SUNLIGHT AS A DISINFECTANT 49 



light. The distinguished German bacteriologist, 

 Dr. Robert Koch, reported, some years since, the 

 results of his experiments with the tubercle bacil- 

 lus. He found that the time required to kill this 

 bacillus varies from a few minutes to several hours, 

 depending upon the thickness of the layer exposed. 

 Even diffused daylight exerts a certain germicidal 

 action, although the time of exposure is very much 

 longer five to seven days for the tubercle bacillus. 

 In the writer's experiments made in 1892 it was 

 found that two hours' exposure to direct sunlight 

 was fatal to the cholera spirillum suspended in a 

 liquid medium. 



The electric light, and even gas-light, have also 

 a germicidal action upon certain disease germs, al- 

 though very much less in degree than sunlight. It 

 has been ascertained that the rays at the violet end of 

 the spectrum have the greatest disinfecting power, 

 while the red rays are comparatively inert. 



The facts stated fully sustain the popular idea that 

 the exposure of infected articles of clothing and bed- 

 ding in the sun is a useful sanitary precaution. Re- 

 peated and prolonged exposure will, however, be 

 necessary to ensure safety. In the case of such dis- 

 eases as smallpox, diphtheria, and scarlet fever more 

 speedy and reliable measures of disinfection will be 

 required. Exposure to the sunlight is nevertheless a 



