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  Gerald Elliott: But first I want   
 you to look at a phenomenon which was…

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  ..first seen by the botanist   
 Robert Brown looking down his microscope…

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  ..at suspensions of pollen grains   
 some hundred years ago.

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  He saw that they were constantly   
 in motion, jiggling around in this way, and…

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  ..he asked himself whether this   
 was because there was a living system,

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  so he did an experiment with   
 suspensions of non-living particles of about 
  the same size,

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  gold particles in fact, and   
 convinced himself that it wasn’t a function of…

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  ..whether the thing was living or   
 not, he could get…

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  ..just the same molecular motion   
 going on with gold particles.

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  So he said there must be some   
 sorts of forces which were…

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  ..causing these particles in   
 suspension to jiggle around like this.

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  What then are these forces? To   
 show this I’ve…

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  ..set up an analogy using our old   
 friend, the air table.

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  (Gerald Elliott) You see the   
 motion of this white puck.

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  It’s moving in a random way around  
  the field of view…

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  ..and there are apparently no   
 forces to cause this motion.

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  But if we change the illumination   
 conditions we will see that the…

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  ..motion of the white puck is in   
 fact caused because round it there are…

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  ..lots of other black pucks and   
 these are impacting with it in a random way…

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  ..and it’s the motion of the black  
  pucks which is forcing…

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  ..the random motion of the white   
 puck in the system.

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  This is, of course, a straight   
 analogy with Brownian motion…

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  ..as Brown saw it in his microscope.  

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  You’ll see that the particles are   
 moving about at random…

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  ..as they moved about at random in  
  our analogue.

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  You’ll see that they are going in   
 and out of focus in our microscope…

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  ..as they go up and down in the   
 solution.

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  So we see that the effect of   
 Brownian motion is caused by…

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  ..the impact of literally billions  
  of billions of atoms and what we’re 
  seeing here is…

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  ..the influence of the atomic   
 motion of the molecules in our fluid, the gas
   or the liquid.