ACTION OF LIGHT ON MICRO-ORGANISMS 355 



typhoid bacillus, and his first experiments were made to 

 ascertain if this bacillus suffers any diminution in its 

 vitality when exposed to diffused daylight. It was found 

 that, when typhoid growths were exposed on sloping 

 surfaces of gelatine in a cold room the temperature of 

 which was about 12 E., growths were visible on the 

 third day in the tubes protected from light with black 

 paper, but that in the case of the exposed tubes nothing 

 was observed until the fifth day. Again, when in the 

 same room at 12 E., typhoid bacilli were introduced 

 into sterile bouillon, rendered as clear as possible by 

 the addition of four parts distilled water and 0*5 per 

 cent, sodium chloride, turbidity did not commence in 

 the case of the exposed flasks and tubes until from 

 twenty-four to twenty- eight hours after inoculation, 

 whereas in the protected flasks turbidity was noted 

 sixteen to twenty hours after inoculation. These differ- 

 ences are not very great, but were maintained through- 

 out a large number of experiments. 



In direct sunlight the results were more striking. 

 The temperature registered during the experiments 

 never rose above 40 C. The same kind of broth was 

 used as in the previous experiments, and the tubes, 

 after inoculation, were exposed to direct sunlight on 

 the roof from eight in the morning until seven in the 

 evening ; during the night they were placed in a re- 

 frigerator. In the protected tubes turbidity com- 

 menced after eight hours, whilst in the insolated tubes 

 the liquid remained perfectly clear. After two days' 

 insolation the tubes were placed in an incubator at 

 37 C., but no turbidity made its appearance. This 

 result was invariably obtained throughout a large series 

 of investigations. In order to ascertain whether the 

 bouillon had undergone some chemical change during 

 insolation unfavourable for the development of the 



