DISINFECTANTS 291 



and in that case the disinfectant contained in the loopful 

 might fail to 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 disinfectant, an amount which on previous 

 knowledge would be unlikely to arrest growth when com- 

 posed 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. 



