BACTERIOLOOICAL EXAMINATION. 291 



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 iiecessaiy 

 to properly interpret the results obtained. 



Bacteriological Examinations. 



A very simple examination is all that is usually necessaix'. 

 A sample of the water for bacteriological cxaminatinn must lie 

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

 plugged with cotton wool, and the stopper is wrapjied in cottcm 

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

 hours at a temperature of 150" C. (.'i5(.) 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 quickly removed 

 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 'li" C. and a search for microbes of intesthial origin. 



Preparation of Nutrient Media — Xuliient Gelatine. — 1-20 

 f^rammes of gelatine (Coiguet's Extra Gold Label) are dissolved 

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

 extract of meat and 10 grammes of peptone are added, and 

 dissolved bv further heating ; the whites and shells of two egi^s, 

 stirred up together to make an intimate mixture are next added, 

 and the heating on the water-bath continued till the liquid is 



