larly with the acetate. The normal acetate gives
solutions of high viscosity, films of considerable tenacity, and when
those are saponified the cellulose is regenerated as an unbroken film.
The acetates of hydrolysed celluloses manifest a retrogradation in
structural and physical properties, proportioned to the degree of
hydrolysis of the original.

We may take this opportunity of pointing out that the celluloses not
only suggest with some definiteness the connection of the structural
properties of visible aggregates--that is, of matter in the mass--with
the configuration of the chemical molecule or reacting unit, but supply
unique material for the actual experimental investigation of the
problems involved. Of all the 'organic' colloids cellulose is the only
one which can be converted into a variety of derivative forms, from each
of which a regular solid can be produced in continuous length and of any
prescribed dimensions. Thus we can compare the structural properties of
cellulose with those of its hydrates, nitrates, acetates, and benzoates,
in terms of measurements of breaking strain, extensibility, elasticity.
Investigations in this field are being prosecuted, but the results are
not as yet sufficiently elaborated for reduction to formulae. One
striking general conclusion is, however, established, and that is that
the structural properties of cellulose are but little affected by
esterification and appear therefore to be a function of the special
arrangement of the carbon atoms, i.e. of the molecular constitution.
Also it is established that the molecular aggregate which constitutes a
cellulose is of a resistant type, and undoubtedly persists in the
solutions of the compounds.

It may be urged that it is superfluous to import these questions of
mass-aggregation into the problem of the chemical constitution of
cellulose. But we shall find that the point again arises in attempting
to define the reacting unit, which is another term for the molecule. In
the majority of cases we rely for this upon physical measurements; and
in fact the purely chemical determination of such quantities is
inferential. Attempts have been made to determine the molecular weights
of the cellulose esters in solution, by observations of depression of
solidifying and boiling-points. But the numbers have little value. The
only other well-defined compound is the sulphocarbonate. It has been
pointed out that, by successive precipitations of this com