BACTERIOLOGICAL STUDY OF WATER. 449 



mine by counts whether there is an increase or diminu- 

 tion in the number of organisms i. e., if they are grow- 

 ing 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 collec- 

 tion as possible. 



For the collection of water for this purpose, a con- 

 venient vessel to be employed is a glass bulb (Fig. 91) 

 or balloon, which one soon learns to make for oneself 

 from glass tubing. 



FIG. 91. 



g 

 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 exist- 

 ence of the negative pressure within. The negative 



