189G.] on the Past, Present and Future Water Supphj of London. 69 



comparatively small thickness, before they reach the living organisms. 

 By a series of ingeniously devised experiments, Mr. Burgess has 

 demonstrated the correctness of this view. 



A sterile bottle, about half filled with Thames water, was violently 

 agitated for five minutes to insure equal distribution of the organisms. 

 Immediately afterwards, a number of sterile glass tubes were partially 

 filled with this water and sealed hermetically. Three of these tubes 

 were immediately packed in ice, and the remainder were attached in 

 duplicate at definite distances apart to a light wire frame which was 

 then suspended vertically 

 in the river. The experi- 

 ments were made near 

 the Grand Junction Com- 

 pany's intake, at a place 

 favourable for the sun's 

 rays to fall on the river 

 without any obstruction. 



The river was, at the 

 time, in a very clear con- 

 dition and contained but 

 little suspended matter, 

 whilst the day was fine, 

 although clouds obscured 

 the sun occasionally. 

 The tubes were exposed 

 to light in the river for 



Fig. 16. 



4i hours, from 10.30 a.m. 

 toSp.M. onMayl5, 1895. 

 At the end of this time 

 they were packed in ice 

 for transport to my labo- 

 ratory, where the cultivation was started immediately. The colonies 

 were counted on the fourth day, and yielded the results given in the 

 following table : — 



No. of colonies 

 per c.c. 



Thames water packed in ice immediately after collection 

 „ „ alter exposure to sunlight for ij hours at 



surface of river 



„ „ after exposure to sunlight for 4| hours at 



6 in. below surface of river 



„ „ after exposure to sunlight for 4J hours at 



1 ft. below surface of river 



„ „ after exposure to sunlight for 4| hours at 



2 ft. below surface of river 



„ „ after exposure to sunlight for 4| hours at 



3 ft. below surface of river 



2127 

 1140 

 1940 

 2150 

 2430 

 2440 



These experiments show that on May 15 the germicidal effect 

 sunlight on Thames microbes was 7iil at depths of 1 foot and 



of sunlight on 



