COLLECTION OF WATER. 3 



reducing and oxidising powers really belong to distinct groups 

 of micro-organisms. Still, the fact that a potable water may 

 undergo entirely opposite chemical changes according as one 

 group or the other gains the ascendency shows how frail a basis 

 there is for the belief that the determination of ammonia, 

 nitrites and nitrates will enable a reliable opinion to be given 

 as to " previous sewage contamination." At the same time, it 

 is certain that in surface waters ammonia and nitrites are 

 scarcely ever present to any extent without sewage contamina- 

 tion. Consequently, if a chemical examination is made imme- 

 diately after collection of the sample it will assist us in arriving 

 at a proper judgment ; this is especially the case when the 

 pollution is solely from urine, under which condition bacteri- 

 ology is often at fault. Much patient work on sewage organisms 

 has shown that a recent contamination of a water supply by 

 sewage can easily be detected even when the pollution is so 

 slight as one part per million. Chemistry is quite unable to 

 indicate such a slight contamination as this. If, however, a 

 considerable time has elapsed since the pollution occurred it 

 becomes more and more difficult to detect it by bacteriological 

 processes ; and this is especially the case when waters of great 

 original purity are under examination. It is therefore evident 

 that a bacteriological examination has its limits of usefulness, 

 and a slavish adherence to it under all conditions, combined 

 with neglect of the hints to be obtained by chemical means, may 

 lead to a perfectly erroneous judgment. Still, there is one 

 branch of hygienic study in which bacteriology must always 

 reign supreme ; it is now acknowledged on all sides that the 

 working of sand filters for public water supplies cannot be 

 properly kept under control except by appealing to bacteriological 

 methods of examination. 



From what has been said it is obvious that a bacteriological 

 examination of water consists essentially of two parts viz., (a) 

 the quantitative analysis or determination of the number of 

 bacteria in a known volume of water, and (b) the qualitative 

 analysis or determination of the species of bacteria present in 

 the sample. 



Before proceeding to the consideration of these analyses it 

 will be advisable to mention a few points in relation to the 



