24 LABORATORY EXERCISES IN BACTERIOLOGY. 



again closed with its cotton plug. A third tube is heated similarly for ten 

 minutes, bouillon added, and the tube closed. A fourth tube is heated for 

 twenty minutes, bouillon added, and the tube closed. The fifth tube is 

 heated for thirty minutes and similarly treated ; and the last tube is heated 

 for twenty minutes each day for three days, to the same temperature, after 

 which sterile bouillon is added and the tube closed and placed in the incu- 

 bator, which should also have been done with the former tubes. The re- 

 sults of such heating in each case should be noted and compared with the 

 results obtained in the unclean and unheated tube, at the close of twenty - 

 four, forty-eight, and seventy-two hours. 



Exercise 8. A tube containing some marked growth is selected. In- 

 oculation is performed from this material to a fresh tube, the latter closed 

 and placed in the incubator. The original tube is then subjected to dry 

 heat for five minutes, ten minutes, fifteen minutes, twenty minutes, twenty- 

 five minutes, and thirty minutes. At the close of each interval a fresh tube 

 of sterile medium is inoculated from the original tube, and after being 

 closed with its sterile cotton plug, is placed in the incubator, and results are 

 noted as usual, at the close of each twenty-four hours, for three days. That 

 tube in which growth first fails to occur probably corresponds with the lethal 

 exposure to the temperature to which the tube was heated (vide Thermal 

 Death-point) . 



3. Boiling. The temperature of boiling water (100 C.) is easily destructive of 

 bacterial life if long enough continued or if the conditions for penetration of the heat are 

 favorable. This ease of penetration is present best when the bacteria to be destroyed 

 are suspended in the fluid (rather than when inclosed in fabric or other solid) which 

 is being boiled, the hot liquid coming into close contact with the organisms in every 

 part and on every side. As a rule it may be said that sterilization by boiling is quite as 

 efficient as by exposure to a temperature of 150 C. of dry heat. Boiling is particularly 

 useful in the sterilization of articles likely to be destroyed by dry heat, as clothing, non- 

 coagulable nutrient fluids, glassware which may be cracked by any irregularities in the 

 application of the oven heat, etc. 



Exercise 9. Boil water for five, ten, fifteen, and twenty minutes, and at 

 the close of each interval add several drops to a tube of sterile medium; 

 close, place in incubator, and at intervals of twenty -four hours for three 

 days note the results obtained. Compare with a tube inoculated with 

 water not previously boiled. 



4. Steam Sterilization. Steam heat is commonly used for the sterilization of such 

 substances as are likely to be injured by heating in the dry-air oven, and which cannot 

 conveniently be boiled or sterilized by chemical or mechanical means. It is most fre- 

 quently used in connection with the various culture media, and in practical medicine and 

 surgery in the sterilization of dressings for wounds, of clothing and similar substances. The 

 temperature of steam unconfmed by the lid of the steam chamber is approximately 85 



