XI 



DEPOSITION IN RAIN, SNOW, AND HAIL 



Airborne microbes can be deposited direct or they may be washed out 

 of the air in raindrops, hailstones, or snow-flakes. Trillat & Fouassier 

 (19 14), from their laboratory experiments with artificial fogs condensing 

 on a suspension of pathogenic bacteria in small vessels, thought that air- 

 borne microbes act as condensation nuclei. Condensation nuclei are now 

 thought to be small hygroscopic particles, and it seems more likely that 

 droplets already formed collect spores by impaction {see Chapter VII). 

 McCully et al. (1956) estimate that, over all land areas of the globe, from 

 35 to 50 per cent of the total atmospheric dust load is washed out each 

 day. Here we will consider the results of rainwash in nature and the spore 

 content of precipitation water. 



Over the last 300 years about a score of people are known to have sought 

 microbes in precipitation water. Collecting the sample has some pitfalls, 

 however; the vessel must obviously be clean, but the danger of contamina- 

 tion by rain-splashed soil has not always been anticipated though, with 

 current knowledge of the magnitude of splash and its part in soil erosion, 

 the danger is now clear (Laws, 1940). Much of the early work summarized 

 below, however, is clearly trustworthy. 



Animalcules in rain-water deHghted Leeuwenhoek (1676, in Dobell, 

 1932). Rain was collected in a clean porcelain dish set on a wooden tub 

 to avoid earth being splashed by rain. Minute organisms were searched 

 for in vain until after the rain-water had stood for some days, by which time 

 it would also have been contaminated by dry deposition — so we do not 

 know whether or not Leeuwenhoek found microbes of precipitation water. 



Rain 



The only systematic study of precipitation micribiology comes from 

 Miquel (1884, p. 597; 1886, p. 530) at the Pare Montsouris, Paris. Miquel 

 caught his rain in a metal funnel fixed at 1.7 metres above ground-level 

 on a pillar, well away from trees and buildings. Rain falling into the funnel 

 was collected in a platinum crucible with a cover, both funnel and crucible 

 having been heated to redness just before sampling. The sample was then 

 sown, drop by drop, in 50 to 100 flasks of beef broth. Miquel also designed 

 apparatus which placed raindrops on a moving band of nutritive paper. 

 After 6 days' incubation, the paper was dried and kept as a record of 



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