206 ANNUAL, REPORT OF THE Off. Doc. 



polluted, will purify itself by percolating through a few yards of 

 earth has .led to the commission of many blunders. Entirely 

 too much confidence has been placed and is still placed in the power 

 of the earth to purify water by filtration. 



In order to illustrate this fact I propose to compare the results of 

 analyses of water from polluted wells with those of pure waters 

 from the same localities. In order to understand the significance 

 of these results it will be necessary first, to explain the meaning of 

 the terms used in the report of a chemical analysis of water. 



A chemical analysis of water for sanitary purposes is simply a 

 means of obtaining comparative measurements of the amount of filth 

 which has been dissolved by the water and the extent of its decom- 

 position. 



In the report of a water analysis we usually find figures represent- 

 ing the amounts of the following substances expressed either in parts 

 per million or in parts per hundred thousand; chlorine, nitrogen as 

 nitrites, nitrogen as nitrates, nitrogen as ammonia, organic nitrogen, 

 and total solids. Sometimes temporary and permanent hardness, 

 oxygen absorbed and other data are also added. 



A bacteriological examination may be made consisting of an 

 estimation of the number of bacteria in one c. c. of the water and a 

 search for such bacilli as occur in the intestines of man and the lower 

 animals (bacillus coli communis or other bacilli of the colon group). 

 From the data furnished by such an analysis and an examination 

 of the surroundings of the water supply, an intelligent judgment 

 can be formed as to the sanitary condition of the water. 



Chlorine usually exists in water combined with sodium in the form 

 of common salt. As sewage and the tissues and excreta of man and 

 many of the domestic animals are rich in salt, any excess of 

 chlorine above the amount found in the purest water of a given lo- 

 cality is generally due to contamination with animal matter and its 

 presence in excess unless it can be otherwise accounted for is re- 

 garded as an indication of pollution of a dangerous character 



Nitrogen is a constituent of many organic compounds especially 

 those of animal origin. The figure given under nitrogen by per- 

 manganate is regarded as a measure of the amount of decomposing 

 organic matter in the sample. 



Nitrates are formed by the decomposition and complete oxidation 

 of nitrogenous organic matter. Nitrogen in. this condition repre- 

 sents the final stage in the putrefaction of animal matter. In the 

 final disposition by the processes of nature of organic matter rich in 

 nitrogen; nitrites represent an intervening stage between ammonia 

 and nitrates. Their presence in water may be due either to the par- 

 tial oxidation of organic nitrogen or to the abstraction of oxygen 



