EXAMINATION OF AIR, SOIL, AND WATER 221 



a water stand walled up in cisterns or wells, 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, allowing the products of germs of refuse 

 water to find their way into the well. The casing may not be 

 water-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, in a certain measure, render these waters harm- 

 less. 



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. 



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 very 

 large filters, sand and gravel give the best results; the number 

 of germs in a cubic centimeter is reduced to forty or fifty and 

 kept at that number. This is a very pure water for a city water, 

 though, as we stated before, not a safe one, for among those 

 forty germs very dangerous ones may be found. It is then 

 necessary for the users to refilter the water, before drinking it, 

 through a material which will not allow any germs to pass, or, 

 in the presence of an epidemic, to boil all water used for 

 drinking purposes. 



Pasteur-Chamberland Filter. This very perfect filter 

 consists of a piece of polished porcelain in the form of a 



