SEDIMENTATION IN STILL AIR 



2 a — p 



Vs = -. .gr- 



9 /^ 



where, in C.G.S. units at ordinary surface temperature and pressure: 



Vs = terminal velocity (velocity of sedimentation) in cm. per sec. ; 



a = density of sphere in gm. per cc. (water = i-oo); 



p = density of medium (air = 1-27 X lO"^ gm. per cc); 



g = acceleration of gravity (981 cm. per sec.^); 



ju, = viscosity of medium (air at i8°C. = i-S X iQ-^gm.percm.sec); 



r = radius of sphere in cm. {N.B. radius = \ diameter). 



For a water droplet falling in air, Vs = 1-2 X io~^ r^ cm. per sec, when 

 the radius is expressed in microns (/it). A fog droplet of 10 p- radius (20 p. 

 diameter) has a calculated terminal velocity of 1-2 cm. per sec. 



The pollens and spores with which we are concerned belong to the 

 size-range where Stokes's law is valid, but they are seldom anything like 

 smooth spheres. Stokes's law has given values of at least the right 

 order, however, for spores whose terminal velocities have been measured 

 experimentally. At first sight the pollen grains of some species of conifers 

 appear to fall unexpectedly slowly, but these grains have conspicuous 

 air sacs which greatly reduce the density of the individual particle. 



The diameters of particles constituting the air-spora vary from approxi- 

 mately I ^ to lOO/Lt or more for the largest pollens and spores {see 

 Appendix I, p. 207, Plates 5-7). Some spores are filamentous, perhaps one 

 hundred times as long as wide. Although the densities of the spores of 

 very few species have yet been measured, there are reasons for expecting 

 them to be much less dense than mineral particles and indeed to resemble 

 water droplets in density. The few determinations which have been made, 

 relative to water = i, are as follows: 



Pohl (1937) 



Zeleny & McKeehan (19 10) 



15 



