PREPARA TION OF NUTRIENT MEDIA. 47 



ia wHch it is left for exactly five minutes. This 

 operation is repeated for six consecutive days, after 

 ■which it is certain that the test-tubes are sterilised. 

 The gelatine may also be sterilised by boiling it for 

 about an hour and a half on end in the test-tubes; but 

 by this means it is deprived of its most valuable pro- 

 perty, its power of solidification — thus becoming use- 

 less as a solid transparent medium. Boiling for five 

 minutes, even when it takes place on several consecu- 

 tive days, does not essentially aS'ect its power of 

 solidification, whilst it completely sterilises it. On 

 the first day all the bacteria which are in a vegetative 

 condition are killed in a body. Only the so-called 

 "resting' spores" are uninjured; these, on the con- 

 trary, appear to be stimulated to germination, especi- 

 ally as they are richly provided with nourishment. 

 Germination takes place during the following days, 

 chiefly during the first and second, but in a somewhat 

 desultory manner during the later ones. But when 

 the bacteria have once forsaken the resistant spore 

 membrane, and have gone over into the vegetative 

 condition, they succumb to this application of heat for 

 five minutes ; and hence the gelatine, by this method 

 of fractional sterilisation, is completely freed from all 

 living organisms. 



If the gelatine is not required for immediate use, it 

 ia well to keep it in Florence flasks, containing about 

 i to 1 litre. These must be stopped up like the test- 



