ISOLATING THE TYPHOID BACILLUS. 437 



mixed with the water by means of a sterile glass rod. 

 The cylinder is then placed in the ice chest. Precipi- 

 tation is complete in two to three hours. The over- 

 standing water is syphoned off, and the precipitate or 

 portions thereof are poured into sterile test-tubes. To 

 this precipitate is now added about a half volume of a 

 25 per cent, solution of neutral potassium tartrate. The 

 test-tube is closed with a sterile rubber cork and the 

 mixture thoroughly agitated, whereby the precipitate is 

 completely dissolved. With a sterile pipette one part 

 of the mixture is mixed in a test-tube with two parts 

 of sterile bouillon, and this mixture is distributed over 

 a series of Drigalski-Conradi plates. Ficker advises 

 when possible the use of a centrifuge for the separation 

 of the precipitate, as he believes the results are likely 

 to be more satisfactory. 



NOTE. Obtain a pure culture of typhoid bacilli, and 

 from this make inoculations upon a series of potatoes 

 of different ages and from different sources. Do they 

 all grow alike ? 



Before sterilizing render another lot of potatoes slightly 

 acid with a few drops of very dilute acetic acid ; render 

 others very slightly alkaline with dilute caustic soda. 

 Are any differences in the growths noticeable ? 



Make a series of twelve tubes of peptone solution to 

 which rosolic acid has been added. Inoculate them all 

 with as nearly the same amount of material as possible 

 (one loopful from a bouillon culture into each tube) ; 

 place them all in the incubator. Is the color-change, 

 as compared with that of the control-tube, the same in 

 all cases. 



Compare the morphology of cultures of the same age 

 on gelatin, agar-agar, and potato. 



