270 MICROBIOLOGY OP WATER AND SEWAGE 



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 hy- 

 gienic 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. 

 Hypochlorite, however, is now used more commonly in municipal water 

 supplies where they can not be otherwise controlled. 



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 con- 

 tamination as outhouses, cesspools, stables, or barnyards; and those 

 who locate theln 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 he from the well. 



Sketches 113, 114, and 115 illustrate these points, the upper part of 

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

 the dotted Une marked on the plan. Fig. 113, 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 



