STANDARDIZATION OF DISINFECTANTS 235 



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 all excess of the disinfectant 

 on the outside of 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 con- 

 tents 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 dis- 

 infectants 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 i : 100 in accordance with the second table of dilu- 

 tions. 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 dilu- 

 tions made from this. 



We had adopted the following scale for making dilutions: 

 For dilutions up to 1:70, increase or decrease by a difference 

 of 5 (i.e., 5 parts of water). 



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



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



From : 200 101:400 by a difference of 25 



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



From :goo to i: 1800 by a difference of 100 



From : 1800 to 1:3200 by a difference of 200 



and so on if higher dilutions are necessary. 



It is important that the cylinders used for making the dilutions 

 be correctly graduated, as we have found disregard of this factor 

 an important source of error. It is preferable to use standardized 

 cylinders and pipettes, and we recommend that they be used 

 whenever possible. They of course should be perfectly clean. 

 For making the dilutions in accordance with the above scheme we 

 have found the following table of much service: 



