486 BACTERIOLOGICAL ANALYSES 



b. Trial Trip. 



Determine the lethal effect of a series of five solutions 

 of germicide-x (say 1:100, 1:250, 1:300, 1:500, 

 1:600) at contact times of 2j, 5, 25 and 30 minutes 

 in the following manner: 



1. Arrange five test-tubes marked A to E in the lower 

 tier of the test-tube rack. 



2. Into tube A pipette 5 c.c. germicide-x 1:100 

 solution. 



Into tube B pipette 5 c.c. germicide-x 1:200 solution. 

 Into tube C pipette 5 c.c. germicide-x i :3oo solution. 

 Into tube D pipette 5 c.c. germicide-x i :5oo solution. 

 Into tube E pipette 5 c.c. germicide-x i :6oo solution. 



3 . Arrange 20 tubes of bile salt broth in the upper tier 

 of the test-tube rack in two rows, those in the front 

 row numbered consecutively from left to right i-io, 

 those in the back row 11-20. 



4. Place a square wire basket of about 50 tubes 

 capacity close to the left of the test-tube rack, for the 

 reception of the inoculated tubes. 



5. Take a sterile i c.c. pipette from the case, pick up 

 a sterile rubber washer with forceps and push the point 

 of the pipette into the central hole. 



6. Put down the forceps on the bench with the sterile 

 points projecting over the edge. Without taking. the 

 tube from the rack remove the cotton- wool plug from 

 tube A, and lower the pipette, with the rubber washer 

 affixed, on to the open mouth of the tube; with the 

 help of the forceps to steady the washer, push the 

 pipette on through the hole until the point of the 

 pipette has reached to within a few millimetres of the 

 bottom of the tube (see fig. 219). 



7. Adjust in the same way a pipette and a washer in 

 the mouth of each of the other tubes, B, C, D and E. 



8. Set the electric signal clock to ring for the com- 

 mencement of the experiment and at subsequent inter- 

 vals of 2j, 5, 25 and 30 minutes. 



