392 MICRO-ORGANISMS IN WATER 



6. There is no evidence that the virulence of an- 

 thrax undergoes any permanent attenuation through 

 exposure to light, for although the bacilli or spores 

 which have been insolated, short of being killed, may 

 be incapable of producing a lethal effect on animals, the 

 cultures which can be obtained from them are found to 

 be fully virulent (Eoux and Momont). 



7, As regards the effect of light on bacteria present 

 in water, distilled or potable, the experiments hitherto 

 made have been principally limited to anthrax spores. 

 Although the exact time that such spores will endure 

 insolation in water varies greatly according to their 

 previous history, the balance of evidence tends to show 

 that they are less rapidly destroyed than when exposed 

 in culture media or in an insolated condition. (The 

 greater power of resisting sunshine in water was indeed 

 already shown for mixtures of bacteria by Downes and 

 Blunt.) Their endurance is particularly long-continued 

 when they are insolated in distilled water in the absence 

 of air, resistance to upwards of 110 hours' exposure 

 having been observed by Momont. It has moreover 

 been shown by one of us that the addition of a halogen 

 salt, in the shape of sodium chloride, to the distilled 

 water materially increases the injurious effect of the 

 sunlight, whilst the addition of an oxy-salt, in the form 

 of sodium sulphate, is practically without influence in 

 this respect. 



Of great importance in connection with the deport- 

 ment of bacteria under insolation in water is the depth 

 to which the sun's rays can take effect. On this point 

 the evidence is very contradictory. Thus, whilst Arloing 

 found that a stratum of water two centimetres in thick- 

 ness was almost an efficient protective screen for anthrax 

 against the sun's rays, Buchner found that the latter 

 penetrated to a depth of half a metre without having 



