V 



HORIZONTAL DIFFUSION 



We have now described the particles composing the air-spora and the 

 relevant properties of the atmosphere. What happens to the particles 

 after they have been launched into the atmosphere? Common-sense 

 tells us that they become dispersed — in the sense that their concentration 

 per unit volume of air decreases with increasing distance from the point 

 of liberation. 



Tyndall (1881) believed that airborne microbes float through the 

 atmosphere in miniature clouds. He explained Pasteur's demonstration 

 of non-continuity in the spontaneous generation controversy by postulat- 

 ing that Pasteur sometimes opened his flask in the midst of a bacterial 

 cloud and obtained life, and sometimes in the interspace between two 

 clouds and obtained no life. In hospital practice, opening a wound during 

 the passage of a bacterial cloud would have an effect very different from 

 opening it in an interspace between clouds. 



It was not necessary to draw this conclusion, however, as Pasteur's 

 results could be explained equally well if microbes were randomly dis- 

 tributed in the air. Evidence for random distribution was obtained by 

 Home (1935), who applied Fisher's x^ test to catches on 1,000 or more 

 Petri dishes of sterile media which had been exposed in Kentish orchards 

 by N. W. Nitimargi. The observed frequencies of total bacteria or total 

 moulds, or of any genera or species tested separately, did not depart 

 significantly from the Poisson distribution. Home concluded that micro- 

 organisms are distributed at random in the air, and that, for making valid 

 comparisons between populations of airbome microbes at different places 

 and times, analysis of variance could legitimately be applied to plate 

 counts. 



Dispersion of the Spore-cloud 



It is still convenient to speak of clouds of spores — not, indeed, keeping 

 together in the manner of locust swarms, but tending to become dispersed 

 while suspended passively in the atmosphere. Sampling a region small 

 enough in relation to the size of the cloud may then reveal a random 

 distribution of particles. 



Dispersion of the spore-cloud can be deduced from early observations 

 on the distribution of rust on rye by Windt (1806), who observed that 



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