Determination of Chemical Constants. 17 



Tabl#V t . 





X . 



^mai. 



X-M.Pt. 



^B.Pt. 



X-oalc. 



Mercury 



. 15530+ 15 



15646 



14884 



14161 



14900 



Cadmium .. 



. 26770+750 



26941 



26292 



— 



269C0 



Zinc 



. 31330+750 



31570 



30871 



— 



31200 



Constants of other Metals. 



Measurements of the vapour pressure of other monatomic 

 substances which have been made do not lend themselves 

 readily to a calculation of the constant, as most of them 

 would involve a knowledge of the specific heat of the 

 molten metal. 



Langmuir and Mackay (Phys. Zeit. xiv. p. 1273, 1913, 

 and Phys. Rev. 4, ii. p. 377, 1914) have, however, investi- 

 gated the vapour pressure of solid tungsten, platinum, and 

 molybdenum by measurement of the loss of weight of a 

 heated filament in vacuo. Using a vapour-pressure formula 

 of the Hertz type, the results are expressed : 



Tungsten, logp (mm.) = 15*502 -47440/T- 0*9 log T. 

 Molybdenum, log p = 17-354 -38600/T- 1*26 log T. 



Platinum, logp = 14'09 -27800/T- 1*26 log T. 



By differentiation, 



Tungsten, = 216850-1*79 T. 

 Molybdenum, = 177000-2-5 T. 

 Platinum, = 128000-2'5T. 



In the latter cases, if (c p —C p ) is constant throughout 

 the temperature scale to T° absolute, then the constants 

 177000 etc. will represent the value of \ ; and as C p = 4*963, 

 c p is taken as 7*64 in the last two cases and as 6*75 in the 

 case of tungsten. Langmuir and Mackay have applied 

 their results to the formula : 



log^-^ + l-TSlogT-^T+C, 



and have obtained values for the constants as follows : — 



Tungsten .... 3*5 

 Platinum .... 1*5 

 Molybdenum . . . 4' 4 

 Phil. Mag. S. 6. Vol. 39. JS T o. 229. Jan. 1920. C 



