CHAPTER XV

THE COLLOIDAL CONDITION

Reference has frequently been made, in preceding chapters, to the fact that
proteins, enzymes, lipoids, etc., exist in the protoplasm of plants and
animals in the colloidal condition. The properties and uses of these
compounds by plants depend so much upon this fact that, before proceeding
to the consideration of the actual physical chemistry of protoplasm itself,
it will be appropriate and profitable to give some attention to the nature
and significance of the colloidal condition of matter and of some of the
phenomena which grow out of it.

Every discussion of the colloidal condition in general properly begins with
reference to the work of the English physicist, Thomas Graham, who carried
on his investigations of the so-called "colloids" through a period of forty
years, beginning with 1851. His most important results were published,
however, from 1861 to 1864. Graham studied the diffusibility of substances
in solution through the parchment membrane of a simple dialyzer. As a
result of his earlier investigations, he divided all the chemical compounds
which were known to him into two groups, which he called "crystalloids" and
"colloids," respectively, the first including those substances which
readily diffused through the parchment membrane and the second those which
diffused only very slowly or not at all. He at first thought that
crystalloids are always inorganic compounds, while colloids are of organic
origin. He soon learned, however, that this distinction in behavior is not
always related to the organic or inorganic nature of the compound. He
further discovered that the same individual chemical element or compound
may exist sometimes in crystalloidal, and sometimes in colloidal, form.
This latter discovery led to the conclusion that diffusibility depends upon
the _condition_, rather than upon the _nature_, of the material under
observation.

As a result of the long series of investigations which were stimulated by
Graham's work, the modern conception is that diffusibility is a
_condition_ of matter when in minute subdivision, or in solution, in some
liquid, as contrasted with its _state_, or condition, when existing alone.
That is, the _state_ of a substance may be either gaseous, liquid, or
solid; and its _condition_ when in solution may be either crystalloidal or
colloidal. Substances which are in crystalloidal