THE COLLOIDAL STATE 95 



to enter a beam of light in a partially darkened room, we see the 

 smoke brilliantly illuminated. If, instead of smoke particles 

 in the air, metal particles in water are illuminated laterally 

 against a dark background, an even more striking cone of light 

 is obtained (Fig. 70). It was such a cone of light that Faraday 

 first saw, and Tyndall later interpreted. 



A dispersion of silver oxide prepared by reduction of a silver 

 salt with dextrin gives a particularly interesting Tyndall cone, in 

 that the cone is green where the light beam enters, then yellow- 

 green, yellow, yellow-orange, orange-red, and brilliant red at the 

 far end. This is probably due to color absorption by the solution. 



Fig. 70.— The Tyndall cone (in a silver solution). 



There are a number of other phenomena similar to the Tyndall 

 effect which are not to be confused with it, viz., luminescence, 

 opalescence, iridescence, fluorescence, and the Raman effect. 

 Luminescence is a purely chemical (oxidation) process. Opal- 

 escence and iridescence are due to colloidal structure but of 

 plates, or lamellae, and not particles. Fluorescence is unpolar- 

 ized light coming from certain substances (sodium salicylate, 

 quinine sulphate in water, mercury vapor) when illuminated, the 

 light given off being of a different (greater) wave length than that 

 illuminating the substance. (The light from colloidal dispersions 

 shows characteristic polarization.) A number of light effects 

 result in a change of wave length. This is true of fluorescence, 

 the Compton effect (scattered homogeneous X rays which give 

 rise to a change of wave length) , and the Raman effect. (The last 

 will be referred to again presently.) • 



