1895.] affecting Bacterial Life in Thames Water. 449 



Then came the tremendous flood of November, the highest on record, 

 the flow of the river over Teddington Weir, on the 18th, having 

 reached nearly 20,136 millions of gallons. The sample for microbe 

 cultivation was, however, taken on the 5th, when the flow only 

 amounted to 4,462 millions of gallons per day, but this increase from 

 596 millions of gallons in October to 4,462 millions on the 5th of 

 November was accompanied by an increase in the number of microbes 

 from 3,800 on the 10th of October to-20,080 per cubic centimetre on 

 the 5th of November ; whilst, in December, the flow of the river had 

 fallen to 2,058 millions of gallons per day, and the number of 

 microbes to 16,300 per cubic centimetre. 



The only exception of any importance to the rule, that the number 

 of microbes varies with the flow of the river, occurring during 

 the thirty-two months through which these observations have been 

 continued, happened in November, 1892, when the flow increased 

 from 501 millions of gallons in October to 1,845 millions in November, 

 whilst the microbes actually diminished in number from 2,316 to 

 1,868 per cubic centimetre. Neither the sunshine nor the temperature 

 records of these two months, however, afford any explanation of this 

 anomalous result, for there was a good deal of sunshine in October 

 before the collection of the sample, and the temperature was higher ; 

 whilst in November, no ray of sunshine reached the Thames during 

 the three days preceding the taking of the sample, and the temperature 

 was nearly 4 C. lower than in the preceding month. 



These comparisons, therefore, demonstrate, I think satisfactorily, 

 that the number of microbes in Thames water depends upon the 

 rate of flow of the river or, in other words, upon the rainfall, and 

 but slightly, if at all, upon either the presence or absence of sunshine 

 or a high or low temperature. 



With regard to the effect of sunshine upon bacterial life, the in- 

 teresting researches of Dr. Marshall Ward leave no doubt that sun- 

 light is a powerful germicide ; but it is probable that its potency, in 

 this respect, is greatly diminished if not entirely annulled, when the 

 solar rays have to pass through a stratum of water even of compara- 

 tively small thickness before they reach the living organisms. If 

 this be the case, it can be no matter for surprise that the effect of 

 sunshine upon bacterial life in the great mass of Thames water should 

 be nearly, if not quite, imperceptible. 



Note added March 16. 



Since this paper was written, I have ascertained that between 

 October 17 and November 22, 1892, when the sample for microbe 

 cultivation was collected, the river had several times been in such 

 high flood as to be bank-full. Thus, between October 30 and 



VOL. LVII. 2 K 



