554 BACTERIA IN WATER. 



Later, on account of the exhaustion of organic pabulum, the num- 

 ber is again reduced as the bacteria present gradually lose their 

 vitality. Under these circumstances it is evident that an estimate of 

 the number of bacteria present in water from a given source can 

 have no value, unless a sample is 'tested by bacteriological methods 

 within a short time after it has been collected. Not more than an 

 hour or two should be allowed to elapse, especially in warm weather. 

 By placing the water upon ice the time may be extended somewhat, 

 but Wolffhugel has shown that the number of germs is gradually 

 diminished when water is preserved in this way, and it will be safest 

 to make an immediate examination when this is practicable. 



The collection may be made in a sterilized Erlenmeyer flask pro- 

 vided with a cotton air filter, or in a bottle having a ground-glass 

 stopper which has been wrapped in tissue paper and sterilized for an 

 hour or more at 150 C. in the hot-air oven. Or the small flasks with 

 a long neck may be used, as first recommended by Pasteur. These 

 are prepared as follows : The bulb is first gently heated, and the ex- 

 tremity of the tube dipped into distilled water, which mounts into 



FIG. 194. 



the bulb as it cools ; the water is then made to boil, and when all 

 but a drop or two has escaped and the bulb is filled with steam the 

 extremity of the tube is hermetically sealed. When the steam has 

 condensed by the cooling of the bulb a partial vacuum is formed, 

 and the tube is ready for use at any time. It is filled with water by 

 breaking off the sealed extremity under the surface of the water of 

 which a sample is desired. This is done with sterilized forceps, and 

 care must be taken that the exterior of the tube is properly sterilized 

 before the collection is made. The end is immediately sealed in the 

 flame of a lamp. A difficulty with these vacuum tubes is that they 

 are so completely filled with water that this cannot be readily drawn 

 from them again in small quantities. The writer therefore prefers 

 to make the collection in a tube shaped as shown in Fig. 194, in which 

 a partial vacuum is formed just before the collection by heating the 

 air in the bulb. The water mounts into the tube as the air in the 

 bulb cools, and is readily forced out again for making cultures by 

 applying gentle heat to the bulb. As a lamp is needed to seal the end 

 of the tube in either case, there is 110 special advantage in having a 

 vacuum formed in advance, and, as stated, the vacuum tubes are So 



