80 



BIOLOGY AND TECHNIQUE 



disinfectant is employed has already been discussed. For ordinary 

 work it is customary to express absolute and comparative antiseptic 

 and bactericidal values in terms of percentages based upon weight, and 

 this, beyond question, is both simple and practical. For strictly scien- 

 tific comparisons, however, as Kronig and Paul ' have pointed out, it 

 is by far more accurate to work with equimolecular solutions. 



Rideal and Walker ^ have devised a method of testing disinfectants, 

 in which an attempt is made to estabhsh a standard for comparisons. 

 They choose, as the standard, carboUc acid, and estabhsh what they call 

 the "carboUc-acid coefficient." This coefficient they obtain in the fol- 

 lowing way: the particular dilution of the disinfectant under investiga- 

 tion which will kill in a given time, is divided by the strength of carboUc 

 acid which, under the same conditions, will kill the same bacteria in 

 the same time. We quote an example of such a test, given by Simpson 

 and Hewlett,' comparing formaUn and carbolic acid. 



BACILLUS PESTIS. 



In the above table, formahn 1 in 30 killed in the same time as 

 carbolic acid 1 in 110. Thus the carbolic-acid coefficient of formahn 

 in this test = ^/no = .27. 



The Rideal- Walker method has been much used and is recommended 

 by many workers.^ 



The most precise method of standardizing disinfectants is that now 

 in use in the U. S. Public Health Service. It is a modification of the 

 Rideal-Walker procedure devised by Anderson and McClintic' 



Stock 5 per cent solutions of the disinfectant in question and of the 



1 Kronig und Paid, loc. oit 



2 Rideal and Walker, Jour, of the Sanitary Ins. London, xsdv. 

 ' Simpson and Hewlett, Lancet, ii, 1904. 



* Sommerville, Brit. Med. Jour., 1904. 



' Anderson and McClintic, Jour, of Inf. Dis., 1911, viii, 1. 



