154 BACTERIOLOGY. 



of the substance estimated by the absence of 

 cloudiness, odour, or other sign of development 

 of bacteria in the inoculated fluid. Koch pointed 

 out the errors that might arise in these experiments 

 from accidental contamination, or from there being 

 no evidence of the destruction of spores ; and we 

 are indebted to him for a complete and careful 

 series of observations with more exact methods. 



Instead of employing a mixture of bacteria, 

 Koch's plan was to subject a pure cultivation of 

 some well-known species with marked character- 

 istics to the reagent to be tested. A small 

 quantity was then transferred to fresh, nourishing 

 soil, under favourable conditions, side by side with 

 nutrient material inoculated from a cultivation 

 without treatment with the disinfectant. The latter 

 constituted a control test, which is most essential in 

 all such experiments. To test the resistant power 

 of bacteria which are easily destroyed, two species 

 were selected, the so-called Micrococcus prodigiosus, 

 and the bacillus of blue pus. These were culti- 

 vated on potatoes, the surface of which was sliced 

 off and dried. A fragment transferred to freshly- 

 prepared potato gave rise to a growth of the 

 particular micro-organism ; but if after treatment 

 with some reagent no growth occurred, the conclu- 

 sion was drawn that the agent was efficacious in 

 destroying the vitality of the bacteria. 



Anthrax bacilli in blood withdrawn from an 

 animal just killed were taken to represent spore- 



