1 00:00:00,000 --> 00:00:03,000 Gerald Elliott: But first I want you to look at a phenomenon which was… 2 00:00:03,000 --> 00:00:07,000 ..first seen by the botanist Robert Brown looking down his microscope… 3 00:00:07,000 --> 00:00:11,000 ..at suspensions of pollen grains some hundred years ago. 4 00:00:12,000 --> 00:00:16,000 He saw that they were constantly in motion, jiggling around in this way, and… 5 00:00:16,000 --> 00:00:19,000 ..he asked himself whether this was because there was a living system, 6 00:00:20,000 --> 00:00:25,000 so he did an experiment with suspensions of non-living particles of about the same size, 7 00:00:25,000 --> 00:00:29,000 gold particles in fact, and convinced himself that it wasn’t a function of… 8 00:00:29,000 --> 00:00:31,000 ..whether the thing was living or not, he could get… 9 00:00:32,000 --> 00:00:36,000 ..just the same molecular motion going on with gold particles. 10 00:00:37,000 --> 00:00:40,000 So he said there must be some sorts of forces which were… 11 00:00:40,000 --> 00:00:44,000 ..causing these particles in suspension to jiggle around like this. 12 00:00:45,000 --> 00:00:50,000 What then are these forces? To show this I’ve… 13 00:00:51,000 --> 00:00:56,000 ..set up an analogy using our old friend, the air table. 14 00:01:01,000 --> 00:01:05,000 (Gerald Elliott) You see the motion of this white puck. 15 00:01:06,000 --> 00:01:10,000 It’s moving in a random way around the field of view… 16 01:01:11,000 --> 00:01:16,000 ..and there are apparently no forces to cause this motion. 17 00:01:17,000 --> 00:01:22,000 But if we change the illumination conditions we will see that the… 18 00:01:23,000 --> 00:01:27,000 ..motion of the white puck is in fact caused because round it there are… 19 00:01:28,000 --> 00:01:32,000 ..lots of other black pucks and these are impacting with it in a random way… 20 00:01:33,000 --> 00:01:36,000 ..and it’s the motion of the black pucks which is forcing… 21 00:01:37,000 --> 00:01:41,000 ..the random motion of the white puck in the system. 22 00:01:42,000 --> 00:01:46,000 This is, of course, a straight analogy with Brownian motion… 23 00:01:46,000 --> 00:01:50,000 ..as Brown saw it in his microscope. 24 00:01:51,000 --> 00:01:54,000 You’ll see that the particles are moving about at random… 25 00:01:55,000 --> 00:01:58,000 ..as they moved about at random in our analogue. 26 00:01:59,000 --> 00:02:03,000 You’ll see that they are going in and out of focus in our microscope… 27 00:02:03,000 --> 00:02:06,000 ..as they go up and down in the solution. 28 00:02:08,000 --> 00:02:12,000 So we see that the effect of Brownian motion is caused by… 29 00:02:13,000 --> 00:02:18,000 ..the impact of literally billions of billions of atoms and what we’re seeing here is… 30 00:02:20,000 --> 00:02:28,000 ..the influence of the atomic motion of the molecules in our fluid, the gas or the liquid.