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these two conditions is often very indefinite. Certain chemically definable
classes of substances, such as proteins, occur only or chiefly in the
colloidal condition while others, such as the inorganic salts, occur as
crystalloids. Finally we find others that can occur in both forms, namely
the soaps (page 17). In short the difference between the crystalloid and
colloidal condition may be considered in that the crystalloids occur in
solution as molecules of medium size while the colloids are either very large
molecules, molecular aggregations or at least particles of a larger spacial
volume than the crystalloids. According to such a conception many
properties of the colloids can be explained.
In order to give a better review we will give a classification of the
colloids which seems, for the present, to be rather universally accepted.
This was first suggested by Perrin[1]and later accepted by Höber,[2]
A. Müler,[3]and Wo. Ostwald,[4]although different authors use different
names for the two classes. The classifications of Hardy [5]and
Zsigmondy[6]have also much in common with the classification given below.
One of the two groups of colloids is called hydrophile colloids
(emulsion colloids, emulsoides) because in the aqueous solution a certain
relation still exists between the dissolved substance and the solvent which
is evident especially by a certain viscosity of the solution. The
hydrophile colloids often gelatinize on cooling, the gel is again soluble in
water (reversible), and in general the hydrophile colloids are separated
from their solution by electrolytes with greater difficulty than the
colloids of the second group. Bodies of the greatest importance for
physiological chemistry like the proteins, starch, glycogen, and soaps in
watery solution belong to the hydrophile colloids.
Contrary to the hydrophile colloids, the colloids of the colloidal metal
type are called suspension colloids (suspensoids) as they must be
considered as suspended solid particles in a solvent and have no close
relation to the solvent. The viscosity of the solution does not differ
much from that of the pure solvent; besides this, the suspension
colloids do not gelatinize, do not swell up, and are readily precipitated
by electrolytes. To this group belong the metallic sols, the colloidal
metallic sulphides, and certain typical suspensions obtained by dissolving
water-insoluble substances in another liquid (alcohol, acetone) and then
pouring this solution into a large volume of water. In this way the
substance is precipitated in a finely divided condition. Such suspensions
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