lxxxvi. ABSTRACT OF PROCEEDINGS. 



media, the aeration of water, and the necessity of keeping 

 the upper layer of sand in a condition most favourable to 

 filtration, quoting at length the opinions of many eminent 

 authorities on all these questions. He concluded his 

 remarks with an allusion to the Sydney Water Supply and 

 the many difficulties the authorities had in preserving its 

 purity without resorting to filtration. 



Dr. Tidswell, m.b., M.ck., discussed the question as a 

 bacteriologist, and as a useful preliminary to further studies 

 proposed to refer to the behaviour of bacteria in unfiltered 

 water and subsequently to tlie changes they undergo during 

 the process of filtration. He stated that they reach the 

 water mainly with washings from the adjacent land, but in 

 a less degree from the air with dust and rain, and even 

 snow and hail. Bacteria are most numerous in river water 

 during winter, interpreting this time as meaning the rainy 

 season, when the land washings are most voluminous. The 

 number of bacteria in a river will be materially affected by 

 the character of the land through which it flows. The 

 character of the contiguous land may be expected to influ- 

 ence not only the number but the kinds of bacteria washed 

 in from it. The continuous existence of these bacteria in 

 water has been mainly studied under laboratory conditions 

 and their behaviour has been closely examined. It is 

 commonly observed initially that a more or less marked 

 and rapid increase in numbers is followed sooner or later 

 by a gradual decline and not reaching the length of extinc- 

 tion for months at least. The rapidity of the increase is to 

 some extent dependent upon the character of the water ; 

 where this is originally rich in bacteria the multiplication 

 is less marked than where the initial number of the bacteria 

 is low. Though possibly influenced by what may be called 

 elbow room, the usual explanation of the fact just given is 

 that it depends upon available food supply in relation to 



