ANALYSIS OF WATER. 179 



made after the water has been in a vessel for a day or 

 two are often very different from those which would 

 have been obtained on the spot. 



Where it is not possible, however, to make the analysis 

 on the spot, the sample of water should be collected and 

 packed in ice and kept on ice until ready for use, which 

 should in all cases be as soon after its collection as 

 possible. 



For the collection of water for this purpose, the best 

 vessel to be employed is a glass bulb (Fig. 26) or 

 balloon, which one soon learns to make for himself 

 from glass tubing. 



It consists simply of a round glass sphere blown on 

 the end of a glass tube, which latter is subsequently 



Fio. 26. 



drawn out into a fine capillary stem and sealed while 

 hot. As it cools, the contraction of the air within the 

 bulb results in the production of a negative pressure. If 

 the point of the stem be broken off under water, the 

 bulb is quickly filled because of the existence of the 

 negative pressure within it. 



A number of them may be blown, sealed, and kept 

 on hand. They are sterile so long as they are sealed, 

 because of the heat that is employed in their manu- 

 facture. 



When a sample of water is to be taken, the point of 

 a bulb is simply broken off with sterilized forceps under 

 water at the place from which the sample is to be taken. 

 It rapidly fills with water. This may serve as a sample. 



