EXAMINATION OF WATEE 391 



extract gelatine with peptone, 10 c.c. of which is used for 

 each experiment. 



' Two samples of the water under examination are to be 

 taken, [one of 1 c.c. and one of J c.c. 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, and 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 c.c. of the water is 

 greater than about 100, the counting must be done with 

 the help of Wolifhiigel's apparatus.' 



In those cases where there are no previous records 

 showing the possibilities of the works and the influence of 

 the local conditions, especially the character of the raw 

 water, and until such information is obtained, it is to be 

 taken as the rule that a satisfactory filtration shall never 

 yield an effluent with more than about 100 bacteria per 

 cubic centimetre. 



The filtrate must be as clear as possible, and the colour, 

 taste, temperature, and chemical composition, must be in 

 every way satisfactory. 



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 



