TYPHOID FEVER 83 



in the same place, so that the junction of the lowest two columns 

 corresponds exactly to the unit mark. Then get a second person 

 to make an ink or grease-pencil mark at the junction of each of 

 the columns. Expel the fluids, and you will have a Wright's 

 pipette graduated into equal parts. 



If you cannot suck up enough alternating layers to fill the tube, 

 suck up as many as you can, remove the pipette, and mark it as 

 before. Then relax the pressure so that the whole column is 

 drawn upward, and adjust it until the lowest column just coincides 

 with the uppermost mark you have just made, and the rest of the 



Fig. 27. — Showing Method of Graduating a Wright's Pipette in 

 Equal Parts. 



column extends into the ungraduated part of the tube ; make 

 marks against the intersections as before. 



Now proceed to put the emulsion into the sedimenting-tubes 

 with the mercury. If you wish to prepare tubes to give you a 

 dilution of i in 20 (and this is the most convenient for general 

 work), put as] much^as will fill nineteen divisions of the pipette, 

 sucking it up into the pipette by using an indiarubber nipple. If (as 

 is likely) your pipette does not hold ig units, it will be necessary 

 to take two or more pipettes full — say 10 and 9 units respectively. 

 Then seal the open end of the sedimenting-tube in the flame, and 

 heat the whole to 60° C. for half an hour to insure sterility. 



Lastly, cut the pipette up into lengths by notching it with a file 



6—2 



