WATER FILTRATION. XLVII. 



2. To allow of a complete and constant control of the 

 bacterial efficiency of filtration, the filtrate from each single 

 filter must be examined daily. Any sudden increase in the 

 number of bacteria should cause a suspicion of some un- 

 usual disturbance in the filter,, and should make the super- 

 intendent more attentive to the possible causes of it. 



3. Filters must be so constructed that samples of the 

 effluent from any one of them can be taken at any desired 

 time for bacteriological examination. 



4. In order to secure uniformity of method, the following 

 is recommended as the standard method for bacterial 

 examination : — The nutrient medium consists of 10°f meat 

 extract gelatine with peptone, 10 cubic centimetres of 

 which is used for each experiment. Two samples of the 

 water under examination are to be taken, one of 1 cubic 

 centimetre and one of h cubic centimetre. The gelatine is 

 melted at a temperature of 30° to 35° C, and mixed with 

 the water as thoroughly as possible in the test-tube by 

 tipping backwards and forwards, and is then poured upon a 

 sterile glass plate. The plates are put under a bell- jar 

 which stands upon a piece of blotting paper saturated with 

 water, and in a room in which the temperature is about 

 20° C. The resulting colonies are counted after forty-eight 

 hours with the aid of a lens. If the temperature of the 

 room in which the plates are kept is lower than the above, 

 the development of the colonies is slower, and the counting 

 must be correspondingly postponed. If the number of 

 colonies in 1 cubic centimetre of the water is greater than 

 about 100, the counting must be done with the help of the 

 Wolffhugel's apparatus. 



5. The person entrusted with the carrying out of the 

 bacterial examinations must present a certificate that he 

 possesses the necessary qualifications, and wherever pos- 

 sible, he shall be a regular employee of the waterworks. 



