372 



MICRO-ORGANISMS IN WATER 



tained none whatever. In the control vessel exposed 

 under the same conditions, but from which the light 

 was excluded by drawing a covering of black paper 

 over the flask, the contained organisms had undergone 

 a slight increase. Diffused daylight had a less powerful 

 effect than sunlight, but also produced a marked effect 

 after a few hours, and in some cases succeeded in de- 

 stroying all the organisms present. 



In a more recent paper ^ Buchner has endeavoured 

 to ascertain at what depth in water the bactericidal 

 action of light ceases. For this purpose he used the 

 method described on p. 360. Eecently infected agar- 

 agar dishes, partly covered with a leaden cross, were 

 lowered to particular depths in the Starnberger Lake, 

 near Munich ; the site selected for the experiments was 

 the starting-place of the steamers, and the water was 

 not quite clear, 

 obtained : — 



The following table shows the results 



Action of SunligJit During dj Hours on Bacteria at Different 



Deaths of Water (Buchner) 



(Temp, of Water = 15° R.) 



At a depth of 1'6 m. the bactericidal action of the 

 sun's rays is as strong as outside the water, but at 2"6 

 the action was much less apparent, whilst at 3-1 it was 

 only just perceptible. 



^ ' Ueber den Einfluss des Lichtes auf Bakterien und tiber die Selbst- 

 reinigung der Fliisse,' Archiv filr Hygiene, 1893. 



