12 PHYSIOLOGICAL CHEMISTRY. 



are not so desirable. These are various partly decomposed 

 organic matters of animal or vegetable origin and, what is 

 more important, minute living vegetable cells, mostly bacteria, 

 which are capable of causing much mischief when taken into 

 the stomach of man. It is very generally believed that sev- 

 eral diseases in man have their origin in the consumption of 

 water contaminated in this way. 



Natural Purification of Water. But it must not be sup- 

 posed that these bacteria are always harmful. On the con- 

 trary some of them are the common agents which effect the 

 natural purification of waters containing organic matter, in 

 which they incite destructive fermentation or putrefactive 

 changes and finally oxidation. As a result of these changes 

 harmless inert substances such as nitrogen or nitrates, carbon 

 dioxide, methane and water are produced from the relatively 

 complex waste or excreta of the higher organisms. When 

 we speak of the spontaneous or self-purification of water we 

 refer to a series of changes in which these bacteria play a 

 leading part. 



Artificial Purification of ' Water. On a smaller scale 

 water may be rendered safe and suitable for household use by 

 several methods. By distillation all objectionable matters 

 may be rejected and a wholesome drinking water obtained. It 

 is possible, also, to separate practically all bacteria and other 

 solid matters by filtration through beds of fine sand. In this 

 way the supplies of many cities are obtained at the present 

 time. Frequently the filtration is preceded by coagulation or 

 precipitation of the organic substances by means of some 

 suitable agent such as alum or salts of iron, or lime. 



Tests of Drinking Water. In the sanitary examination 

 of water it is not necessary to make very full analyses to deter- 

 mine its value for household use. A few tests usually suffice 

 to discover the presence or absence of objectionable substances. 

 For example, in uncontaminated waters from ordinary springs, 

 lakes, rivers or wells, chlorine is present in small amount only. 

 Any excess of chloride suggests contact with sewage or house- 



