374 MICRO-ORGANISMS IN WATER 



be undertaken with extreme precaution, and that in the 

 interpretation of the results it must not be forgotten 

 that numerous other factors besides the presence or 

 absence of light contribute to bring about numerical 

 differences in the bacteria present. It is therefore 

 desirable that further confirmation of the above results 

 should be forthcoming before any decided inference can 

 be drawn. In this connection it may be pointed out 

 that we have repeatedly found about twenty times as 

 many microbes in the waters of the rivers Thames and 

 Lea during the winter as in the summer months (see 

 table, p. 123). Here, again, we must recognise that 

 there are other agencies besides the greater duration 

 and potency of the sun's rays in the summer also tend- 

 ing to reduce the number of bacteria present at this 

 season of the year, although sunshine may undoubtedly 

 have contributed to bring about the observed contrast. 

 Procacci ^ has still more recently investigated the 

 depth to which the lethal action of the sun's rays ex- 

 tends in water. For this purpose ordinary drain-water 

 was employed. Cylindrical glass vessels about 60 cm. 

 high and 25 cm. wide were filled with drain-water; 

 from some of these the light was excluded, whilst in 

 others it was allowed free access. The temperature in 

 the exposed vessels never exceeded that of the darkened 

 by more than from 2° to 4° C, and in neither case did it 

 ever rise beyond from 40° to 42° C. In every instance a 

 marked diminution in the bacterial contents was observed 

 in the insolated vessels, whilst at the same time a more or 

 less decided increase took place in the darkened ones, the 

 duration of the experiments varying from 1^ to 9 hours. 

 In order to ascertain to what depth the antiseptic action 

 of sunlight extended, and what part- was played by the 



^ * Influenza della luce solare suUe acque di rifiuto,' Annali delV In- 

 stituto d'Igiene Sperimentale di Boma, vol. iii., 1893, p. 437. 



