STERILISATION BY STEAM 29 



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 satur- 

 ated 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 perforated diaphragm 

 placed in the upper part of the steam chamber. 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 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 twenty minutes on each of three 

 succeeding days. 



This is a modification of what is known as " Tyndall'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 sterilisation 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 condition. 



Steam at 100 C. is therefore available for the sterilisation of 

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

 when a large bulk is to be sterilised, it is best to put the 

 medium in while the apparatus is cold, in order to make 

 certain that the whole of the food mass reaches the tempera- 

 ture of 100 C. The period of exposure is reckoned from the 

 time boiling commences in the water in the steriliser. At any 

 rate allowance must always be made for the time required 

 to raise the temperature of the medium to that of the steam 

 surrounding it. 



B (3). Sterilisation by Steam at High Pressure. This is 

 the most rapid and effective means of sterilisation. It is effected 

 in an autoclave (Fig. 4). This is a gun-metal cylinder supported 

 in a cylindrical sheet-iron case ; its top is fastened down with 

 screws and nuts, and is furnished with a safety valve, pressure- 

 gauge, and a thermometer. As in Koch's steriliser, the contents 



