530 



Dr. P. F. Fraiikland. 



[June 10, 



Thus in the one case there is a slight diminution in the number, 

 and in the other case increase ; but in neither is the alteration very 

 considerable. Similar samples collected on the 19th February, 1886 y 

 were examined after exposure to various conditions, as specified 

 on p. 528. 



Further experiments were made with a view of testing this beha- 

 viour of the micro-organisms present in river- water. The results 

 obtained were as follows : — 



Number of Colonies obtained from 1 c.c. 



Description. 



April 13/86. 



On day 

 of collection, 

 temp. 8° C. 



April 15/86. 

 Standing in dark 

 at 20° C. for 



2 days. 

 (Same bottle.) 



April 17/86. 

 Standing in dark 

 at 20° C. for 



4 days. 

 (Same bottle.) 



River Thames at Hampton 

 River Lea at Chingford. . . 



12,250 

 7,300 



4,386 

 2,148 



2,018 

 1,286 



Thus in both cases there was a marked reduction in the micro- 

 organisms after storing for two and four days respectively. 



From the experiments made with these river- waters, it would 

 appear that there is a decided tendency for the micro-organisms 

 which they contain to become reduced in number when the waters 

 are kept at the ordinary temperature of the air (20° C), whilst the 

 numbers may l?e temporarily enormously increased by exposing them 

 to an incubating temperature. 



A very large number of experiments was also made with the filtered 

 river- waters, as supplied to the metropolis. These waters have sub- 

 stantially the same chemical composition as the rivers from which, 

 they are derived, but the number of micro-organisms which they con- 

 tain at the time of collection rarely exceeds on the average 5 per 

 cent, of that present in the raw river-waters. The results obtained 

 are recorded in the following tables : — 



