747 
I have repeated this determination in ‘the following somewhat differ- 
ent way. I shall consider a gram molecule, so that MN becomes 
Avogapro’s constant and £N the gas constant fh. 
Let, at the temperature 7’, p be the vapour pressure of fluid mer- 
cury, S the entropy of the vapour, S’ that of the fluid, v the 
-volume of the vapour and #' that of the fluid. Then we have according 
to a well known thermodynamic relation 
for which we may write 
as v is much greater than v’. 
If the vapour pressure is very low we may treat the vapour as 
an ideal gas, so that 
RT 
Dit . = e . > ° . . . (14) 
P 
and 
l 
See Rian (15) 
dT 
if 
M 
If now in (6) we substitute A for kN, (14) for »v, a for m, M 
being the molecular weight, and } RT for #, we get 
5 3 5 
S=R }— log(RT)—log p—AlogN + 5 log 2a) +. 5 — Blog(wh){. 
By substituting this in (15) we find 
3 1 Ha (Ti | 16 
UN AE Eee nch pl tain di 
gy Rd? og p) + (16) 
where for shortness’ sake | have put 
5 3 5 
Peo = log (RT) — 4 log N + = log (2 aM) + 5 — Blogh . (17) 
This quantity is completely known. Thus we can calculate the 
coefficient w as soon as we know as a function of 7'and besides 
the entropy 5’ of the fluid. 
$ 10. For the pressure we may use Hertz’s formula *) 
logp = a— Blog T — —. 
9) [2 605 7 
1) H. Herzz, Ann. d. ‘Phys. 17 (1882), p. 198. 
