BACTERIOLOGICAL EXAMINATION OF WATER. 155 



tion is to be done with, great care. Unless very great 

 exactitude is required, glass flasks containing about 

 106 grms. and provided with ground glass stoppers, 

 may be selected for the reception of the water. The 

 stoppers must fit so well, that it is impossible for 

 water to trickle out of the flasks, even if they are 

 kept upside down for hours when full. If it is neces- 

 sary to sterilise these vessels quickly, it may be done 

 by rinsing them and their stoppers with corrosive 

 sublimate solution, and then keeping them closed 

 with a sterilised india-rubber cap drawn closely over 

 the stopper. If there is no such hurry, the closed 

 flasks may be kept in the hot-air steriliser for some 

 hours at a temperature of about 160°. In this case, 

 the stoppers must not be too firmly inserted in the 

 flasks, as otherwise these may crack. It is very 

 important to make sure, before making use of them, 

 that the flasks which have been sterilised in this 

 manner are quite sound. After they have cooled, a 

 sterilised india-rubber cap should be drawn over them 

 as before. 



However, although this method is so simple, and 

 quite answers the purpose in most cases, there are 

 some drawbacks to it, which may make an accurate 

 examination impossible. Although the stopper may 

 appear to fit well, yet there may be a small chink or 

 crevice between it and the flask, sufficient to establish 

 communication between the water and the outside 



