Isolation of Specific Pathogenes. 75 



mentally, 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 mix- 

 ture was centrifugalized 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 pointed out 

 .that the hypolsuphite should be in excess. In his ex- 

 periments 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 hyposulphite 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 degrees for 24 

 hours. Ficker (Ficker, 1904) modified the process still 

 more by using ferric sulphate, and dissolved the precipi- 

 tate 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), after comparing different precipi- 

 tation methods, adopted ferric oxychloride as the most 

 suitable precipitant, because of its quicker and less de- 



