208 MICROBIOLOGY OF WATER AND SEWAGE. 



well known. It is used in the purification of the water supply of some 

 towns Nice, Chartres, etc. Ozone used for this purpose is usually 

 obtained by means of the electric current; and a flowing film of water is 

 brought into contact with an upward current of air charged with ozone, 

 which current makes the water almost completely sterile. This method 

 of purification is efficient, but rather expensive. 



PURIFICATION BY HEAT. By bringing water to the boiling point, all 

 harmful bacteria are destroyed; a few spores may resist this treatment, 

 but they are harmless. Boiled water is of a flat, insipid taste, due to the 

 driving out of the contained gases. The taste may be improved by cooling 

 and shaking. The boiling of water is often resorted to as a hygienic 

 measure in times of epidemic, and for the supply of armies in the field. 



PURIFICATION BY CHEMICALS. The addition of a small amount of 

 calcium hypochlorite, or potassium iodide, etc., purifies water; but these 

 methods are seldom used, except for the use of soldiers on campaign. 



LOCATION AND CONSTRUCTION OF WELLS. 



Farms in many sections of this country are practically all supplied 

 with surface water collected in shallow wells. Hence farmers should 

 understand the principles involved in the location and construction 

 of wells. 



Many farm wells are badly located too near such sources of contami- 

 nation as out-houses, cess-pools, stables, or barn-yards; and those who 

 locate them give too little attention to the slope of the ground, and the 

 nature and slope of the subsoil. There should be at least 22 to 

 30 M. (75 to 100 feet) between the well and all probable sources of 

 contamination; and this distance is too small, if the soil is very porous, 

 or if the surface and subsoil drainage is toward the well, or if the well 

 is sunk in fissured rock as it is obvious that there are serious chances of 

 contamination in each of the above circumstances. 



In all cases, the surface drainage should be away from the well; and, 

 as far as possible, the subsoil drainage also should be from the well. 



Sketches 60, 61, and 62 illustrate these points, the upper part of each 

 drawing showing the plan and the lower portion a section through the dotted 

 line marked on the plan. Figure 60, shows that the surface drainage is 

 from the house, privy, stables, and barnyard toward the well. The section 

 through the line " A" shows the relation of the impervious subsoil "B " to 



