TESTS OF DISINFECTION 33 



that the means of disinfection resorted to were effi- 

 cient. Negative evidence should be received with 

 great caution. In the first place, the question as to 

 whether susceptible individuals have been fairly ex- 

 posed in the disinfected room must be considered. 

 Then it must be remembered that susceptible persons 

 do not always contract a disease, even when they are 

 exposed in a locality known to be infected. A further 

 difficulty in estimating the value of evidence obtained 

 in practice arises from the fact that in connection 

 with the special means of disinfection resorted to, 

 such as fumigation, hanging up cloths saturated with 

 a disinfecting solution, etc., it is customary to resort 

 to additional precautionary measures, such as wash- 

 ing surfaces with soap and hot water, whitewashing 

 plastered walls, and free ventilation. It is apparent 

 that under these circumstances it would be unsafe to 

 accept the fact that no other cases occurred in a room 

 treated in this way as evidence that the particular 

 disinfectant used is efficient for the destruction of the 

 infectious agent of the disease in question. The fond 

 mother who attaches a charm to her child's neck to 

 protect it from evil also takes the precaution of guard- 

 ing it from contact with other children who are sick 

 with any infectious disease. If her child fortunately 

 grows to manhood or womanhood without having 

 suffered an attack of scarlet fever or diphtheria, she 



