STERILIZATION OF CULTURE MEDIA. 51 



The spores of bacilli have a much greater resisting power, and 

 the vitality of some of these reproductive bodies, from known spe- 

 cies, is not destroyed by a boiling temperature maintained for sev- 

 eral hours. Thus Globig found that the spores of a certain bacillus 

 from the soil his " red potato bacillus " required six hours' exposure 

 to streaming steam in order to destroy it. Steam under pressure, at 

 a temperature of 115 C., killed it in half an hour ; at 125 C. in five 

 minutes. This extreme resisting power is exceptional, however, 

 and many spores are destroyed in a few minutes by the boiling tem- 

 perature of water. 



In practice we assume that some of the more resistant spores, 

 which are frequently present in the atmosphere, may have fallen 

 into our culture material, and to insure its sterilization we subject it 

 to a temperature which can be depended upon to destroy these ; or 

 we resort to the method of discontinuous heating. This method 

 was first employed by Tyndall (1877), and is now in general use in 

 the bacteriological laboratories of Germany, having been adopted by 

 Koch and his pupils ; while in France a single sterilization by means 

 of steam under pressure, securing a higher temperature, is still the 

 favorite method with many. 



In the method by discontinuous heating we subject the culture 

 material for a short time to the temperature of boiling water, thus 

 destroying all bacteria in the vegetative stage. After an interval, 

 usually of twenty-four hours, we repeat the operation for the pur- 

 pose of destroying those which in the meantime have developed 

 from spores which may have been present. Again the material is 

 put aside, and after twenty-four hours it is again heated to the 

 boiling point. This is usually repeated from three to five times. 

 The object in view is to kill the growing bacteria which are de- 

 veloped from spores which were present ; and, as a matter of expe- 

 rience, we find that this method of sterilization is more reliable than 

 a single prolonged boiling, unless this be effected at a higher tem- 

 perature than that of boiling water at the ordinary pressure of the 

 atmosphere. Discontinuous heating is especially useful for the sterili- 

 zation of liquids which would be injured by prolonged boiling as is 

 the case with solutions of gelatin or which are coagulated by the 

 boiling temperature. By means of a water bath, the temperature 

 of which is regulated automatically, we may conduct the operation 

 at any desired degree. Thus in sterilizing blood serum we use a 

 temperature a little below that at which coagulation occurs (about 

 70 C.). 



Test tubes, flasks, and apparatus of various kinds are commonly 

 sterilized by dry heat in a hot-air oven. This is usually made of 

 sheet iron, with double walls, and shelves for supporting the articles 



