Testing Germicidal Value of Liquids 261 



washed in clean, sterile water, transferred to fresh culture-media, and 

 their growth or failure to grow observed. This method also deter- 

 mines the time in which a certain solution will kill micro-organisms, so 

 is advantageous. 



Sternberg suggested a method by which the dilution necessary to 

 kill the bacteria could be determined, the time remaining constant 

 (two hours' exposure) in all cases. "Instead of subjecting test- 

 organisms to the action of the disinfecting agent attached to a silk 

 thread, a certain quantity of a recent culture — usually 5 cc- — is 

 mixed with an equal quantity of a standard solution of the germi- 

 cidal agent, . . . and after two hours' contact one or two loopf uls 

 are transferred to a suitable nutrient medium to test the question 

 of disinfection." 



A very simple and popular method of determining the germicidal 

 value is to make a series of dilutions of the reagent to be tested;- add 

 to each a small quantity of a fresh liquid culture, and at varying in- 

 tervals of time transfer a loopful to fresh culture-media. By a little 

 ingenuity this method may be made to yield information as to both 

 time and strength. 



Hill* has suggested a convenient method of handling the cul- 

 tures, which are dried upon the ends of sterile glass rods and can then 

 be transferred from one solution to another or otherwise manipulated. 



The Modem Method of Testing the Germicidal Value of Liquids. 

 —The methods of testing germicidal strength given above are uncer- 

 tain and inaccurate, and can only be looked upon as "rough and 

 ready" methods, that should be willingly abandoned for anything 

 better. Three methods are now offered that hold out the promise of 

 scientific accuracy through an established standard of comparison. 

 In the order of their appearance, which is also, probably, the order of 

 their importance, these are the method of Rideal and Walker,! 

 "The Lancet Method,"t and the method of Anderson and McChntic. § 

 The methods are similar in general principles, and have the same 

 object in view, i.e., the expression of the germicidal value of any sub- 

 stance as the carbolic acid or phenol "coeflScient." Experience 

 with the methods leads to the conviction that the Rideal and 

 Walker method is the more easy to execute, but that the Anderson- 

 McClintic method is the more accurate. As the latter in addition to 

 its accuracy has now become the standard method of the United 

 States Government, it is the method with which the student should 

 be acquainted and which wiU be given in detail. 



/. The Apparatus, Reagents, etc., Required for the Test. — r. A 

 Phenol Solution that shall act as the standard of comparison. In the 

 preparation of this solution, pure phenol — as free from cresols, etc., 



* "Public Health," vol. xxiv, p. 246. 



t Journal of the Royal Sanitary Institute, London, 1903, p. 424. 

 I "The Standardization of Disinfectants" (unsigned article), Lancet, London, 

 vol. CLXXVTi, Nos. 4498, 4499, and 4SOO. 

 § Bulletin No. 82 of the Hygienic Laboratory, Washington, P. C, 191 2. 



