STERILISATION BY STEAM 29 



by which water may be supplied or withdrawn.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 sur- 

 rounded during sterilisation by an atmosphere saturated with 

 \\atrr 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 ;i modification of what is known as "Tyndall's intermittent 

 >t< rilisation." The fundamental principle of this method is that all 

 ba'-tt'ria in a non-spored form are killed by the temperature of boiling 

 \\ater. while if in a spored form they may not be thus killed. Thus by 

 the sterilisation mi 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 

 wlu-n 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 temperature of 100 C. The 

 I'fiiod of exposure is reckoned from the time boiling commences 

 in the water in the steriliser. At any rate allowance must 

 always In- made for the time required to raise the temperature 

 of the medium to that of the steam surrounding it. 



B. (:\) 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- 

 ^aiige, and a hole for thermometer. As in the Koch's steriliser, 

 the contents are supported on a perforated diaphragm. The 



