ON EVAPORATION AND DISSOCIATION. 
95 
XIY. Temperature, 340°. Rate, 0T91 millim. per minute. 
Pressure read. 
Time x rate. 
Pressure recalculated. 
Time. 
millims. 
767-4 
0 
millims. 
767-4 
0 
800-3 
35-1 
765-2 
184 
805-0 
40-3 
764-7 
211 
809-0 
48-9 
760-1 
256 
839-2 
77-0 
762-2 
403 
845-9 
86-5 
759-4 
453 
857-9 
95-1 
762-8 
498 
866-9 
103-3 
763-6 
541 
870-1 
110-0 
760-1 
•576 
§ 47. As in all these experiments the volume was kept nearly constant, a constant 
increase of pressure denotes a constant evolution of gas. This would not have been 
the case had the volume been allowed to increase. It seemed impossible that such 
results could have been produced by dissociation; and as it was believed that this 
constant evolution of gas was due to the action of hydrochloric acid on mercury, this 
supposition was put to the test. On heating mercury with ammonium chloride for 
three hours to 330-340° in a sealed tube, it was proved, on treating the contents 
with water, that mercury had entered into solution as chloride. This has also been 
noticed by Than (‘ Annalen/ 131, p. 131). It would appear that the rate of action 
depends on the quantity, and therefore on the surface of the solid ammonium chloride 
present; for it diminished after each escape of gas. It was, therefore, possible to 
calculate the pressure due to the vapour of ammonium chloride or of its products of 
dissociation; for it may be considered certain that the rate of action of ammonium 
chloride on mercury during one experiment is constant. By deducting the product of 
the constant rate of increase of pressure into the time, from the read pressures, the 
true vapour-pressure, or dissociation-pressure of ammonium chloride at each time of 
reading could be calculated. It will be seen that at 320° the vapour-pressure is 
constant throughout the whole experiment; while at 340° the pressure due to 
ammonium chloride rises rapidly during the first twenty minutes, and then remains 
constant. Now, although both the initial and final pressures with clifterent experi¬ 
ments at 340° differed widely from each other, yet, calculating in this way, the result 
varies within very small limits. We are unable to account for the rise in vapour- 
pressure during the first twenty minutes at 340°, and not at 320°; had it occurred 
also at 320° we should have ascribed it to the usual rise of pressure accompanying 
dissociation before equilibrium is established; but, as will afterwards be shown, 
dissociation is complete at both temperatures; there is no doubt, however, that the 
temperature of the ammonium chloride must have been greatly lowered at first by 
volatilization, and it may be due to this cause. 
Since at 320° the initial pressure is the same as that obtained by recalculation, the 
