THE FLUIDITY OF SOLUTIONS

183

Jones and Veazey (1907) observed that potassium, rubidium
and caesium are the elements with the largest atomic volume and
they therefore reasoned that their salts would also be relatively
fluid. From what has preceded we are prepared to find relations
between fluidity and volume, but as a matter of fact the fluidity
of the pure salts in the molten condition is very low. For
example, Foussereau (1885) found the fluidity of ammonium
nitrate to be 0.505 at 185°C and 0.4037 at 162°C, so that at ordi-
nary temperatures the fluidity of the salt in the undercooled
condition would certainly be very low, probably negligible as
compared with water. Furthermore, there are salts which show
negative curvature but in which the metal has a small atomic
volume such as silver nitrate, mercuric chloride, and thallium
nitrate. In view of the periodic relationship of the elements,
the same coincidence noted by Jones and Veazey would occur
with many other properties. Finally there are several salts of
potassium and ammonium which have not been found to show
negative curvature hence the explanation proposed by Jones and
Veazey is not satisfactory.

EXPLANATION OF THE INFLECTED CURVE

As to the reason for positive curvature, it seems probable
from what precedes that it is due to combination between
the solvent and the solute. That many of the salts of potassium,
rubidium, caesium and ammonium exhibit so slight positive
curvature is due to their smaller tendency to form hydrates
than is usually the case in aqueous solution. In contrast with
the salts of potassium, no sodium salts show " negative viscosity."
Perhaps the most striking difference between the salts of sodium
and potassium, generally so similar, is the greater affinity for
water on the part of sodium salts. None of the salts which show
negative curvature crystallize from water with water of crys-
tallization, and the few salts of potassium and ammonium which
do not show negative curvature do exhibit a tendency to form
hydrates. Examples are potassium carbonate, ferrocyanide and
sulfate, and ammonium sulfate. It is true that hydrobromic
acid solutions are probably hydrated, but according to the
measurements of Steele, Mclntosh, and Archibald (1906) anhy-
drous liquid hydrogen bromide has a high fluidity. The small-