494 BAGTEBIOLOOY. 



growing or dying. Represent the results graphically, 

 and it will be noticed that in many cases there is at first, 

 during the first three or four days, a multiplication, 

 after which there is a rapid diminution; and, if the 

 organism does not form spores, usually complete death 

 in from ten to twelve days. This is not true for all 

 organisms, but does hold for many. 



Where it is not convenient, however, to make the 

 analysis on the spot, the sample of water should be col- 

 lected 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, a coo- 

 venieut vessel to be employed is a glass bulb (Fig. 98) 

 or balloon, which one soon learns to make for one's 

 self from glass tubing. 



Fig. 98. 



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 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 water is pressed up into the bulb, because of the 

 existence of the negative pressure within. The nega- 

 tive pressure obtained in this way is frequently in- 

 sufficient to permit of the bulb being completely filled, 



