INTERMITTENT STERILISATION. 105 



minutes' exposure to 120 than by three hours' exposure in the 

 Koch apparatus. 



On the other hand, there are liquids which are so readily 

 decomposed that neither of the above methods of treatment can 

 be thought of. An example of these is afforded by the medium 

 so frequently employed in mycological laboratories under the name 

 of nutrient gelatin ; a solution of bouillon, wort, &c., containing 

 8-1 o per cent, of gelatin. This mixture, which sets at the ordinary 

 temperature of a room to the consistency of soft glue, and liquefies 

 at about 25 C., would lose its property of setting if exposed to 

 such degrees of heat, and would thereby become useless. In 

 such cases another method of killing the germs must be employed, 

 namely, that first proposed by Tyndall, and known as 



78. Intermittent Sterilisation. 



The powerful methods hitherto described have been considered 

 necessary, for the sole reason that the sample to be sterilised had 

 to be regarded as presumably containing highly resistant bacterial 

 spores. In the absence of such forms, the object in view is attain- 

 able by much milder means, and the liquids, &c., to be sterilised 

 can be converted into this more favourable condition by causing 

 the spores (possibly present therein) to germinate. It then be- 

 comes a much easier task to deal with the resulting vegetative 

 forms, since these latter perish at temperatures below 100 C., and 

 therefore so much the more certainly in a current of steam. For 

 this reason then the sample to be sterilised which, as before, is 

 supposed to contain the most highly resistant types of bacterial 

 spores, in addition to the comparatively feeble vegetative forms 

 is exposed at first to a temperature of 100 C. in the Koch steril- 

 iser for a short time. The duration of this first treatment de- 

 pends on the volume of liquid in the individual samples. For 

 flasks containing a charge of 10-15 c - c - each, fifteen minutes will 

 suffice ; larger quantities warm through more slowly, and must be 

 left in the steamer for a correspondingly longer time. In every 

 case the liquid should remain at a temperature of 100 C. for 

 about fifteen minutes. By this treatment only the vegetative forms 

 and weaker spores are killed, and the next step is to ensure that 

 the still living spores germinate, which is generally effected by 

 simply leaving the samples to stand at room temperature. At the 

 end of twenty-four hours the first treatment in the steamer is 

 repeated, whereby the vegetative forms that have in the meantime 

 developed from the spores are killed. It being, however, possible 

 that, owing to the known irregularity of germination, some of the 

 spores have not developed, the samples are again left at rest for a 

 day and thereafter steamed a third time to kill the residual cells 

 proceeding from these tardy spores. The medium, liquid, &c., 

 will in this manner be entirely freed from living germs without 



