STANDARDIZATION OF DISINFECTANTS 247 



tent by mixing with distilled water in proper proportion, in each 

 case adding the measured disinfectant to the measured quantity 

 of water to make the desired dilution. 



The Standard. Merck's pure phenol is diluted by volume 

 by weighing out any desired quantity, dividing by the specific 

 gravity, 1.08, and dissolving in distilled water, diluting to twenty 

 times the volume of the carbolic acid used, to make a 5 per cent. 

 solution by volume in volume. Further dilutions can be made 

 from this as desired, since the solution is practically permanent. 



The dilutions of carbolic acid ordinarily used with the results 

 to be expected are as follows: 



The sample is usually diluted in such a way that not more 

 than a half unit in the coefficient lies between two dilutions. 

 For example 



1-480 1-540 1-600 1-660 1-720 



the intention being to compare with the carbolic acid dilution 

 1-120. A disinfectant value based on growth at i min. only 

 would be far from exact. Much more accurate results can be 

 obtained where the dilutions compared are those which kill the 

 organism in from 3 to 5 min. 



IV. The proper mixture of culture and disinfectant is carried 

 out as follows: five drops of bacterial filtrate are introduced into 

 5 cc. of germicidal solution, contained in test-tubes 5^/2 in. long 

 by % in. in diam. This is added drop by drop in rapid suc- 

 cession, by means of a sterile pipette, 8^ in. long and 0.219 in. 

 in diam., the narrow end measuring about 0.075 in. in diam., 

 held vertically at the mouth of the test-tube containing the 

 germicidal solution, so that the pipette with the germs will not 



