THE WATER SUPPLY OF LONDON. 183 



experiments were made near the Grand Junction Company'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 condition 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 four and a half 

 hours — from ICV30 a.m. to 3 r.M. on 15th May, 1895. At 

 the end of this time the tubes were packed in ice for trans- 

 port to my laboratory, where the cultivation was started 

 immediately. The colonies were counted on the fourth day 

 and yielded the results given in the table : — 



No. of Colonies 

 per c.c. 

 Thames water packed in ice immediately after collection - 2127 

 Thames water after exposure to sunlight for 4^ hours at 



surface of river ------- 1140 



Thames water after exposure to sunlight for 4^ hours at 



6 in. below surface of river ----- i^o 



Thames water after exposure to sunlight for 4^ hours at 



1 ft. below surface of river - - - - - 2150 

 Thames water after exposure to sunlight for 4J hours at 



2 ft. below surface of river ----- 2430 

 Thames water after exposure to sunlight for 4! hours at 



3 ft. below surface of river ----- 2440 



These experiments show that, on 15th May the 

 germicidal effect of sunlight on Thames microbes was nil 

 at depths of one foot and upwards from the surface of the 

 water. It cannot, therefore, excite surprise, that the effect 

 of sunshine upon bacterial life in the great mass of Thames 

 water should be nearly, if not quite, imperceptible. It is 

 thus ascertained that sunlight can only kill the germs, or 

 microbes, near the surface of the water, whilst those at any 

 depth, for the most part, escape destruction. 



On the other hand the enormous effect of floods in 

 augmenting the number of microbes can hardly surprise us, 

 for when a great body of water has flowed over the banks 

 of the river, which are at other times dry and exposed, it 

 carries along with it countless impurities — an effect common 

 both to the main stream and its tributaries. The Thames 



