182

FLUIDITY AND PLASTICITY

is weak, as with potassium iodide, silver nitrate and urea, the
positive curvature quickly shows itself.

The first important attempt to explain the lowering of the
viscosity of water was made by Arrhenius (1887), who thought
that it might be due to electrolytic dissociation. Wagner and

24-0

40 50 60
Per Cent
FIG. 70.—Fluidities of potassium halide solutions in water at various tempera-
tures. The curves show negative curvature which is most marked for the
chloride, and at low temperatures and at low volume concentrations of the salt.
At high concentrations or at high temperatures all of these solutions may show
positive curvature, but the nitrate and iodide most readily. (After Gorke.)
Miihlenbem (1903), however, showed that the dissociation
hypothesis was by itself insufficient as an explanation since salts
like NaN03 and K2S04 are highly ionized and yet do not show
negative curvature as does KN03. Now that it appears that
urea and mercuric chloride solutions both show negative curva-
ture, it would seem probable that electrolytic dissociation is
not necessary for the phenomenon. Since these substances in
solution are practically unionized.