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 HANDOVSjcY 1 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 light 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 TYND ALL'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 ultramicroscorje 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 ca\\ed_submicrons, while those that gradually 

 disappear on dilution are called amicrons. 



The investigations of ZSIGMONDY and others upon the giowth 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) . 



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. 



4 Zsigmondy, Zeitschr. f. physik. Chem.. 56, 65 (1906). 



5 Zsigmondy and Lottermoser, ibid., 56, 77 (1906). 



