348 MICRO-ORGANISMS IN WATER 



septic-us, and the Staphylococcus pyo genes albus. These 

 investigations were carried out at Naples in the zoo- 

 logical station there and were pursued throughout the 

 year, care being taken that in every case a control 

 experiment was made with cultures preserved in the 

 absence of light, contained in blackened glass vessels, 

 arid placed side by side with those exposed to the sun- 

 light. The temperature never rose beyond 45 C., and 

 the difference of temperature in the darkened and the 

 insolated flasks respectively never varied more than one 

 degree. In this manner the possibility of temperature 

 being responsible for the effect produced was disposed 

 of. Pansini exposed, first, various culture media re- 

 cently inoculated with different microbes ; secondly, well- 

 developed cultivations ; thirdly, hanging drop cultures of 

 infected bouillon. The second of these methods, which 

 consisted in exposing well-developed cultivations on 

 gelatine or potatoes, is obviously unsatisfactory, inas- 

 much as by reason of the thickness and opacity of the 

 growth the sun's rays could not penetrate equally to all 

 parts of the cultivation. 



In the first method, test-tubes plugged with cotton- 

 wool, some blackened and others not, containing gelatine 

 or slices of potato, were inoculated with various mi- 

 crobes and exposed to the sun's rays. Every half-hour 

 one tube from each series was removed and placed in 

 the incubator, to watch its further development:. In 

 this manner it was found that light invariably exercised 

 a retarding influence on the development of the or- 

 ganisms, and this effect was the more rapid the more 

 nearly normal the angle at which the rays fell on the 

 surface of the culture material. The rate of destruction 

 varied, however, according to the micro-organism and 

 the medium in which it was present. The Bacillus 

 pyocyaneus was found to resist light generally better 



