177 
temperature would yet render the drawing up of a formula by the 
aid of the caloric data impossible. 
We will only calculate the value of the heat of evaporation from 
our vapour-tension determinations by the aid of the equation of 
CLAPEYRON, which can only be applied for low pressures, because 
the specific volumes along the border line are unknown at higher 
pressure. From the equation : 
dP Q 
ped eee 
we find, neglecting V; with respect to Vyas and applying the law 
of Borre-Gay-Lussac : 
PV (asa) sheer, 
in which a represents the degree of dissociation 
gm CH RT ap 
B dr 
ry? 
im 
In order to calculate Fe have represented our determinations 
at low pressure by an empirical formula. By the aid of the data: 
fm 23,p— 10 mi; t= 11,0, “p= 4639 mid; t= 48.1, p= 2418 ZE 
from our former communication we derive the values a= 1325.6, 
6 = 3.354 , c = — 0.8950 for the constants a, 6, and c in the equation: 
logp = — 7 + blog T + ¢ 
This equation represents our observations of the preceding com- 
munication very well. It may be remarked here in passing that this 
expression can represent the observations at higher pressure even up 
to about 120° and 36 atmospheres. At higher pressures the curve 
calculated from the equation deviates slightly towards lower pressure ; 
in the immediate neighbourhood of 7), the deviations become greater ; 
still even at 7}. the deviation amounts only to about three atmospheres. 
It is remarkable that this formula drawn up from observations below 
3 atmospheres, is able to represent the vapour-tension line of this 
complicated substance so accurately. 
If we now differentiate the obtained expression we find: 
TPE de 
ET i fi 
which yields after substitution: 
1325.6 
esas + #)R+ 3.354 (1 + 2) RT, 
