156 BACTERIOLOGY. 



various strengths. A thread was removed from 

 each on successive days, and transferred to nutrient 

 gelatine, and the result noted. It was found that 

 immersion of the thread in a 5 per cent, solution 

 of carbolic acid was sufficient in two days to effect 

 complete sterilisation, and seven days in a 3 per 

 cent, solution was equally efficacious. Since for 

 practical purposes a strength should be selected 

 which would be effectual in twenty-four hours, 

 Koch recommended that for general use, allow- 

 ing for deterioration by keeping, a solution con- 

 taining not less than 5 per cent, should be 

 employed, and for complex fluids probably a still 

 higher percentage would be necessary. In the 

 case of sporeless bacilli the results were very 

 different. Blood, containing the bacilli, from an 

 animal just killed, was dried on threads, and after 

 exposure for two minutes to a i per cent, solution, 

 was completely sterilised. Fresh blood mixed with 

 a i per cent, carbolic solution produced no effect on 

 inoculation. If, on the other hand, the blood was 

 mixed with a '5 per cent, solution, the virulence 

 was not destroyed. The facility with which the 

 bacilli are destroyed, compared with their spores, 

 illustrates how easily errors may occur, if mere 

 arrest of growth or loss of motility be regarded as 

 a sign of the efficacy of disinfection. 



To test vapours, Koch exposed anthrax spores 

 or the spores which occur in garden earth by sus- 

 pending them over solutions, e.g., of bromine or 



