178 FLUIDITY AND PLASTICITY
That ethyl alcohol and water are less than 7 per cent combined
is surprising in view of the higher amount of combination in
chloroform and ether, but the temperature of comparison is
very much higher, in the case of water and alcohol being 89°C.
But there is another important disturbing factor which must be
considered, in that water and alcohol are both highly associated,
2.31 and 1.83 respectively, so that when the two are mixed there
is almost certainly dissociation.
That dissociation does occur can be proved as follows: We
have seen that when ethyl alcohol and water are mixed there is
a lowering of the fluidity. There is also a pronounced lowering
of the fluidity when acetic acid and water are mixed. There is
furthermore a lowering of the fluidity when acetic acid is mixed
with ethyl alcohol. Yet when acetic acid solution (C2H402.H20)
is mixed with ethyl alcohol solution (C2H60.3H20), having
practically the same fluidity of 43 absolute units at 25°C, there
is a very noticeable increase in the fluidity as seen in Fig. 66 from
the paper by Bingham, White, Thomas and Cadwell (1913).
IV. INFLECTION CURVES
The discussion of simultaneous dissociation and chemical
combination brings us naturally to the consideration of the
fourth type of fluidity-volume concentration curves. There
are several pairs of non-aqueous mixtures which fall into this
class, such as ethyl alcohol and benzene discovered by Dunstan
(1904); but by far more important are certain aqueous solutions of
electrolytes, notably the salts of potassium, rubidium, caesium
and ammonium. That potassium nitrate added to water lowers
the time of flow was discovered by Poiseuille (1847) although
priority is usually attributed to Hiibener (1873). The list of those
substances which lower the viscosity of water has been added to
by Sprung (1875), Slotte (1883) and many others and is given in
Table XLVI. The phenomenon has been often referred to as
"negative viscosity," but since viscosity is a result of friction,
which is never negative in fact, the use of the term is not happy.
The term "negative curvature," d2 (p/db2< 0, where b is the
volume concentration, is not open to similar objection when dis-
cussing the fluidity-volume concentration .curves of these solutions.