20 



LABORATORY EXERCISES IN BACTERIOLOGY. 



moments during which it is being allowed to cool for use. Thus it is well to hold the 

 platinum needle perpendicularly, point downward, so as to expose the least surface to 

 bacteria settling by gravity through the surrounding air (Figs. 2 and 3), and for the 

 same reason to hold a knife under these circumstances in a 

 similar position or horizontally with the cutting edge downward. 



Exercise 6. Two tubes of sterile nutrient bouillon or 

 other medium are provided. A platinum inoculating 

 needle is purposely contaminated by being thrust into 

 a tube of decomposing meat solution or other similar 

 substance. Without further precaution it is thrust into 

 one of the prepared tubes of sterile medium, moved 

 gently in it for a moment and withdrawn, and the tube 

 resealed with the usual cotton plug. The needle is now 

 heated throughout its entire length to a cherry heat and 

 the lower portion of the glass handle also cautiously 

 heated ; after which the needle is thrust into the second 

 tube of sterile medium, moved about therein for a 

 moment and withdrawn, and the tube closed. Note 

 results at the close of twenty-four, forty-eight, and 

 seventy-two hours so as to determine the efficiency of 

 the flame to destroy the known bacterial contamination 

 upon the needle. 



2. Exposure to Dry Oven Heat. The dry-air oven is em- 

 ployed in the sterilization of such articles as are not liable to be destroyed by drying, 

 but which do not demand rapidity of cleansing, or are, because of their size or for other 

 reason, not easily cleansed by exposure to the direct flame. It is usually used in 

 sterilizing culture tubes and their cotton plugs, 

 for the sterilization of Petri dishes, flasks, glass 

 plates, and similar objects. The articles to be 

 sterilized are placed in the oven, either inclosed 

 in some suitable receptacle, as in wire cages, 

 metal boxes, or folded in wrapping paper (as 

 may be practised with Petri dishes, filtration 

 bougies, glass rods, pipettes, etc.), or loose; 

 and heat is applied in the most convenient 

 manner. The type of oven commonly used in 

 the laboratory is a double-walled sheet-iron 

 box, covered with asbestos card to prevent 

 heat loss, and provided with a suitable door. 

 A large opening at the bottom of the outer wall 



permits the hot air from the flame to enter and circulate between the outer and inner 

 walls, and a series of smaller openings at the top in the outer wall, guarded by a slide 

 valve, allows the escape of the hot air and gas after circulation. In this way the heat 

 is evenly distributed to the entire inclosure. A section of such hot -air oven is repre- 

 sented in figure 4. 



FIG. 2. POSITION 

 IN WHICH INOCU- 

 LATING NEEDLE 

 is HELD WHILE 

 COOLING. 



FIG. 3. PROPER POSITION OF HOLD- 

 ING KNIFE AFTER FLAMING. 



