II 



SEDIMENTATION IN STILL AIR 



All the particles with which we are concerned are heavier than air. 

 In still air they sink with characteristic and constant 'terminal velocity'. 



Stillness as a quality of air is only relative. In the laboratory we can 

 make the air as still as possible by eliminating draughts and convection 

 currents, only to find an intense underlying activity revealed by the 

 scintillation of motes in a beam of light. The motes are small enough to 

 be jerked irregularly by the impact of gas molecules; but they are too 

 large to be transported bodily by molecular diffusion, and most of the 

 phenomena of colloidal suspensions are irrelevant to the air-spora. We 

 shall meet some analogies with the diffusion of a gas, however, in studying 

 the diffusion of a cloud of spores in the atmosphere. 



In this study we usually ignore the underlying molecular activity of 

 the medium, and consider a patch of air as 'still' if it is not being trans- 

 ported bodily at more than a certain speed. Out-of-doors this speed might 

 be 10 cm. per sec; in a room it might be i cm. per sec; and, under 

 carefully controlled conditions in special apparatus, a higher standard 

 might be expected. For the present we must leave the definition vague, 

 and simply regard air as 'still' when, in a particular context, the effects of 

 wind, turbulence, and molecular activity are negligible. Knowledge of the 

 properties of small particles in still air throws light on the behaviour of 

 spores in moving air out-of-doors. 



Factors Determining Velocity of Fall 



One effect of its molecular activity is that the air is viscous, i.e. it 

 resists the movement of solid particles. A small particle liberated into 

 the air from a resting position tends to fall with an acceleration due to 

 gravity; however, the resistance of the air increases faster than the speed 

 of fall, and a state of balance is soon reached in which the particle stops 

 accelerating and continues to fall through the air at a constant terminal 

 velocity. 



The terminal velocity of smooth spheres with diameters of between 

 about I /x* and loo ju, is satisfactorily predicted by Stokes's law (for smaller 

 particles Cunningham's correction becomes applicable, and larger par- 

 ticles have to be treated experimentally). Stokes's law can conveniently 

 be given in the form : 



* « == 1 oVo mm. 

 14 



