THE MICROBIOLOGY OF THE ATMOSPHERE 



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 (1881) 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 (Home, 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). 



TABLE II 



POLLEN DISTRIBUTION AT DIFFERENT ALTITLIDES 

 (After Rempe, ig^j ; day flight, A 6) 



By night, it sometimes happened that pollen grains were partially 

 sorted out according to size even within a single species, as shown by the 



20 



