METHOD OF LA VERAN 



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upper vessel is then opened, and the water allowed to run slowly 

 into the lower one. As soon as the whole of the water has run off, 

 the position of the vessels is reversed and the procedure repeated. 

 By this means any number of litres, being a multiple of 5, can be 

 run through the aspirator, an equivalent number of litres of air 

 being drawn through the filtering tube, depositing upon the 

 filtering medium all organisms floating or suspended therein.^ 



3. Method of Laveran— 



This method will be found of service when the atmosphere 

 is so heavily charged with moisture as not to admit of the em- 

 ployment of soluble filtering medium as 

 above. The apparatus employed is shown 

 in Fig. 31, and consists of 2 tubes of stout 

 glass of 2 cm. internal diameter closed at the 

 bottom, and joined together, as shown by a 

 horizontal tubulure. The orifices of both 

 tubes are fitted with india-rubber stoppers, 

 through which are passed pipettes, the in- 

 ferior extremities of which reach almost to 

 the bottom of the tubes, and of which one 

 is graduated in divisions of a cc. One of 

 the tubes is clearly marked with a line 

 showing the level of 10 cc. of water, and the 

 superior orifices of the pipettes are lightly 

 plugged with cotton-wool. Into the marked 

 tube, 10 cc. of sterile water (the addition of 

 I cc of pure cane sugar can be made if 

 thought desirable) is then placed, and the 

 whole is sterilised in the autoclave at 115'' C. 

 for thirty minutes. When required for use, 

 the graduated pipette in the empty tube is 

 joined up to the aspirating apparatus, the 

 cotton-wool plug of the other pipette removed, 

 and the aspirator set in action. The aspired 

 air bubbles through the sterile water, passing 

 into the first tube, and thence by the hori- 

 zontal tubulure into the second, finally escaping by the pipette 

 in connection with the aspirator. When a suflficient quantity of 

 air has been passed through, the cotton-wool plug is replaced, the 

 apparatus detached from the aspirator, and the water gently drawn 

 ^ Various aspirating pumps are in common use, the particular form used is 

 a matter of personal selection. 



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Fig. 31. — Laveran's 

 tube. 



