BACTERIOLOGICAL STUDY OF WATER. 451 



consists of a piece of glass tubing of about 5 or 6 mm. 

 inside diameter and 36 cm. long, bent in the form of a 

 U, with either extremity of the arms bent again at right 

 angles in the same plane and drawn out to a point and 

 sealed. They are sterilized in the flame as they are 

 made. The sample is collected by breaking off both 

 points, immersing one of them in the water and suck- 

 ing on the other until the tube is filled. Then both 

 points are again sealed in the flame and the tube packed 

 in ice. The objection to this tube is the danger of con- 

 taminating its contents with saliva during the act of fill- 

 ing by suction, though this danger is not so great as 

 might at first appear, as we shall learn in our efforts to 

 cultivate bacteria from the mouth cavity. 



NOTE. Make cover-slips from your own mouth ; 

 make plates on both gelatin and agar-agar, at the same 

 time. Compare the number of bacteria found by micro- 

 scopic examination of the cover-slips with the number 

 of colonies that develop on the plates. 



In beginning the quantitative analysis of water with 

 which one is not acquainted, there are certain prelimi- 

 nary steps that are essential. 



It is necessary to know approximately the number of 

 organisms contained in any fixed volume, so as to deter- 

 mine the quantity of water to be employed for the plates 

 or tubes. This is usually done by making preliminary 

 plates from one drop, two drops, 0.25 c.c., 0.5 c.c., and 

 1 c.c. of the water. After each plate has been labelled 

 with the amount of water used in making it, it is placed 

 aside for development. When this has occurred, one 

 selects the plate upon which the colonies are only mod- 



