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notably Perrin. In such an experiment he noted the position of a 

 particle at the end of equal intervals of time, Fig. 2. He found that 



Fig. 2 — ^A close-up of molecular motion appearing absolutely irregular, yet controlled 



within limits. 



the direction of this motion observed in one interval differed, in general, 

 from that in the next succeeding interval. He found that the direction 

 of the motion presents what we instinctively call absolute irregularity. 

 Let us ask ourselves certain questions about this motion. 



Suppose we fix our attention on the particle at the point A. What 

 made it move to B in the next interval of time? Of course we answer 



