EFFECTS OF SEDIMENTATION The effects of spore fall in still air can be observed indoors, particu larly if a room is left closed and unoccupied—a fact noted early in the study of air hygiene by workers using Hesse's horizontal tube method of air sampling, or some modification of it (see Chapter I, p. 1o). Although all these investigations suffer from the defect of being based on highly selective culture media, all agree that wind or crowds stir up micro organisms, and that these soon settle in buildings when the air is left undisturbed.
At the Royal Institution in London, England, Tyndall (1SS1) made a close study of microbes in the air in relation both to the question of spontaneous generation and to the antiseptic surgery which was being developed by Lister at that period. Tyndall showed that the air of a darkened room scattered a powerful beam of light. Gas molecules did not appreciably scatter light. Scattered light always arose from suspended particles, some of them too fine to be described as motes. By passing a beam of light through windows in the side walls of a glass-fronted box, he showed that, after a day or two, the air became optically empty, the par ticles having settled on the floor and even on the roof of the box. At the same time Tyndall found that the air, previously full of microbes, had become sterile. The ability to generate life was associated with the presence of the light-scattering particles, and the air of small spaces could be sterilized by sedimentation. Tyndall had the curious idea that microbes remained associated in the air in clouds, much as fish are associated in shoals, and he explained that some of Pasteur's flasks (pp. 3-4) must have been opened within clouds, while others were opened between clouds of floating microbes. We now think of micro-organisms as distributed in the air at random (Horne, 1935), but, under certain conditions, it may be that Tyndall was right.
Outdoors the effects of terminal velocity are usually masked by the speed and turbulence of the wind. However, conditions are sometimes tranquil enough for its effects to be detected. One example was found by Rempe (1937), of Gottingen, who made a series of aeroplane flights both by day and by night to study the distribution of tree pollen over German forests. By trapping on sticky cylinders, he obtained evidence that pollen grains of different sizes and terminal velocities differed in their relative abundance with altitude, even by day (Table II).