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COLLOIDS. 19
loids is equal to that of the pure solvent or differs from it only slightly.
On the contrary hydrophile colloids are, in proper concentration, very
viscous which is probably the reason that they gelatinize under certain
circumstances. Pauli as well as Pauli and Handovsky1
have inves-
tigated strongly dialyzed serum in regard to its internal friction. The
addition of a little salt (to 0.05 normal) causes a lowering of the internal
friction below that of a pure albumin solution, while acids and alkalies
in small amounts cause a powerful rise in the viscosity.
Optical Properties. Colloidal solutions are opalescent by reflected
light, which depends upon the fact that the ligljt is reflected by the sus-
pended particles. The reflected light is partly polarized. This phenom-
enon, called Tyndall’s phenomenon, depends upon the presence of
small particles in the liquid, and is considered as a test for colloid solu-
tions. Still there are colloid solutions (certain gold solutions, Zsig-
mondy), which do not give Tyndall’s phenomenon, and on the other hand
we also have solutions of certain high molecular crystalloids (cane
sugar, rafnnose), which produce this phenomenon.2
With the aid of the ultramicroscope of Siedentopf and Zsigmondy,
it has been made possible to see the colloidal particles directly.3
In
this apparatus the colloidal particles are strongly illuminated by direct
light, so that no ray of light falls directly into the eye of the observer.
The particles are hereby made visible on account of the formation of
diffraction disks which are visible through the miscroscope. In colloidal
solutions where the particles are close together, a more or less intense,
homogeneous, polarized sphere of light is seen in the microscope where
the individual particles cannot be distinguished from each other. This
is possible on diluting the solution. Those particles which are only
made visible by dilution are called submicrons, while those that gradually
disappear on dilution are called amicrons.
The investigations of Zsigmondy and others upon the growth of colloidal
metallic particles are also interesting. Thus the reduction of gold chloride by
formaldehyde, whereby colloidal gold is formed, is accelerated by the addition of
colloidal gold, and the added particles indeed grow at the cost of the newly
reduced gold. 4
In a similar manner the reduction of silver nitrate with ammonia
and formaldehyde is helped by the addition of colloidal gold when the reduced
silver precipitates upon the gold particles. 5
In such processes the amicrons can
enlarge so that they can be observed by the ultramicroscope (submicrons).
’Pauli, Koll. Zeitschr., 3, 5 (1908); Pauli and Handovsky, Biochem. Zeitschr.,
18, 340 (1909); 24, 239 (1910).
2
Lobry de Bruyn and Wolff, Rec. trav. chim. des Pays-Bas., 23, 155 (1904).
3
Zsigmondy, Colloids and the Ultramicroscope, translated by Alexander, New York,
1909.
* Zsigmondy, Zeitschr. f. physik. Chem., 56, 65 (1906).
5
Zsigmondy and Lottermoser, ibid., 56, 77 (1906).
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