236 ESSENTIALS OF BACTERIOLOGY 



wells, or run through the wood, gathering the washings from 

 pastures and farm lands, it becomes, as surface water, open 

 to all sorts of impurities, and the bacterial nature of it 

 changes every moment. 



Artesian or Driven Well. The driven well will secure to 

 a certain extent a pure water. It is the only form of well or 

 cistern that will insure this, since the water does not become 

 stagnant in it; but it may connect with an outhouse the soil 

 being very loose and thus bacteria and refuse water find 

 their way into the well. The casing may not be w r ater- tight 

 and surface water can be sucked in. 



Filtered Water. Dangerous as surface water is, the 

 greater quantity used is such, the inhabitants of larger towns 

 and cities using chiefly the rivers and other large waters which 

 course near them for drinking purposes. A purification or 

 filtration can, to a certain extent, render these waters 

 harmless. 



Filtration is carried on on a large scale in the water-works 

 of cities and towns, and bacteriologic examination is here of 

 great service to determine if a water which has been filtered 

 and may have a very clear appearance, and give no harmful 

 chemical reaction, is entirely free, or nearly so, from germs; 

 in other words, if the filter is a germ-filter or not; daily tests 

 are necessary in order to insure safety, and if it is performing 

 this function regularly, a good filter plant should show 99.8 

 per cent, efficiency, removing nearly all the bacteria. 



Filter Materials. When waters are muddy or when rapid 

 filtration is wanted, mechanical filters are employed. The 

 water is first treated with coagulants, like alum, which forms 

 a flocculent precipitate and carries down with the suspended 

 matter much of the bacterial content. This is then filtered 

 through sand and gravel. Sedimentation and filtering 

 slowly through gravel and sand is known as the slow process; 

 the other as the rapid, filtration. 



Charcoal sponge and asbestos, the materials formerly in 

 use, are objectionable because germs readily develop on them 

 and clog them, so that they require frequent renewal. In 



