22 



LABORATORY EXERCISES IN BACTERIOLOGY. 



While there is undoubted advantage in thus evenly distributing the heat to the 

 different parts of the inclosed space of such an oven, it is not absolutely necessary, and 

 the same results may be surely obtained by the employment of any available oven. The 

 oven of any stove may be used for the purpose, a thermometer being introduced to es- 

 tablish certainly the presence of the requisite degree of temperature ; and the same end 

 may be attained with even as simple an apparatus as an ordinary tin cracker box (with 

 unsoldered seams) placed upon the top of a stove or other source of heat, or in an old- 

 fashioned Dutch oven. 



For destruction of most bacteria by dry heat a temperature of 140-150 C. is usu- 

 ally regarded as essential, although many adult forms are killed by a heat much less than 

 this. The spores of bacteria are, however, more resistant than the adult forms, and the 



degree indicated may be accepted as essential 

 for the complete destruction of all forms. 

 For the destruction of the adult varieties, 

 exposure of from twenty to thirty minutes to 

 such temperature is practically always lethal ; 

 but a much higher temperature is required 

 for many of the sporulating forms, and occa- 

 sionally even prolonged exposure to 150 C. 

 will fail to destroy some spores. In order to 

 obviate such difficulty, Tyndall, in 1877, sug- 

 gested the so-called " fractional sterilization " 

 (or interrupted sterilization) in connection 

 with this mode of procedure as well as in 

 case of steam sterilization; and at present 

 this is usually followed. Such method de- 

 mands an exposure of the object to be steri- 

 lized in the oven to a temperature of 140- 

 150 C., for from twenty to thirty minutes 

 each day for three days, the intervals between 

 the heatings being afforded so as to permit 

 the development of any existing spores into 

 adult bacteria and consequent loss of their 

 powers of resistance to the heat. 

 FIG. 4. SECTIONAL VIEW OF HOT-AIR 



OVEN. Exercise 7. Select six contami- 



nated tubes from previous exercises. 



Pour out the clouded bouillon, or wash out with ordinary water the con- 

 taminated solid medium into the disinfecting jars distributed about the 

 laboratory, taking care lest the substance be introduced into any cuts or 

 abrasions on the hands, and at once sterilizing the hands by washing for at 

 least five minutes in a one in two thousand solution of bichloride of mer- 

 cury. Into the first of these tubes, without further preparation, introduce a 

 few cubic centimeters of sterile bouillon, and close with sterile cotton plug, 

 as usual. Heat a second tube in the hot-air oven, in which a temperature 

 of 150 C. has been attained, for five minutes ; and in the same manner there 

 are to be added a few cubic centimeters of sterile bouillon, and the tube 



