66 ELEMENTARY AGRICULTURAL CHEMISTRY 



This in itself is comparatively harmless ; its importance lies 

 in the influence it has upon the kinds of micro-organisms 

 which accompany it. Animal excreta is the most dan- 

 gerous contamination, since the micro-organisms which cause 

 various diseases — e.g., typhoid and cholera — are liable to be 

 thus introduced into the water. Animal organic matter is 

 richer in nitrogen than most vegetable refuse, so that in 

 practice the detection of much combined nitrogen, whether as 

 organic matter, ammonium salts, or nitrates, is regarded as 

 sufficient to indicate that the water has been contaminated 

 with sewage or other animal matter. If much organic matter 

 of animal origin be present there must always be considerable 

 risk of pathogenic (disease-producing) organisms finding their 

 way into the bodies of those who drink it ; and though such 

 contaminated water may be, and often is, drunk for years with 

 impunity its consumption is decidedly dangerous. 



Another substance characteristic of sewage is common salt ; 

 consequently the presence of much chlorine in a water is 

 generally indicative of sewage contamination, unless the water 

 is derived from some rock containing salt or is collected near 

 the sea. 



Analyses of typically good and bad drinking waters are 

 given by Roscoe and Schorlemmer as follows : 



