EXAMINATION OF WATER. 433 



they will live much longer in water having- the ordinary 

 room temperature of about 20°. The interesting studies of 

 Hankin have shown that the water of certain rivers in In- 

 dia will destroy the cholera germ in 3 hours; whereas, if the 

 water is previously boiled it will have no such effect. Simi- 

 lar germicidal substances may be present at tim.es in the 

 water of other localities and thus explain, at least in part, 

 the so-called local immunity. 



\Vater Analysis. 



A bacteriological analysis of water consists: (1) in the 

 determination of the number of bacteria; (2) the identifica- 

 tion of the several species; (3) the recognition of the patho- 

 genic bacteria present. 



Number of bacteria. — The water to be examined should 

 be received in a sterilized bottle or flask and thoroughly 

 protected against subsequent contamination. Furthermore, 

 in view of the rapid multiplication of bacteria, a given 

 sample of water should be examined as soon as possible 

 after collection. The method commonly employed in the de- 

 termination of the number of bacteria present is as follows : 



Several 1 c.c. pipettes, graduated in i^ c.c, are placed 

 in a pipette box and sterilized in the dry-heat oven in the 

 usual way. 3 gelatin tubes are then liquefied and marked. 

 By means of a sterilized, cooled pipette 1 c.c. of the water 

 is transferred into tube No. 1. In like manner i c.c. and 1 

 drop are placed into tubes 2 and 3, respectively. The con- 

 tents of the tubes are gently agitated to secure complete 

 mixture. The gelatin is then poured on sterilized glass 

 plates or into Petri dishes, observing the usual precautions 

 in making plate cultures (p; 175). The gelatin plates thus 

 obtained are set aside for two or three days at 18-20° and 

 and the colonies which develop are then counted. 



28 



