

38 METHODS OF CULTIVATION OF BACTERIA. 



water to the depth of 3 inches be placed in the interior and 

 heat applied, it will quickly boil, and the steam streaming 

 up will surround any flask or other object 

 standing on the diaphragm. It is con- 

 venient to have the cylinder tall enough 

 to hold, standing in its neck, a litre flask 

 with a funnel 7 inches in diameter. With 

 such a " Koch " in the laboratory a hot- 

 water filter is not needed. As has been 

 said, one and a half hour's steaming will 

 practically sterilise any medium, but as 

 some of our most important media contain 

 gelatine, such an exposure is not practic- 

 able, as with long boiling, gelatine tends 

 to lose its physical property of solidifica- 

 tion. The method adopted in this case is 

 to steam for a quarter of an hour on each 

 of three succeeding days. This is a 

 modification of what is known as " Tyn- 

 dall's intermittent sterilisation." The 

 fundamental principle of this method is 

 that all bacteria in a non-spored form are 

 killed by the temperature of boiling water, while if in a spored 

 form they may not be thus killed. Thus by the sterilisa- 

 tion on the first day all the non-spored forms are destroyed 

 the spores remaining alive. During the twenty-four hours 

 which intervene before the next heating, these spores, being 

 in a favourable medium, are likely to assume the non-spored 

 form. The next heating kills these. In case any may 

 still not have changed their spored form, the process is 

 repeated on a third day. Experience shows that usually 

 the medium can now be kept indefinitely in a sterile condi- 

 tion. Steam at 100 C. is therefore available for the sterili- 

 sation of all ordinary media. In using the Koch's steriliser, 

 especially when a large bulk of medium is to be sterilised, 

 it is best to put the media in while the apparatus is cold, in 

 order to make certain that the whole of the food mass 

 reaches the temperature of 100 C. The period of exposure 



