30 



METHODS OF CULTIVATION OF BACTERIA. 



Fig. 3, — Koch's 

 steam steriliser. 



narily used is "Koch's steam steriliser" (Fig. 3). This consists 

 of a tall metal cylinder on legs, provided with a lid, and covered 

 externally by some bad conductor of heat, 

 such as felt or asbestos. A perforated tin 

 diaphragm is fitted in the interior at a little 

 distance above the bottom, and there is a tap 

 at the bottom by which water may be sup- 

 plied or withdrawn. If 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. Here no 

 evaporation takes place from any medium as 

 it is surrounded during sterilisation by an 

 atmosphere saturated with water vapour. It 

 is convenient to have the cylinder tall enough 

 to hold a litre flask with a funnel 7 inches in 

 diameter standing in its neck. The funnel 

 may be supported by passing its tube through a second per- 

 forated diaphragm placed in the upper part of the steam 

 chamber. With such a " Koch " in the laboratory a hot-water 

 filter is not needed. An even more serviceable steriliser is that 

 known as the Arnold steam steri- 

 liser, which, by its peculiar con- 

 struction, effects a greater saving 

 in the time necessary to develop 

 steam than does a similar sized 

 Koch apparatus. As has been 

 said, one and a half hours' steam- 

 ing will sterilise any medium, but 

 in the case of media containing 

 gelatin such an exposure is not 

 practicable, as, with long boiling, 

 gelatin tends to lose its physical 

 property of solidification. The 

 method adopted in this case is to 

 steam for a quarter of an Jioiir 

 on each of three succeeding days. 

 This is a modification of what is known as " Tyndall's intermit- 

 tent sterilisation." The fundamental principle of this method is 



Fig. 4. — The Arnold steam-steriliser. 



