ON EVAPORATION AND DISSOCIATION. 
101 
Pressure 
(uncorr.). 
Correction for 
column of 
hot mercury. 
Pressure 
corrected 
to U°. 
Pressure 
due to air. 
Pressure of 
ammonium-, 
chloride vapour. 
Volume. 
P. V. 
Vapour- 
density. 
millims. 
miilims. 
millims. 
millims. 
millims. 
cub. centims. 
O 
(1) 
146'8 
141 
160-3 
3-7 
41 "66 
(2) 
286-8 
7-6 
293-3 
21 
134-6 
73-52 
9,896 
15-09 
(3) 
286" 5 
7-5 
292-9 
21 
134-2 
74-11 
9,946 
15-06 
(4) 
286-2 
7-4 
292-5 
2-0 
133-9 
74-57 
9,985 
14-96 
(5) 
285-2 
7-2 
291-3 
1-95 
132-75 
75-41 
10,010 
14-92 
(6) 
282-3 
6-7 
287-9 
1-7 
129-6 
77-75 
10,077 
14-82 
(7) 
281-2 
6'45 
286"55 
1-55 
128-4 
78-78 
10,115 
1477 
(8) 
286-2 
7-5 
292-6 
1-6 
134-4 
73-91 
9,934 
15-04 
(9) 
287-0 
7-75 
293-65 
21 
134-95 
72-73 
9,815 
15-23 
00) 
280-1 
6-5 
285‘5 
1-55 
127-35 
78"56 
10,005 
14-94 
Of these determinations, (1) was with mercury alone, in order to determine the 
amount of air present. From the rest it would appear that the vapour of ammonium 
chloride is not wholJy dissociated at pressures near those of saturation ; the per¬ 
centage number of molecules decomposed varying from 75’57 to 81'04 within the 
limits of these experiments. It may also be noticed that, in general, the higher 
the pressure, the higher the vapour-density. 
§ 53. In the case of ammonium chloride it appears that the temperature of 
volatilization depends on the pressure, and that the curve representing the tempera¬ 
tures of volatilization under different pressures is coincident with that representing 
the vapour-pressures at different temperatures. But on inspecting the curve 
(Plate 1) showing the relation between temperature and pressure for ammonium 
chloride it will be noticed that the temperatures of volatilization, represented by 
crosses, lie in all cases lower than the circles representing pressures. Two corrections 
have not been applied, both of which would raise the temperatures of volatilization, 
because we were unable to estimate the exact amount. The temperatures of volati¬ 
lization were ascertained by means of a mercurial thermometer, but the others, 
indirectly by an air thermometer. The thermometers were not compared beforehand, 
and, having been broken during the experiments, a comparison is now impossible. 
Thermometers belonging to the same batch, which were made of German soda-glass, 
have been subsequently compared, and to make their readings coincide with those of 
an air thermometer require to have their readings raised 1'0° at 220°, 1'2° at 230°, 
1'4° at 250°, and 1'5° at 280°. If this correction were applied it is obvious that the 
results of the two methods would form the same curve. Above 280° this comparison 
has not been made. 
Another correction would appear to be requisite for the highest temperatures, owing 
to the temperature of the jacket of mercury-vapour being too near that of the block 
of ammonium chloride. Of this we can form no estimate, but it should be noticed 
that the single observation at the atmospheric pressure, by a method which is free 
