52 MICRO-ORGANISMS AND DISEASE. [CHAP. 



of the air during the preparation of the plate cultivation 

 cannot be prevented, and if the air happens to contain a 

 good many organisms, the total number of colonies appear- 

 ing in the plate cultivation exceeds the number of bacteria 

 present in the water tested. 



Professor Warden of Calcutta (Chemical News, Nos. 1340 to 

 1344) and Dr. Percy Frankland (Proceedings of the Royal 

 Society, No. 238) have made valuable experiments on the 

 examination of bacteria present in water by means of 

 plate cultivations. 



4. Examination of Air. The simplest plan to test for the 

 presence of organisms in the air is to draw out the cotton- 

 wool plug of several test-tubes or flasks containing the sterile 

 nourishing material, or, if this be boiled, potatoe, paste, or 

 gelatine (see p. 20), to expose their surface, and to leave it 

 thus for variable periods, from a few seconds to several 

 minutes. Then replace everything and expose the material 

 to incubation, or keep it only at the ordinary temperature of 

 the room. Another method is to collect the particles present 

 in the air on glasses moistened with pure glycerine (Maddox), 

 and then to make microscopic specimens or inoculate tubes 

 with this glycerine. 



A method which is very useful is the one recommended by 

 Cohn and Miflet. 1 The principle of it is, that by means of 

 an aspirator, an air-pump of any kind e.g. a Sprengel pump, 

 or simply the fall of water air of a particular locality is drawn 

 into one, two, or more Wolff's bottles (each with the ordinary 

 two bent glass tubes), connected with one another by short 

 pieces of indiarubber tubing, and containing the sterile 

 material in which the organisms are required to grow. All 

 bottles and tubes are of course sterile ; the plugging of the 

 tubes after the air has passed is done with sterile cotton- wool. 

 1 Zeitsthr-f. Biol. d. Pft. iii. i, p. 119. 



