Testing Germicidal Value of Liquids 253 



growth or failure to grow observed. This method also determines 

 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 loopfuls 

 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 Modern Method of Testing the Germicidal Value of Liquids.-^- 

 The methods of testing germicidal strength given above are uncertain 

 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 impor- 

 tance, these are the method of Rideal and Walker,f "The Lancet 

 Method, "{ and the method of Anderson and McClintic. 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 "coefficient." 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 ac- 

 curacy has now become the standard method of the United States 

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

 acquainted and which will be given in detail. 



I. The Apparatus, Reagents, etc., Required for the Test. i. A 

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

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

 as possible should be employed. Walker recommends that only 



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



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

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

 vol. CLXXVII, Nos. 4498, 4499 and 4500. 



Bulletin No. 82 of the Hygienic Laboratory, Washington, D. C., 1912. 



