DIFFUSION 



81 



i i a i i i l 



i'','i."^V- - 



* ,. »** . o « i> 



° ' _? — dr-*.-*-:^-2_== 



-» o f m . <9 = H 1 



Position in joined tubes 



x 3/4 x l/4 



x 3/4 



•r 1/4 



Fig. 37. The initial arrangement has all the particles in one half of the setup. 

 The concentration, plotted on the right, is 100% up to the division point, then 

 falls abruptly to zero. After a short time there is a net flow of particles to the 

 right, so that the concentration now drops more gradually to zero. After more 

 time for diffusion, the particles have reached almost to the right-hand end of 

 the tube, and the concentration falls almost linearly to zero. After an infinite 

 time the particles are evenly distributed through the tube which is twice as 

 large as the starting tube, so the concentration throughout is half the starting 

 concentration. The positions in the tube at which the concentration is 75% or 

 25% of the starting value are indicated. The distance between these two points 

 is a direct measure of the time elapsed since diffusion commenced. 



