THE DESTRUCTION OF BACTERIA 



99 



an example of such a test, given by Simpson and Hewlett, 50 com- 

 paring formalin and carbolic acid. 



BACILLUS PESTIS. 



Sample. 



Formalin .... 

 Carbolic acid 



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

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

 in this test = 3 % 10 = .27. 



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

 mended by many workers. 57 



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 Mc- 

 Clintic. 58 



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

 the standard (phenol) are first prepared and a series of accurate 

 dilutions made with distilled water using graduated pipettes. (To 

 make 1 :70 take 4 c.c. of stock and 10 c.c. distilled water ; 1 :80 = 4 

 c.c. of stock + 12 c.c. distilled water ; 1 :90 .= 4 c.c. stock + 14 c.c. 

 distilled water; 1:500 = 2 c.c. of stock + 48 c.c. of distilled water. 

 Complete dilution tables are given in their original article.) The 

 series should include dilutions strong enough to kill B. typhosus 

 in two and a half minutes and weak enough to fail to do so in 

 fifteen minutes. If dilutions greater than 1-500 are required, a 

 second 1 per cent stock solution is prepared. They adopted the 

 following scale for their tests : Dilutions up to 1 :70 should vary 

 from the next in the series by a difference of 5 (i.e., 5 parts of 

 water), and so on if higher solutions are necessary. 



" Simpson and Hewlett, Lancet, ii, 1904. 



57 Sommerville, Brit. Med. Jour., 1904. 



68 Anderson and McClintic, Jour, of Inf. Dig., 1911, viii, 1, 



