36 



VETERINARY HYGIENE 



sewage pollution. A knowledge of the source of supply is neces- 

 sary in this case as with all other deductions concerning possible 

 pollution. As to the limit of permissible ammonia this, as with 

 other impurities, depends upon the other constituents and other 

 factors which the analyst takes into consideration before giving his 

 final decision. " It is often difficult, if not impossible, to judge of 

 the character of a water by the amount of the ammonia which it con- 

 tains, but wherever the amount exceeds 004 per 100,000 an 

 endeavour should be made to ascertain its origin." 



Organic Matter. The amount of oxidisable organic matter in 

 water helps, when taken collectively with the other analytical results, 

 to determine the condition as regards contamination. 



As the test applied is to note the amount of oxygen the water will 

 take up from a substance that readily parts with its oxygen (per- 

 manganate of potassium), the results are expressed in so many parts 

 of oxygen absorbed per 100,000 of water. Sometimes iron and 

 nitrites are found to absorb oxygen from the permanganate. The 

 oxidisable material in water may be of vegetable or animal origin, 

 and the test applied does not distinguish the one from the other. 

 Upland surface waters may contain a very considerable quantity of 

 vegetable matter, and peaty waters especially may absorb a large 

 amount of oxygen. 



Standardisation is difficult. The table of standards here given 

 is that of Sir E. Frankland and Dr. Tidy.* 



Amount of Oxygen absorbed in 100,000 Parts of Water. 



* The Examination of Waters and Water Supplies, J. C. Thresh, 1913. 



