54 BACTERIOLOGY. 



time. It should be remembered that while spores which 

 may be present are not directly killed by the exposure 

 to heat that they experience in the intermittent method 

 of sterilization, still their power of germination is some- 

 what inhibited by this treatment. In this method, 

 therefore, if the temperature of 100 C. be employed for 

 too long a time, it is possible to so retard the germina- 

 tion of the spores as to render it impossible for them to 

 develop into the vegetating stage during the interval 

 between the heatings. By excessively long exposures 

 to high temperature, but not long enough to destroy the 

 spores directly, the object aimed at in the method may 

 be defeated, and in the end the substance undergoing 

 sterilization be found to still contain living bacteria. In 

 this process the plan that has given most satisfactory 

 results is to subject the materials to be sterilized to the 

 action of steam, under the ordinary conditions of atmos- 

 pheric pressure, for fifteen minutes on each of three 

 successive days, and during the intervals to retain them 

 at a temperature of about 25-30 C. At the end of 

 this time all living organisms which were present will 

 have been destroyed, and, unless opportunity is given 

 for the access of new organisms from without, the sub- 

 stances thus treated remain sterile. 



As an exception to this, one occasionally encounters 

 certain species of spore-forming bacteria that are not 

 readily destroyed by this mode of treatment. They are, 

 presumably, of the group of so-called "soil organisms/ 7 

 and represent the forms most resistant to the influence 

 of heat. We are not as yet sufficiently familiar with 

 all their peculiarities to warrant our speaking with cer- 

 tainty as to a means of sterilizing media in which they 

 are present. It does not seem unlikely that they are of 



