BACTERIOLOGICAL STUDY OF WATER. 693 



can be made from it, which should in all cases be as 

 soon after its collection as possible. 



For the collection of water for this purpose a con- 

 venient vessel is a glass bulb (Fig. 102) or balloon, 

 which is easily blown from glass tubing. 



Fig. 102. 



Glass bulb for collecting samples of water. 



It consists simply of a round glass sphere blown on 

 the end of a glass tube, which latter is subsequently 

 drawn out into a capillary stem and sealed while hot. 

 As it cools, the contraction of the air within the bulb 

 results in the establishment of a negative pressure. 



If the point of the stem be broken off under 

 water, the water flows into the bulb, because of the 

 existence of the negative pressure within. The nega- 

 tive pressure obtained in this way is frequently in- 

 sufficient for filling the bulb, and often only a few 

 drops of fluid can be obtained. To obviate this, 

 bulbs may be blown and allowed to cool, but not 

 sealed. After a sufficient number of them are pre- 

 pared they are taken, one at a time, and gently 

 warmed over a flame ; while still warm the extremity 

 of the stem is dipped into distilled water and held 

 there until a few drops have passed up into the bulb ; 

 this is then carefully boiled, or, rather, completely vapor- 

 ized, over the flame, and while steam is still escaping 

 the point is sealed in the gas-flame. All air will 

 thus have been replaced by water-vapor, and if the 

 point of the stem be now broken off under water 

 38 



