84 ELEMENTS OF WATER BACTERIOLOGY 



precipitation have been tested experimentally, with 

 some degree of promise. Vallet (Vallet, 1901) was 

 the first to employ this principle, and made use of 

 sodium hyposulphite and lead acetate. The mixture 

 was then centrifuged and the precipitate dissolved in 

 more hyposulphite. The clear solution was then plated. 



Schiider (Schiider, 1903) observed that the lead 

 salt reacted harmfully upon the bacteria, and that 

 the hyposulphite should be in excess. In his experi- 

 ments water was allowed to stand in tall jars for 24 

 hours. To 2 liters of infected water, 20 c.c. of a 7.75 

 per cent solution of sodium h^^posulphite was added, 

 and after thorough mixing 20 c.c. of a 10 per cent 

 solution of lead nitrate. The precipitate, after 20 

 to 24 hours, was treated with 14 c.c. of saturated sodium 

 hyposulphite solution and shaken. From the clear 

 solution 0.2 to 0.5 c.c. portions were streaked upon 

 Drigalski-Conradi plates which were then incubated 

 at 37° for 24 hours. Ficker (Ficker, 1904) modi- 

 fied the process still more by using ferric sulphate, 

 and dissolved the precipitate with neutral potassium 

 tartrate. The final solution was then plated on 

 Drigalski-Conradi medium. Ficker claimed that this 

 method gives excellent results, 97-98 per cent of the 

 typhoid bacteria being carried down with the precipitate. 



Muller (Miiller, 1905), in comparing different pre- 

 cipitation methods, adopted ferric oxychloride as the 

 most suitable precipitant, because of its quicker and 

 less destructive action. Willson (Willson, 1905) sug- 

 gested the use of alum as a precipitant. He added 

 0.5 gr. alum per liter of water examined. The mixture 



