154 APPENDIX. 



germ cin be proven to be present in that particular water, the 

 epidemic may be past. 



Purity of Waters. The purest water we have is the natural 

 spring water water that lias slowly filtered its way through 

 various layers of gravel and sand and comes finally clear and 

 sparkling from the ground. It is without germs ; but let sudi 

 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 drivrn well will secure to a cer- 

 tain 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. If a chemical examination shows increased 

 amounts of chloride of sodium, a contamination can be mooted. 



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 harmless. 



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

 works of cities and towns. 



Bacteriological examination is here of great service to deter- 

 mine if a water, which has been filtered and may have a very 

 clear appearance, and give no harmful chemical reaction, yet 

 be entirely free, or nearly so, from germs ; in other words, if 

 the filter is a germ filter or not. 



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 centimetre 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 gerins to pass. 



