8 
Field Museum op Natural History 
The following experiment as described by Ostwald 
in 1896 illustrates this process. If silver nitrate is 
introduced into a colloid which contains ammonium 
bi-chromate, silver chromate (a red salt) is at once 
formed, but it does not at first appear in a solid form. 
It is still dissolved. Gradually, however, through the 
continual formation of silver chromate, the solution 
becomes so concentrated that it must somewhere sep- 
arate out. This separation takes place first at an edge. 
Then all the super-saturated substance makes its way 
to this edge and likewise separates out. An essential 
consequence of this is that adjacent to this deposit 
a zone is formed which is free from silver chromate. 
But this zone also does not contain ammonium bi- 
chromate, for this has been used up by the silver ni- 
o- b c d e g 
Fig. 1. Origin of banding in colloids. 
trate. Here, then, no new silver chromate can form 
and the zone becomes colorless. The excess of silver 
nitrate pushes on, however, and by the same reactions 
forms a second band of silver chromate and a color- 
less zone. Continuation of this process gives a banded 
structure. 
The process is illustrated diagramatically in Fig. 1. 
The areas colored yellow represent a mass of gelatine 
which contains ammonium bi-chromate. In a, the 
light-red area at the top represents a zone of dis- 
solved silver chromate. In b, the dark-red edge shows 
ni2] 
