165 
1914-15.] Vapour Densities of the Ammonium Halides. 
is about 39 per cent, at 320°, and diminishes steadily to about 8 to 10 
per cent, at 388°. The dissociation constant also passes through a maximum 
near 320°, and thereafter decreases steadily. There seems to be no escape 
from the conclusion that, above 320°, the heat of dissociation of ammonium 
bromide is positive. A positive heat of dissociation in solution is not 
uncommon, but in the gaseous state it has not previously been observed. 
The case of hydrogen iodide is not nearly so simple a case of dissociation, 
because there the products unite to form molecular hydrogen and iodine. 
