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 (i881) 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 I,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. Horne concluded that micro organisms are distributed at random in the air, and that, for making valid comparisons between populations of airborne microbes at different places and times, analysis of variance could legitimately be applied to plate counts.