594 A MANUAL OF BACTERIOLOGY 



1. Filtration through a porcelain filter. By passing one to two 

 litres of the water through a sterile Pasteur-Chamberland filter, 

 the whole of the organisms present may theoretically be collected 

 in a few c.c.s. Practically, however, a large proportion of the 

 organisms are lost in the process : perhaps they get carried into 

 and remain in the superficial layers of the filter-candle, and for this 

 reason, though sometimes employed, this method has been largely 

 given up. 



2. Concentration. W. J. Wilson 1 has devised the following 

 method : The water is placed in one or two Winchester quart 

 bottles, and 10 c.c. of nutrient broth are added for every litre. The 

 bottles are placed in a water- bath maintained at 37-40 C., and 

 are connected by rubber corks and tubing with a condenser (at a 

 lower level) through which cold water continuously passes, and 

 the tube of the condenser is connected to a large bottle (at a still 

 lower level). This bottle is kept partially exhausted by means of 

 a filter-pump. The water evaporates and is thus concentrated, the 

 evaporated water being condensed and collected in the exhausted 

 bottle. It requires twenty-one to twenty-two hours to evaporate 

 a litre of water. The water remaining in the bottles, now concen- 

 trated to a few c.c.s., is then plated on Conradi-Drigalski or mala- 

 chite-green agar. 



3. Chemical precipitation. These methods depend on the forma- 

 tion in the water of a fine, inert precipitate, which entangles and 

 carries down with it a large proportion of the bacteria present. Thus 

 in the Vallet-Schiider 2 method, to 2 litres of the water are added 

 20 c.c. of a 7-75 per cent, solution of sodium hyposulphite and 

 20 c.c. of a 10 per cent, solution of lead nitrate. The precipitate 

 is allowed to settle or is centrifuged off, is dissolved in a small 

 volume of a saturated solution of the hyposulphite, from which 

 plates are made in suitable media. Ficker 3 uses ferrous sulphate 

 after making the water faintly alkaline with caustic soda ; the 

 ferrous hydrate formed carries down the micro-organisms (this 

 must be a risky procedure, as the typhoid bacillus is very sensitive 

 to caustic alkalies). Iron oxychloride may also be used as the 

 precipitant. H. S. Willson (loc. cit.) employs alum. A stock 

 solution of alum is prepared, containing 10 grm. per 100 c.c., and 

 of this sufficient is added to the water to obtain 0-5 grm. to the 

 litre. After the precipitate of aluminium hydrate has formed, 

 the vessel is well shaken to mix its contents, and the mixture is 



1 Brit. Med. Journ., 1907, vol. i, p. 1176. 



2 Zeitschr.f. Hyg., xlii, No. 2, p. 317. 



3 Hyg. Rundschau, xiv, No. 1, 1904. p. 7. 



