COLLOIDS. 41 



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 colloid 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. l 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. 2 In such processes the amicrons can 

 enlarge so that they can be observed by the ultramicroscope (submicrons). 

 According to the manner of preparation the colloids may have particles of different 

 sizes. (See page 39). 



Submicrons have also been detected in solutions of organic colloids. The 

 work of GATIN-GRUZEWSKA and Bnvrz, 3 who used a specially pure glycogen, 

 must be especially mentioned. They found that the aqueous solution of glycogen 

 contained amicrons as \vell as easily recognizable submicrons, whose presence 

 was only evident by a homogeneous sphere of light, but on the addition of alcohol 

 conglomerate into detectable submicrons, 



Internal Friction. The watery solutions of organic colloids are often character- 

 ized by their great thickness of viscosity. In a strictly scientific manner this 

 is expressed by the statement that the internal friction of the questionable solu- 

 tion is great. It seems generally accepted that the internal friction of suspen- 

 sion colloids is equal to that of the pure solvent, or differs from it only slightly. 

 The view is probably correct for various reasons, but as far as purely experi- 

 mental determinations are concerned it is based upon only a few experiments 

 carried out by FRIEDLANDER 4 with colloidal silver and suspensions of colophony. 

 The concentration of the " solutions " in these experiments was low, so that 

 positive conclusions could not be drawn from them. 



Molecular Movement. R. BROWN 5 first found that small particles 

 suspended in water showed a quivering motion, and this phenomenon 

 has been called, from its discoverer, Brownian molecular motion. This 

 phenomenon has been observed since then by many investigators in 

 fluids having suspended solid particles as well as in substances dissolved 

 in colloid. It is believed by many that this movement is stopped by the 

 addition of electrolytes. 



ZSIGMONDY has found in regard to the molecular movement of colloidal gold 

 that this cannot be caused by a change in concentration due to evaporation, and 

 also not influenced by the duration or intensity of the light applied. Small particles 

 of gold move much more actively than large ones, still sometimes large par- 

 ticles are met which have an active motion. The particles seem to somewhat 

 influence each other, as the activity of the motion generally diminishes on diluting 

 the gold solution. Old gold solutions (several months to 1^ years) may also show 

 active movement. 



The molecular movement has been recently studied by SvEDBERG, 6 who used 

 a new method. He has shown that the molecular motion of silver particles is 

 also perfectly normal in an isoelectric point, i.e., for perfectly uncharged particles 

 (page 46). Electrical charge can therefore not be responsible for the molec- 



1 Zsigmondy, Zeitschr. f. physik. Chem., 56, 65, 1906. 



2 Zsigmondy and Lottermoser, ibid., 56, 77, 1906. 

 3 Pfluger's Arch., 105, 115, 1904. 



4 Zeitschr. f. physik. Chem., 38, 430, 1901. 



5 Edinb. Phil. Journ, 5, 358, 1828; 8, 41, 1830. 



6 Studien zu Lehre von den kolloiden Losungen. Upsala, 1907, 128. 



