390 THE COLLOIDAL STATE 



purple, pink, or red, the latter containing the smallest particles, 

 while silver sols have been obtained brownish-red, yellow, 

 green, grey, or blue. Some of the colour effects occasionally 

 met with in partly developed photographic plates are probably 

 due to absorption compounds of silver particles of various 

 degrees of dispersion with unreduced silver chloride.* 



Most suspensoid sols, however, although appearing clear 

 when examined in the ordinary way, exhibit what is known 

 as the Tyndall phenomenon ; by this is meant the fact that 

 when a beam of light is projected into the solution, in an 

 . otherwise dark room, the path of the beam is rendered visible 

 by the reflection of light from the particles of the disperse 

 phase. If examined through a Nicol prism the luminous 

 beam is found to be polarized, which distinguishes it at once 

 from the similar effect produced by passing a ray of light into 

 a true solution of a fluorescent substance. 



Brownian Movement. — In 1827, R. Brown, the botanist, 

 first observed, by the aid of a compound lens, the peculiar 

 movement of pollen grains suspended in water ; this move- 

 ment was subsequently found to be common to all sufficiently 

 small particles similarly suspended in a liquid of low viscosity, 

 and the phenomenon is now known as Brownian movement. 

 The phenomenon is generally regarded as a manifestation of 

 molecular motion, and, as is to be expected, the smaller the 

 particles the more rapid the movement. Thus for particles of 

 diameter 3 /a it is only a barely perceptible oscillation, but it 

 rapidly increases with diminishing size ; in the case of particles 

 of diameter 10-50 /x/x, which are beyond the range of visibility 

 of the microscope, Brownian movement is manifested by a 

 rapid rectilinear zig-zag oscillation of minute spots of light in 

 a dark field. The general motion resembles that of the flight 

 of gnats and the velocity is of the order of 100 /x per second. 



B. Electrical Properties. 



When a suspensoid sol in pure water free from electrolytes 

 is subjected to the action of a powerful electric field, by dipping 



* Luppo, Cramer, and Reindeers : " KoUoid Zeitschr.," 191 1, 9, 10. 



