A method proposed by Professor Sheridan Delépine is to place some of the suspected water in sterilised test-tubes without further treatment, and incubate at 37° C. for twelve or eighteen hours, and then plate out and estimate the number of bacteria as in the ordinary course. "In polluted water, containing an excess of organic matter," he says, "an extremely rapid multiplication of bacteria is observed. In unpolluted water, containing only water bacteria and a very small amount of organic matter, very little or no multiplication takes place, and the growth of the water bacteria liquefying gelatine is checked to a remarkable extent." Thirdly, by none of these methods should we be able to isolate anaërobic bacteria, and to furnish a complete report these also must receive careful attention. [Illustration: APPARATUS FOR FILTERING WATER TO FACILITATE ITS BACTERIOLOGICAL EXAMINATION] _The Bacteriology of Water._ In many natural waters there will be found varied contents even in regard to flora alone: _algæ_, _diatoms_, _spirogyræ_, _desmids_, and all sorts of vegetable detritus. Many of these organisms are held responsible for divers disagreeable tastes and odours. The colour of a water may also be due to similar causes. Dr. Garrett, of Cheltenham, has recorded the occurrence of redness of water owing to a growth of _Crenothrix polyspora_, and many other similar cases make it evident that not unfrequently great changes may be produced in water by contained microscopic vegetation. With the exception of water from springs and deep wells, all unfiltered natural waters contain numbers of bacteria. The actual number roughly depends upon the amount of organic pabulum present, and upon certain physical conditions of the water. As we have already seen, bacteria multiply with enormous rapidity. In some species multiplication does not appear to depend on the presence of much organic matter, and, indeed, some can live and multiply in sterilised water: _Micrococcus aquatilis_ and _Bacillus erythrosporus_. Again, others depend not upon the quantity of organic matter, but upon its quality. And frequently in a water of a high degree of organic pollution it will be found that bacteria have been restrained in their development by the competition of other species monopolising the pabulum. Probably at least one hundred different species of non-pathogenic organisms have been isolated from water. Some species are constantly occurring, and are present in almost all natural waters. Amongst such are _B. liquefaciens_, _B. fluorescens liq._, _B. fluorescens non-liquefaciens_, _B. termo_, _B. aquatilis_, _B. ubiquitus_, and not a few _micrococci_, etc. Percy Frankland[14] collected water from various quarters at various times and seasons, and some of his results may here be added: RIVER THAMES WATER COLLECTED AT HAMPTON Number of Micro-organisms Obtained from 1 cc. of Water.