BACTERIOLOGICAL EXAMINATION. 291 



figures are often due to vegetable matter. The proportion of 

 oxygen absorbed to albuminoid ammonia is often a useful datum. 

 Where vegetable contamination has taken place the oxygen 

 absorbed is ten times (or more) the albuminoid ammonia ; in 

 polluted waters it is usually less. 



The presence of phosphates is usually regarded as an un- 

 favourable symptom ; this may, however, be due to the use of 

 artificial fertilisers ; the nitric acid may be increased from this 

 cause. 



If waters known to be pure from the same district and from 

 the same geological formation can be obtained, the water can 

 be compared with them ; any marked increase in the figures 

 found must be regarded as evidence of pollution. By this means 

 evidence of contamination is often obtained which would be 

 difficult, or almost impossible, to acquire from chemical analysis 

 alone. 



It must be remembered in comparing waters with a " district 

 standard " that in the autumn the figures for free and albuminoid 

 ammonia, nitric acid, and oxygen absorbed usually are slightly 

 higher than at other times of the year. 



For further information on the subject works on " Water 

 Analysis " must be consulted. It must be borne in mind that 

 the judging of water supplies is not a subject that can be learnt 

 from books entirely, but that prolonged experience is necessary 

 to interpret properly the results obtained. 



Bacteriological Examination. 



A very simple examination is all that is usually necessary. 

 A sample of the water for bacteriological examination must be 

 taken in a sterilised bottle ; a six-ounce stoppered bottle is 

 plugged with cotton wool, and the stopper is wrapped in cotton 

 wool and tied to the neck ; the bottle is sterilised for three 

 hours at a temperature of 150 C. (350 F.). The sample is best 

 taken directly after the sample for analysis has been obtained ; 

 the plug of cotton wool is removed and the bottle filled with 

 water without being rinsed ; then the stopper is removed quickly 

 from its cotton wool wrapping and inserted in the bottle. The 

 examination must be commenced with as little delay as possible ; 

 and, if the sample has to be forwarded by post or rail, it should 

 be packed in ice. 



The examination usually consists in making a gelatine culti- 

 vation at 22 C. and a search for microbes of intestinal origin. 



Preparation of Nutrient Media Nutrient Gelatine. 120 

 grammes of gelatine (Coignet's "Extra Gold Label) are dissolved 

 in 1 litre of water on the water-bath; 5 grammes of Liebig's 



