164 
Proceedings of the Royal Society of Edinburgh.' [Sess. 
between the degree of dissociation and the density, small errors in the 
latter are greatly exaggerated in the former. Hence, the values of a were 
plotted, a representative straight line was drawn through them, and the 
interpolated values given by this line were used in other calculations. 
The degree of dissociation is almost constant, ranging from 66*8 per cent, 
at 280° to 62*8 per cent, at 330°. 
The heat of vaporisation was calculated from the vapour densities, and 
the dissociation pressures of Smith and Calvert, by means of the Clausius- 
Clapeyron relation. The results ranged from 32*5 kg. cal. per mole at 280° 
to 33*3 at 330° (average, 32‘9). This value is lower than the previously 
published values (Horstmann, 37*4 to 43’9 ; F. M. G. Johnson, 37‘8), because, 
in calculating the latter, the erroneous assumption of complete dissociation 
was made. The dissociation constants (K^) were calculated from the 
smoothed values of a, and from these the heat of dissociation was derived. 
In calculating the latter, van’t Hoffs equation, conjunc- 
tion with Arrhenius’'^ relation, U=:Aq— CT^ was employed, and it was 
found that U= — 12,800 — 0*00967T2 g. cal. per mole. 
The cases of ammonium bromide and iodide were treated in the same 
way, with the following results : — 
Ammonium Bromide. 
Temp. 
V.P. 
Density, 
g. per c.c. 
Per cent. Dissociation = a. 
Observed. 
Interpolated. 
300 
55-0 
0-000130 
16-1 
(47-9) 
310 
74-6 
(43-6) 
320 
100-6 
0-000192 
38-7 
39-3 
330 
134-7 
0-000259 
35-7 
34-9 
340 
178-8 
0-000348 
31-6 
30-6 
350 
236-6 
0 000478 
24-7 
26-3 
360 
310-4 
0-000633 
21-7 
22-0 
370 
404-8 
0-000836 
18-4 
17-7 
380 
525-5 
0-00110 
15-3 
13-4 
388 
644-4 
0-00142 
8-0 
10-0 
The densities, observed (I) and calculated (II and III), are shown in 
hg. 2. On account of the low vapour pressure at 300°, and of the small 
weight of material vaporised, the observation at this temperature may be 
neglected for the present. They suggest, however, that the values of a 
reach a maximum at 320°. The consistent results at the other temperatures 
(52 determinations in all) show definitely that the degree of dissociation 
* “ Energieverhaltnisse bei Dampfbildimg iind bei elektrolytiscbe Dissoziation,” Meddel. 
Jr an K. Vet-Ahads. Nobelinstihd, Band ii, No. 8. 
