Report on the Bacteriology of Water. 355 



On September 3 a small loopful of a gelatine culture of the bacillus 

 ft was carefully shaken in about 50 c.c. of sterile water, and distributed 

 equally in two Erlenmeyer flasks labelled ft (1) and ft (2). 



Flask ft (1) was wrapped in foil and black paper; ft (2) was ex- 

 posed over mirror from 9.30 A.M. to 5 P.M., and about five hours of 

 these seven and a half were good strong sunshine. Thermometers in 

 control flasks went up to 34 35 C. as the highest temperature regis- 

 tered in the afternoon. 



At 9.30 a plate was prepared from each flask, and gave something 

 like 3,000,000 per c.c. as the average numbers to start with. 



After exposure, two plates were prepared from each. Of the two 

 plates from the exposed flask, one plate a 1-drop plate of a l/10th 

 dilution yielded no colonies at all ; the other gave 21,941 per c.c., 

 pointing to a profound light-action. 



Of the two plates from the darkened flask, one gave 5,400,000, 

 and the other 5,000,000 as the nearest estimate per c.c. 



The two flasks meanwhile stood over-night in the laboratory at 

 18 C. for about fourteen hours, and at 7 A.M. next day (September 4) 

 1 made two plates from the exposed flask and then again put them out 

 as before. 



These two plates gave 42,000 and 59,220 respectively; total, 

 101,220 ; mean, 50,610 bacteria per c.c., a prefectly natural rise in the 

 numbers having occurred during the night. 



No plates were taken from the other flask, as I had no particular 

 need for the numbers "known to be very high and wished to 

 reserve the counting for other plates. 



After exposure to the bright sunshine of September 4, from 8 to 

 4.30, say eight hours' sunshine on the exposed flask, two new plates 

 were made from each flask. 



The two plates from exposed flask gave 3050 and 4200 ; total, 

 7250 ; mean, 3625 bacteria per c.c., again showing a marked reduc- 

 tion in the sunlight, and the plates were singularly free of liquefying 

 centres, but showed many colonies of the kind I had previously sus- 

 pected as being the representatives of ft. 



Of the two plates from the unexposed flask, although made from 

 one drop each of a l/20th dilution, the numbers were again so large 

 and the liquefaction so rapid that no reliance can be placed on them, 

 except that they prove that no essential diminution was to be traced 

 to the action of the water or temperature in the absence of light, but 

 only the natural fall in numbers always found when water stands for 

 some time. The numbers actually calculated were 660,000 and 

 3,300,000 ; total, 3,960,000 ; mean, 1,980,000 per c.c. 



The flasks stood in the laboratory at 18 C. through the night, and 

 I repeated the exposure on September 5, putting the uncovered flask 

 into the bright sunlight of that day without a mirror. It received a 



