13 
It follows from this that at 0° C. Hg-vapour is not yet completely 
Hg,, nor completely Hg, at 500° C. Yet it seems that between 
the boiling point (857°) and 500° the greatest deviation occurs 
between the real vapour curve and that which would hold, when 
mercury were Hg, all over the temperature range. From this moment 
the approach to the latter curve will become closer and closer, 
and /,, will gradually increase to a value in the neighbourhood of 
3, nesp...f-to 7. 
The vapour pressures used are Ramsay and Youne’s. At 50° the 
mean value has been taken between that of R. and Y. (0,015) and 
that of Hertz (0,013); at O° C. a value between that of Hertz 
(0,00019) and that of v. p. Praars (0,00047). For log pe has been 
taken /og'® (192 X 760) = 5,1641. Further f,, has been calculated 
from fi, = (5,1641 — log’ p): eld 
d) Calculation of pr and Tr from vapour pressure observations. 
Reversely, when 7% is known and pz unknown, we can determine 
the value of pj from the vapour pressure course by approximation. 
For log pr —logp, =f(Ti_-T): F, and log p,-—log p,=f (Ti-T): T, 
follows from the vapour pressure formula for two temperatures 7’, 
and 7’, so that 
log pr—log p, en Ie G— 
log pr—log p, 7G ‘ie 2077 4 
from which pz can be determined. If for one of these temperatures, 
eg. for 7,, we take the boiling point 7, then in atm. p, = 1, 
log p, =O, and (if we simply write 7 for 7,) we get: 
log pe—logp _ Ts Ty—1 
log pk Bui TR IN 
from which follows: 
EAT, 
log Pk = log p ed Te OEE ite Or (3) 
In this it has of course been supposed that the value of f does 
not appreciably differ for the two temperatures. The greatest chance 
to realise this exists, when we remain in the neighbourhood of the 
minimum of f. This may be illustrated by a few examples. PELLATON 
has found for chlorine (These, Neuchatel, 1915, p. 20) p= 2766 mm. 
at O° C. At this temperature and at the boiling point — 34°,5 f is 
near the minimum (somewhat above 6° C.). For 7% was found 
144°,0 C., so that we have in atm: 
