74 BACTERIA IN DAILY LIFE 



tion which have been shown to take place during 

 this period of repose are to a large extent 

 responsible for the diminution in the number of 

 bacteria present, yet it is also highly probable 

 that insolation assists considerably in this im- 

 provement, at any rate, in the upper layers of 

 the water. As the depth of the water increases 

 the action of light is necessarily diminished. In- 

 deed, exact experiments conducted in the Lake 

 of Geneva to ascertain by means of photographic 

 plates the depth to which the sun's rays pene- 

 trate showed that they did not reach beyond 

 five hundred and fifty-three feet, at which depth 

 the intensity of the light is equal to that which 

 is ordinarily observed on a clear but moonless 

 night, so that long before that their bactericidal 

 potency would cease. 



It is the more important that this limit to the 

 powers of sunshine in water should be duly 

 recognised, inasmuch as solar enthusiasts, when 

 first the fact became known, rashly jumped at 

 the convenient hypothesis, based on very slender 

 experimental evidence, that the sun's rays were 

 possessed of such omniscient power to slay 

 microbes, that they might safely be relied upon 

 to banish all noxious organisms from our streams, 

 and that local authorities might therefore com- 

 fortably and without any qualms of conscience 



