430 BACTERIOLOGY. 



U, with either extremity of the arms beat again at 

 right angles in the same place 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 into the water and 

 sucking 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 contaminating its contents with saliva during the 

 act of filling 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 

 microscopic 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 de- 

 termine the quantity of water to be employed for the 

 plates or tubes. This is usually done by making pre- 

 liminary plates from one drop, .two drops, 0.26 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 colo- 

 nies are only moderate in number — about 200 to 300 

 colonies presenting — and employs in the subsequent 



