 

 dC_ T, . . ft J - Tt-T, log T/Ts-TA 



1 A,-A 3 r 3/2T 1 -5/2T 2 TP + T^ - ] 



d( ■ 

 From which it is clear, that in order to reduce the error -^, 



al 



'Ti — T 2 must be as large as possible and T 2 as small as 



possible. 



Neglecting the last two terms, because (Ax — A^ is small 



and a is a very small constant, about 10~ 5 , it is seen that, if: 



T\ and T 2 are separated by about 50° and T 2 is about 250° 



.and log — about 2, j~ will be about 0*2, and so, supposing 



the constant to have a value of about 1*5, the error may be 

 40 per cent, for a one per cent.. variation in 1\. Thus the 

 accuracy with which the vapour pressure must be measured, 

 particularly regarding the fixing of the correct temperature, 

 is very important, if true values of the constants are to be 

 obtained. 



Chemical Constant oj Mercury. 



These considerations will now be applied to the deter- 

 mination of the chemical constant of mercury. There 

 appear to be no other cases in which the vapour pressure 

 and specific heat of a monatomic substance have been 

 measured with the same accuracy. 



A number of determinations of the vapour pressure of 

 mercury have been recorded. Those of Hertz, Ramsay 

 and Young, Gallender and Griffiths?, Pfaundler, Morley, 

 Gebhardfc, Cailletet, Oolardeau, and Riviere have been 

 discussed by Laby (Phil. Mag. Nov. 1908), who concluded 

 that the following formulae summarized the work of these 

 investigators, and gave the closest approximation to the 

 true vapour pressures : — 



From 15° to 270° 0., 

 logp = 1 5-24431 -£'367233 log - ; 



and from 270° to 450° C, 

 loo?) = 10*04087 -0-7020537 log tf- 3 - 71 ^^. 



In 1900, Knudsen {Ann. d. Phys. (4) xxix. p. 179, 1909) 



