GERMICIDAL POWER OF CHEMICAL DISINFECTANTS IQI 



the platinum loop is laid so that the end of the loops extend about 5 inches 

 beyond the side of the block. The first step in the operation is to sterilize each 

 loop by flaming with a fantail Bunsen burner before beginning the experiment. 

 When ready to begin the operation the loop farthest from the operator is 

 taken in the right hand and the inoculation made. It is then replaced in the 

 groove with the right hand and the Bunsen burner (fantail) placed under it 

 with the left hand. The next loop is then used, replaced in its groove, and the 

 Bunsen burner placed under it with the left hand, the first loop having been 

 heated to redness while the second loop was in use. This procedure is then 

 continued until all the inoculations have been made. The time required in 

 making the inoculations and in replacing the loop is short, it being found that 

 15 seconds is ample. 



INCUBATION 



The subcultures are incubated 48 hours at 37C., and the results then read 



off and tabulated. 



i 



DILUTIONS 



Capacity pipettes for the original dilutions are invariably used for the phenol 

 controls a standard dilution of pure phenol ("Merck's Silver Label") is made 

 and standardized by the U. S. P. method (Koppeschaar) to contain exactly 

 5 per cent, of pure phenol by weight. From this stock solution the higher dilu- 

 tions are made fresh each day for that day's test. 



For the dilutions of the disinfectant a 5 per cent, solution is made by adding 

 5 cc. of the disinfectant to 95 cc. of sterile distilled water. A standardized 

 5 cc. capacity pipette is used for this, and after filling the pipette is wiped off 

 with sterile gauze. The contents of the pipette are then delivered into'a cylinder 

 containing 95 cc. of sterile distilled water, and the pipette washed out as clean 

 as possible by aspiration and blowing out the contents of the pipette into the 

 cylinder. The contents of the cylinder are then thoroughly shaken and the 

 dilutions up to i : 500 made from it, using delivery pipettes for measuring. For 

 those disinfectants which do not readily form a 5 per cent, solution we make 

 a i per cent, stock solution, and from this make the dilutions greater than 

 1:100 in accordance with the second table of dilutions. If greater dilutions 

 than i : 500 are to be made, a i per cent, solution is made from the 5 per cent, 

 solution, and the higher dilutions made from this. 



We have adopted the following scale for making dilutions: 



For dilutions up to i : 70, increase or decrease by a difference of 5 (i.e., parts 

 of water). 



From i : 70 to i : 160 by a difference of 10. 



From i : 160 to i : 200 by a difference of 20. 



From i : 200 to 1 1400 by a difference of 25. 



From i : 400 to i : 900 by a difference of 50. 



