CHAPTER VI. 



OPTICAL AND ELECTRICAL PROPERTIES^OF COLLOIDS. 

 Optical Properties. 



COLLOIDAL solutions, e.g., albumin, always show a slight turbidity. 

 If a strong ray of light is passed through such a solution, its path 

 may be seen as a bright band. (See Plate I.) The "Tyndall phe- 

 nomenon," as it is known, is much more distinct if a ray of light is 

 passed through smoke or through a turbid suspension, in which case, 

 the reflected light is polarized. This phenomenon manifests itself 

 if a sunbeam passes into a dark room. The illuminated dust par- 

 ticles (motes) appear bright against the dark background. 



MICHAEL FAKADAY observed the phenomenon in the case of gold 

 hydrosols and he was led to the opinion that such solutions, which we 

 nowadays call colloids, were nothing but extremely finely divided 

 suspensions (dispersed systems). It was a great service to science, 

 when H. SIEDENTOPF and R. ZSIGMONDY recognized the importance 

 of this phenomenon for the investigation of colloids, and constructed 

 an instrument adapted to this purpose by passing the reflected light 

 into a microscope. In this way they obtained bright pictures of the 

 suspended particles on a dark ground. Since neither of these in- 

 vestigators paid any attention to the representation of the shape of 

 the particles, and devoted their attention only to the reflection of a 

 point of light, it was possible, by utilizing the strongest sources of 

 light (sun, arc lamp), to perceive particles lying below the limit of 

 microscopic visibility. They, therefore, called the apparatus the 

 ultramicroscope. 



The great number of fundamental observations with the ultra- 

 microscope which we owe to R. ZSIGMONDY * 2 and his followers have 

 been repeatedly mentioned. R. ZSIGMONDY called the particles 

 which he could definitely distinguish against the dark field, but 

 which were far below the limit of microscopic visibility, submicrons 

 (from 6 to 250 /*ju). If only a faint cone of light could be seen, it 

 is to be assumed in many cases, that the smallness of the individ- 

 ual particles precludes the recognition of each one. Such particles 

 (under 6 ju/z) he called amicrons. 



Most inorganic hydrosols, especially metals, form characteristi- 

 cally colored solutions, e.g., silver hydrosols are brown, platinum 



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