382 PUBLIC HEALTH BACTERIOLOGY 



hours as a purple tinge, becoming pink and then red in 

 another two hours. The results are filled into a tabular 

 form, with a plus sign for growth and a zero for no growth. 

 The coefficient is thus calculated : The weakest dilution 

 of the sample under test, giving no growth at 2-5 minutes, 

 is divided by the weakest dilution of carbolic acting 

 similarly in the same time ; the weakest dilution giving 

 no growth at thirty minutes is divided by the same for 

 carbolic acid ; the two results are averaged, and the 

 average or mean is taken as the carbolic acid coefficient. 

 Thus, if Disinfectant B gives no growth with 1-220 in 

 2 -5 minutes and with 1-340 in 30 minutes, and if the 

 dilutions for carbolic acid are 1-110 and 1-180, then 

 the coefficient would be : 



(H + Hi) - 2 or (2 + i-88) + 2 = 1-94. 



The Commission prefer to express their dilutions as 

 percentages, and when this is done the mode of division 

 is reversed ; that is, the weakest percentage (of carbolic, etc.) 

 is divided by the weakest percentage of the test substance, 

 etc. The above dilutions become for B, 0-454 per cent 

 and 0-294 per cent, and for carbolic, 0-909 per cent and 

 0-555 per cent respectively. The coefficient therefore is 

 (%m + %' 5 U) - 2 or (2 + 1-85) 4- 2 * 1-92. The tem- 

 perature of the room averaged about 62 to 67 F. 



By this method the coefficients ranged from 0-025 to 

 9-8 for the usual coal-tar disinfectants on the market. 

 Applied to corrosive sublimate, the coefficient at 2-5 

 minutes was about 2000, and at 30 minutes about 6000, 

 giving a mean of about 4000. Chloride of lime similarly 

 gave figures of 45 and 93, or a mean of 69. For formalin 

 the coefficient was o-6. 



Among the conclusions reached were the following : 

 That results obtained in such bacteriological experiments, 

 although giving a germicidal value to a disinfectant under 

 the most favourable conditions, afford little indication of 

 their germicidal value when used in practical disinfection. 

 That much remains to be done in the solution of such 

 problems (amongst others) as arise, due to : (1) The 

 presence of foreign substances in the material to be dis- 

 infected ; (2) The temperature at which the disinfecting 



