IN the determination of the size-frequency distribution of a particulate material by sieve analysis, it is often assumed1 that the rate of sieving is proportional to the quantity of material on the sieve. If y is the amount of material of size smaller than the sieve mesh at any time t, this assumption leads to the equation y/y0 = exp (− at), where a is a constant, and y0 is the value of y at t = 0. In practice, the equation y = at−m + b (equat. 1) has been found to apply2.