VETERINARY HYGIENE 



SECTION I. 

 WATER. 



WHEN rain falls upon the earth it may be evaporated to become 

 once more atmospheric moisture; it may run on the surface until 

 it reaches a stream, river or lake; or it may sink into the soil. 



Rain Water as it descends from the clouds is pure and whole- 

 some, but as soon as it comes in contact with the earth, or passes 

 through the lower air strata over populous places it becomes altered 

 in character. The modifications to which it is subjected from time 

 to time vary greatly, and as it may be so changed as to become 

 unwholesome or dangerous it is necessary for the hygienist to 

 understand the causes and nature of the variation in quality in 

 order that he may be in a position to adjudicate on the fitness or 

 otherwise of a supply. 



During its passage through the atmosphere rain absorbs oxygen, 

 nitrogen and carbonic acid gas. Of these three it absorbs oxygen 

 in greatest amount. Owing to its freedom from mineral matter 

 it has an insipid taste and is not therefore so palatable as water 

 that has percolated into the soil, but is specially suitable for washing 

 purposes, as water containing much dissolved mineral is very 

 destructive to soap. Since rain will absorb to itself any gases or 

 particulate matter that may be in suspension in the air, it follows 

 that the rain over country districts is much cleaner than that which 

 falls over towns where the air is subjected to much pollution. 

 Sodium chloride is to be found in the rain that falls over or near 

 the sea. Snow water differs from rain water in that it has not 

 had the same opportunity to absorb gases. 



Surface Water. This is rain water that has fallen on the 

 earth, washed its surface, and has not yet penetrated sufficiently 

 deep to rid itself of such impurities as it may have gathered. The 

 character of such a water will vary with the nature of the ground 

 on which it has fallen. In country districts not greatly polluted 

 by animals or people, it will be comparatively free from con- 

 tamination other than that of vegetable origin and such mineral 

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