THE PURITY OF DRINKING-WATERS. I2Q 



age contains every form of human excretions; and since it is by 

 means of the excretions that the pathogenic bacteria find exit 

 from the diseased patient, it will thus be easily understood that 

 sewage-contaminated water is likely to contain bacteria which are 

 pathogenic for man. Such water is therefore always dangerous, 

 a fact abundantly proved by the great prevalence of water-borne 

 diseases in cities whose water-supply is contaminated with sewage. 

 When the bacteria in water are in the thousands per c.c., they render 

 the water unsafe, not because this number of bacteria is injurious, 

 but because such water is commonly sewage-contaminated. 



When we recognize the great chance which sewage-contaminated 

 water has of becoming impregnated with the germs of human 

 diseases, it is a little surprising to learn that the number of kinds of 

 disease actually distributed by water is very small. Only one of 

 our common diseases is known to be frequently distributed by 

 water. This is typhoid fever, in regard to which the evidence is 

 abundant and conclusive. This evidence need not be given here, 

 but it is sufficient to demonstrate that typhoid fever is very commonly 

 acquired from drinking-water, that the danger comes wholly from 

 water which has in some way become contaminated with human 

 excrement, usually through sewage, and that the drinking of sewage- 

 contaminated water is probably the most prolific source of this 

 dreaded and serious disease. 



Other water-borne diseases are of less importance. Asiatic 

 cholera is distributed by water, but this is, at least in this country, 

 of no significance. Certain forms of dysentery are probably distrib- 

 uted by water, but little is known of this matter as yet. No'other 

 diseases are known to be thus distributed. 



Detection of Sewage Contamination. Sewage contamination 

 is a rapidly growing danger. As population increases, the amount 

 of sewage also increases, and it becomes more and more difficult 

 to dispose of it so as to prevent its contaminating the sources of 

 drinking-water. Many a stream formerly used for drinking purposes 

 has had to be abandoned because it has become so polluted with 

 sewage as to be no longer safe. It is thus a matter of prime necessity 

 to find some delicate means of determining whether water is sewage 



