THE MICROBIOLOGY OF THE ATMOSPHERE 



Hirst trap slides, but addition of paraffin wax (i2| per cent) is often 

 necessary to harden it. The coating must be kept as soft as possible, how- 

 ever, and for extremes of temperature Pady & Kelly (1949) introduced 

 silicone grease for coating trap surfaces. When spores have to be picked 

 off and transferred to culture media, pectin jelly is recommended by 

 Prof. A. J. P. Oort (personal communication). 



Thermal Precipitation 



A hot body placed in a dust-laden atmosphere produces a dust-free 

 space around itself (Watson, 1936). This well-known phenomenon has 

 been used in the thermal precipitator, in which a dust-laden airstream 

 flows slowly past a wire heated electrically to ioo°C. above the ambient 

 temperature, depositing dust particles on glass slips for examination. 

 It has been little used for aerobiological work, but is highly efficient for 

 sub-microscopic particles and larger ones up to 5 /x diameter. It is most 

 suitable for use when the particles are in high concentrations, as the 

 volume of air sampled is only about 7 cc. per min. (see Green & Lane, 



1957)- 



Electrostatic Precipitation 



The movement of charged particles in an electrical field is widely 

 used in industry to extract dust from air because the pressure drop im- 

 posed by the requisite apparatus is small, even w ith high rates of air-flow. 

 Berry (194 1 ) realized that an efficient sampling method could be developed 

 on this principle, and the General Electric electrostatic air-sampler was 

 devised by Luckiesh et al. (1946). Petri dishes of culture medium are 

 placed on flat metal plates (electrodes) which are oppositely charged to 

 7,000 volts from a half-wave rectifier. Air enters through the apex of a 

 fairly flat inverted metal cone extending to near the edge of each dish, 

 and each cone carries a charge opposite to that of the electrode under its 

 dish. A small pump draws air at 14 litres per min. over each dish, and 

 particles move in the electrostatic field and are deposited on the agar 

 surface. 



When Escherichia coli in aqueous suspension was atomized into a 

 room, the dish on the positive electrode collected nearly ten times as many 

 cells as that on the negative. With naturally-occurring airborne bacteria, 

 30 per cent more were deposited over the negative electrode. Presumably 

 each dish collects a proportion of the uncharged particles by gravity and 

 impaction, as well as collecting the charged particles moving towards it. 

 The positions and dimensions of the upper electrodes have been decided 

 by empirical tests and may need modifying for mould spores and pollen. 

 For naturally airborne bacteria the concentration, based on the sum of the 

 counts on the two dishes, was from 2 to 3 per cent higher than simul- 

 taneous tests with the 'duplex radial-jet air sampler'. How far particles 



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