ON EVAPORATION AND DISSOCIATION. 
117 
sections : that in which pressure has no apparent influence on the temperature of 
volatilization., and that in which its influence is partial. 
§ 77. Judging from the determinations of vapour-density which have been made, it 
may fairly be stated that those substances which show the greatest amount of disso¬ 
ciation also show the greatest divergence between the curves, and the least connexion 
between pressure and temperature of volatilization. Thus with chloral hydrate, the 
dissociation of which is almost complete, the temperature of volatilization is indepen¬ 
dent of pressure, and with ammonium carbamate, which is completely dissociated, this is 
also noticeable. Again, with succinic and phthalic acids, the former of which, judging 
from its melting-point, is probably more stable than the latter, the curve representing 
temperatures of volatilization of the former is more obvious than that of the latter; 
and with aldehyde ammonia, the two curves of which have the same general form and 
which are not widely divergent, the determination of vapour-density shows that the 
dissociation is only 70 per cent, at 78 , 25°, under greatly reduced pressure. Again, 
with the alcoholates of chloral, which appear to show the same behaviour as chloral 
hydrate in the still, the melting-pressure is so low, and the rate at which they volati¬ 
lize so rapid, that any small indication of temperature being influenced by pressure 
might well have escaped observation. Moreover, we are not able at present to 
compare the relative stability of these bodies with that of phthalic and succinic acids. 
Had we to deal with this class alone it would have appeared that the statement at 
the beginning of this paragraph might be general; but ammonium chloride forms so 
marked an exception that this view can hardly be sustained without qualification. 
§ 78. It is also to be noticed that the chloral compounds with water and with the 
alcohols belong to the class of dihydric alcohols, or their methyl or ethyl derivatives. 
Thus aldehyde itself, when mixed with water, evolves heat, and undergoes con¬ 
traction ; and this behaviour would point to the formation of the dihydric alcohol 
riiT nvx I OH 
k±i 3 k±i 
Now such a union, if this constitution be accepted as correct, involves the breaking 
down of a molecule of water, and the partial molecular rearrangement of aldehyde, so 
that from CH 3 CHO there is formed CH 3 CH(OH) 3 . With the methyl and ethyl 
compounds the substance produced is at once a monohydric alcohol, and a methyl or 
ethyl ether, thus :— 
cc1 3 ch {och 3 - cc1 3 ch {8c s h 5 - 
In the case of ammonium carbamate its formation involves the breaking down of 
one molecule of ammonia, and the formation of an ammonium group, NH 4 , and of an 
amido group, NH 2 : and this necessitates the actual transfer of an atom of hydrogen 
from one nitrogen-atom to the other, thus :— 
OCO+2NH 3 = OC 
JONH, 
1nii 3 • 
