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substances are not poisonous, but they impart a disagreeable taste of 

 flatness to the water. Perhaps some water may contain the highly- 

 poisonous ptomaines, which, unfortunately, cannot be easily determined 

 yet, but whose ever-present products of decomposition, the ammonia, can 

 be readily shown to be extant in the water. The microscopical examina- 

 tion was shown by the lecturer not to stand in any direct connection to the 

 findings by chemical analysis. The microscope will reveal suspended 

 mineral matters, or perhaps eggs of parasitical worms, or— and this is the 

 most important point— a larger amount of micro-organisms. Among 

 these there may possibly be found some pathogenetic microbes, especially 

 where the water has been drawn from wells, &c., in the vicinity of humau 

 abodes. Such a water is naturally unfit for use. But it is not easy to 

 identify such pathogeneous germs, and very often the examination will 

 come either too late or will be without a tangible result, notwithstanding 

 well-founded suspicion, as those germs may have passed away in some way 

 or other. Therefore it will be incumbent on us to prevent the possibility 

 of any germs getting into the water, or to remove those present. It follows 

 that open water ought to be carefully filtered. This is most effectually 

 done by the huge sand filters now in vogue. Eecent experiments, carried 

 out with sterilised sand filters, have shown that it is not the sand which 

 filters effectually, but that it is the thin film of bacteria and other— mostly 

 organic— substances on the top of the sand beds which sieves off the 

 microbes. Unless such a film has been formed, the filtering beds do not 

 work efficiently; for, during the first days or weeks they allow 

 all the microbes to pass through, and as soon as the really effective 

 film has been formed, the water trickles so slowly through the 

 filters that they generally get cleaned out. The water from springs or 

 deep wells will usually be free from microbes, as the latter cannot live in 

 any great depth, and if wells be guarded against contamination of any 

 kind, they will yield the best and most wholesome water. 



