ANTISEPTICS AND DISINFECTANTS, 155 



less bacteria, while silk threads steeped in an 

 artificial cultivation of the bacilli and dried, afforded 

 a means of testing the vitality of spores. 



Even by employing pure cultivations on solid 

 media, great precautions were necessary to avoid 

 mistakes. If, for instance, a large quantity of the 

 growth which had been subjected to some chemical 

 solution were carried over to the fresh tube con- 

 taining the nutrient medium, or if a silk thread, 

 which had been dipped in a solution, were directly 

 transferred to the new soil, enough of the supposed 

 disinfectant might be mechanically carried over to 

 retard the development of the bacteria, though 

 it was ineffectual in destroying them. From a 

 growth not appearing, the conclusion might be 

 drawn that the spores or the bacteria had been 

 affected, and so a mistake occurs. To avoid this 

 Koch made a point of transferring a minimum of 

 the disinfected growth to as large a cultivation 

 area as possible, so that any chemical substance 

 mechanically carried over, would be so diluted as 

 to be inert. For the same reason threads, after 

 withdrawal from the disinfecting solution, were 

 rinsed in sterilised water, or weak alcohol, and 

 then transplanted ; or, instead of judging from the 

 development on nutrient gelatine, the effect of 

 inoculation in a healthy animal was made the test. 



A few examples may be quoted in illustration. 

 Silk threads, impregnated with anthrax spores, 

 were placed in bottles containing carbolic acid of 



