72 



DISPERSE SYSTEMS 



will " select " light of certain wave-length for absorption, and, 

 as a consequence, colour may result from " selective " absorption, 

 reflection or diffraction. 



In the following table, from Ostwald, is given the relationship 

 between size of particle and colour (a) from light absorbed, and 

 (b) from light transmitted. 



TABLE XL 

 CORRESPONDING ABSORBED AND SUBJECTIVE COLOURS. 



One must, however, take into account the other optical com- 

 ponents, e.g. refractive index of medium. The absorbed colour 

 given above does not necessarily indicate the colour of light 

 scattered by the particles. 



As the particle becomes smaller, the colour alters to light of 

 longer wave-length, e.g. from blue or green, through various 

 shades of yellow and orange to red. If the suspended particles 

 are very fine, blue light is, as we have noted above, scattered 

 laterally, while red light is transmitted. Such a system will 

 appear red by transmitted light and blue by reflected light (e.g. 

 skim-milk, tobacco smoke and colloidal gold). A very interesting 

 series of coloured plates to illustrate this will be found at the 

 end of Zsigmondy's monograph. It has also been shown that, 

 as the particles decrease in size, the absorption bands in the 

 spectrum of the solution shift towards the ultraviolet (Ostwald). 



The amplitude of vibration of a particle is a function of its 

 mass, temperature being kept constant. As the mass alters so 

 will the period of vibration. According to Wood, metallic 

 particles, if highly dispersed, owe their colour not to ordinary 



