;;- APPLICATIONS OF THE VISCOMETRIC METHOD 293

t

Fig. 91, so that at about 43°C nitrocellulose in acetone would
appear to have the properties of a true fluid. If above 43° we
have a true solution, this temperature is a transmission point
which is analogous to the melting point of a solid. This does
not mean however that there would be any marked change in
the working properties as nitrocellulose solutions below 43° are
extremely soft solids.

50

0

40

<u o

.120

10

10

60

70

80

20 30 40 50
Friction ih dtjnes per cm2
FIG. 91.—Temperature-friction curve for a colloidal dispersion of 7.708 weight
percentage of nitrocellulose in acetone.
Solubility and Plasticity.—It is of course well-known that the
so-called "solutions'7 of nitrocellulose, gelatine and other colloids
are not true solutions, nevertheless the term solution as applied
to colloidal dispersions often leads to confusion. Thus acetone
is one of the best " solvents" for nitrocellulose, being superior
to let us say, amyl acetate. But what does this statement
mean? It cannot possibly mean that acetone will actually
dissolve more nitrocellulose than will a similar amount of amyl
acetate, for there is no point of saturation for either, i.e., both
liquids will "dissolve" or better disperse an indefinite amount of
colloid; hence the term solubility has here a very special, albeit
a very definite, meaning, viz., that dispersive medium is the best
solvent which with a given amount of colloid gives an emulsion
having the maximum mobility. Here however there enters the