TESTING OF DISINFECTANTS. 523 



for 2 minutes no longer kill guinea-pigs, but nevertheless 

 they can grow in tubes, even after 5 minutes' exposure. On 

 the other hand, positive results may be obtained by inocu 

 lating white mice or guinea-pigs with the mixture of bac- 

 teria and disinfectant; whereas, the same material trans- 

 planted to a nutrient medium may fail to grow owing to the 

 antiseptic power of the disinfectant which is carried over. 



10. Control experiments. A dozen or two of the uninocu- 

 lated bouillon tubes should be placed in the incubator, to- 

 gether with the sub-cultures proper, in order to eliminate 

 possible error. Moreover, as an additional control, at least 

 one tube of bouillon should be inoculated with the original 

 untreated culture, cover-glass or silk -thread. 



Frequently, the sub-cultures yield doubtful results. 

 The bouillon in that case is slightly clouded and the growth 

 itself is rather uncertain. In this case, the tubes should be 

 returned to, and kept in the incubator for about a week. 

 Or, one or two loopsful should be transplanted to freshly 

 inclined agar. 



If the growth that develops shows the slightest varia- 

 tion from the normal one, it should be examined under the 

 microscope in order to exclude contaminations. 



The action of a disinfectant on bacteria may be studied 

 by bringing it into contact with infected silk-threads, mus- 

 lin squares, cover-glasses, or by mixing direct with the bac- 

 terial suspension. The preparations of these specimens 

 has been given on p. 517. 



Silk-threads. This method was introduced by Koch and has been 

 used extensively. Silk, linen or cotton threads may be used. They 

 are cut up into convenient lengths, placed in a plugged tube, steril- 

 ized and kept for future use (p. 517). 



To ascertain the disinfecting- action of a solution, a dried thread, 

 impregnated with the bacteria to be tested, is immersed in it for a 

 given length of time, as for instance, 2 minutes. It is then removed 

 with sterilized forceps and gently washed in sterilized water or in al- 



