1505 
In fig. 4 I have indicated the experimental results by drawing 
the logarithm of the vapour pressure as ordinate, the reciprocal 
value of the absolute temperature as abscissa. This way of repre- 
sentation has the advantage that the vapour pressure lines thus 
obtained are almost straight, and interpolations and extrapolations 
are more easily and more accurately executed graphically than with 
curves as in fig. 3. That the lines become almost straight with the 
indicated mode of representation, follows from: 
d log P 7: dlog P d log P Q 
= Ee — an == — Ie 
dlr dT dT ens 
which latter equation holds for substances which evaporate without 
dissociation. Hence the slope of the lines amounts to — 4 and is 
therefore constant, when the heat of evaporation does not change 
with the temperature, which is often true in a first approximation '). 
If, namely, the change of Q with the temperature or in other 
words the difference in specific heat between solid and gas is not 
too great, the influence of the temperature is generally small in 
proportion to the great heat of evaporation. This even appears to be the 
case in the observations of somewhat humid NH,Cl, which have been 
carried out by Sita, and which are indicated by asterisks in fig. 4, 
though here a degree of dissociation varying with the temperature 
d log R Q t d log Je: Q 
ar T rr PO ar ~G4e@kT” 
and the change of « with the temperature will therefore also cause 
a deviation from a straight line in fig. 4. The vapour tensions of 
somewhat humid NH,Cl have also been measured by JonnsoN and 
are indicated by triangles in fig. 4. These two series of observations 
correspond therefore with the line OB of tig. 3. There appears to 
be a discrepancy. between the observations of SmirH and JoHNson, 
which for the greater part will no doubt have to be ascribed to 
inaccuracies in the temperature measurement. Besides the results of 
the remarkable experiment with dry ammonium chloride have been 
indicated by squares in fig. 4; they belong, therefore, to the line 
OD DE Ot feed: 
When we compare figures 3 and 4, the given explanation seems 
already little probable; fig. 4, namely, sooner makes the impression 
that the observations of dry NH,CI coincide with those of humid 
NH,Cl than that intersection of the vapour pressure lines would take 
changes the simple equation 
1) The common logarithm has been used in the figures and tables; the modulus 
has therefore been taken into account. (R = 4.571). 
