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 io 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.