380 PUBLIC HEALTH BACTERIOLOGY 



arranged in a series of tubes, each containing 5 c.c., and 

 the dilution of the disinfectant is marked on each tube. 



To 5 c.c. of a particular dilution, five drops of the filtered 

 culture are added ; the tube is shaken and set down. 

 Now repeat with the next dilution, and so on. At the 

 end of 2-5 minutes, a subculture is made from the tube 

 first inoculated, into 5 c.c. of sterile broth, and similarly 

 with all the dilutions, in the proper order. At the end of 

 5, 7-5. 10, 12-5, and 15 minutes, put up further subcultures 

 thus, making six series of subcultures in all from each tube. 

 The same process is carried out, at the same time, with the 

 same culture, with dilutions of carbolic acid. All the sub- 

 cultures are incubated for at least forty-eight hours at 

 37 C., and the presence or absence of growth is noted. 

 From the table of results, the two dilutions doing the same 

 work in the same time are seen. Say that 1-500 of Disin- 

 fectant A inhibited growth in 2-5, 5, 7-5 minutes' exposures, 

 and that carbolic acid i-no did the same ; then the 

 carbolic acid coefficient of Disinfectant A is 500 -MIO = 4-5. 



Since its introduction this test has been much used. It 

 has also been subjected to much criticism. It deals with 

 " naked " bacteria. The other objections are that it must 

 be carefully done, all the dilutions should be accurate, the 

 organism used should be the same throughout, etc. These 

 are not indictments of the test in careful hands, and where 

 the tests on different disinfectants are made by the same 

 individual. Still the results obtained by this test should 

 not be taken for more than they pretend to be, the relative 

 power of different bodies to carbolic acid under the same 

 conditions. The application of such results to ordinary 

 purposes must be made very cautiously. 



The "Lancet" Commission Test. This is a modified 

 Rideal-Walker test, in which the number of dilutions is 

 increased up to nine, and the time periods correspondingly 

 up to thirty minutes ; B. coli is used as the test organism ; 

 MacConkey's bile-salt litmus glucose peptone water is used 

 as the medium for subcultures instead of broth ; platinum 

 spoons are used instead of loops, and the method of calcula- 

 tion is different. 



The B. coli used is cultivated for twenty-four hours at 



