CHEMICAL PHENOMENA IN LIFE 



From particles which were too heavy to remain 

 suspended and which sank quickly to the bottom, 

 a continual graduation was observed down k to 

 particles which were so small that they passed 

 through paper niters and were not even micro- 

 scopically visible. Bredig's experiments on plati- 

 num dispersed by the electric arc in water clearly 

 demonstrated that metallic platinum may be 

 obtained there in every imaginable size of particles. 

 The coarsest particles form a brown precipitate. 

 The finest of them stain the water dark brown 

 without any trace of turbidity, are not retained 

 by any filter, and no particle is microscopically 

 visible. The liquid has all the properties of a 

 colloidal solution of platinum. 



The metal-sols, of which a large number have 

 already been obtained, are of great interest, since 

 we possess a new experimental help for studies of 

 colloids in the so-called Ultramicroscope. Tyndall 

 drew attention to the remarkable phenomenon that 

 rays of light remain visible in a liquid only when 

 particles suspended therein reflect the light. When 

 water is carefully freed from any trace of particles 

 of dust, we cannot follow the course of rays of light 

 through the liquid. The water rather appears to us 

 as itself diffusely lighted without showing the 

 stripes of light which are produced by a ray of 

 sunlight or electric light thrown upon a vessel 

 containing water. Colloidal solutions always 

 show TyndalTs Phenomenon. This experiment, 

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