6 VETEEINAEY HYGIENE 



imparts a taste and odour to the water is said to be 



innocuous. 



A scum like green paint growing on the surface of a lake 

 in Australia was found to be very poisonous to animals, 

 but as a rule it may be said growing vegetable matter in 

 water is harmless. It may even be productive of good, for 

 it was shown some years ago in India, that after all living 

 vegetable matter had, in accordance with an order, been 

 cleared out of the lakes (tanks) used for storing drinking 

 water, the same became unfit for use. It was apparently 

 the oxidizing power of the water-plants which kept the 

 water pure. 



Eiver Water is as a rule pure enough at its source, but 

 gradually becomes more and more contaminated on its way 

 to the sea, depending upon the number of towns on its banks. 

 Prior to the water supply of towns and cities being 

 taken in hand by legislation, these derived their drink- 

 ing water from, and emptied their sewage into, the same 

 river. It was usual to empty the sewage below the 

 point at which the town supply of water was drawn, but 

 inasmuch as there might have been a dozen towns on the 

 same river each pursuing this course, the element of 

 safety was a slender one. Such a system converted a 

 river into an open sewer, and it is important, as the 

 system still exists, to inquire whether rivers so polluted are 

 capable in any way of purification and by what means. 



It is certain that the process of oxidation and sunlight 

 can work great changes, especially if there is a good 

 current in the river, or agitation of the water by means of 

 weirs or flowing over rapids ; these are capable under 

 the influence of bacteria of considerably purifying the 

 water, perhaps even of rendering it harmless. Not that 

 drinking excreta-infected water is dangerous or harmful in 

 itself, it is so enormously diluted as to be harmless so 

 far as poisonous properties are concerned, but if such 

 water is infected with enteric or cholera the matter is 

 quite different. The specific microbe of these diseases, it 

 is true, appears to have a very short life in rivers, but 



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