BACTERIOLOGICAL TESTING OE DISINFECTANTS 379 



restrain the growth of the unattenuated organisms, and yet 

 might be capable of doing so in the case of those which had 

 been treated with disinfectant. In the process which we 

 suggest, 50 c.c. of the original culture will contain only a 

 loopful of disinfectant solution ; and supposing, for example, 

 that such solution was as strong as 5 per- cent., there would 

 therefore be a total proportion of 1 to 500,000 of disinfec- 

 tant, an amount which on previous knowledge would be 

 unlikely to arrest growth when composed of any disinfectant 

 which requires as much as 5 per cent, for germicidal action. 

 Similarly, if a dilution of 1 to 500 of a substance having 

 the same disinfectant value as perchloride of mercury were 

 employed, it would be present in the 50 c.c. of broth in the 

 proportion of 1 to 12,500,000. 



Instead of using broth flasks for observing the result 

 previously obtained from the action of the disinfectant, we 

 have found in practice that it is safe to use streak cultures 

 on gelatine tubes, making three strokes on each tube in the 

 same way as in the examination of membrane for the Klebs- 

 Loffler bacillus. The probable reason why this method is 

 satisfactory is that in the passage of the needle-point over 

 the gelatine the organisms have the best possible chance of 

 being at all events at some points deposited out of contact 

 with the disinfectant ; and in practice where the disinfection 

 has failed, we usually obtain discontinuous growths. 



In some cases a disinfectant has to be used under con- 

 ditions where an exposure of ten minutes would be incon- 

 venient or impracticable, as, for instance, in the disinfection 

 of the hands of surgeons. In examining a disinfectant for 

 such purpose, the standard time must be fixed at fifteen or 

 thirty seconds, as the case may be. 



An alternative method, which has been considerably used, 

 is to impregnate silk threads in an emulsion of organisms, 

 either in broth or in sterile distilled water, and to inoculate 



