GERMICIDAL POWER OF CHEMICAL DISINFECTANTS 195 



When everything is in readiness the culture is added to the disinfectant solu- 

 tions with a sterile pipette in quantities of Y\ cc. to each dilution. 



To add the culture, the seeding tube containing the disinfectant is removed 

 from the water bath with the left hand and slanted at an angle of about 45, 

 and with the right hand the end of the pipette containing the culture is intro- 

 duced and lightly touched against the side of the tube where the liquid has run 

 away on account of the slanting. At the proper time the culture is allowed to 

 run into the disinfectant solution, the pipette removed, the tube straightened 

 up, gently shaken three times, and replaced in the water bath. The other 

 tubes are done the same way in succession, and it will be found that 15 

 seconds is ample time for each tube. By adding the culture to the 

 disinfectant with a pipette touched against the side of the seeding tube, 

 accurate measurements can be made and each|tube receive exactly the 

 same amount of "seeding," which is not the case when the culture is added 

 by the "drop." 



If 10 tubes are to be inoculated, only a few seconds will remain after 

 inoculating the last tube before a plant from the first tube will have to 

 be made. 



The mixing tubes are not removed or disturbed in making the planting except 

 to insert the loop or spoon into them, touch the bottom, withdraw, and then 

 make the plant in broth. Every effort is made to insert and withdraw the loops 

 or spoons in a uniform manner. The loops and spoons are bent to an angle of 

 about 45, where. they are joined to the shank, and therefore are always filled 

 with the mixture when withdrawn from the seeding tubes. After making the 

 plants the loops or spoons are flamed as already described. 



After an experiment is finished the date and any necessary details can be 

 marked on one of the broth tubes and the rack placed in the incubator at 37C. 

 for 48 hours. At the end of this time the results are recorded on a chart specially 

 devised for the purpose. 



DETERMINING THE COEFFICIENT 



After a large number of experiments we have concluded that the method 

 employed by the Lancet commission, with certain modifications, is the best one 

 for determining the coefficient i.e., the mean between the strength and time 

 coefficients. 



In performing the test, plants are made every 2^ minutes up to and includ- 

 ing 15 minutes. To determine the coefficient, the figure representing the degree 

 of dilution of the weakest strength of the disinfectant that kills within 2^ min- 

 utes is divided by the figure representing the degree of dilution of the weakest 

 strength of the phenol control that kills within the same time. The same is 

 done for the weakest strength that kills in 1 5 minutes. The mean of the two 

 is the coefficient. The method of determining the coefficient will be seen in 

 Table 17. 



