PREPARATION OF NUTRIENT MEDIA. 37 



cylinder, which they keep standing for a long time in a 

 warm chamber at a temperature of 80°, until the im- 

 parities have settled down at the bottom. The clear 

 liquid is then drawn off with a pipette. By this 

 means a nutrient medium quite suitable for many 

 purposes is obtained ; but the beginner is strongly 

 advised not to try this method, as the preparation is 

 never absolutely perfect, and at first he must never be 

 satisfied with less. Delicate little flakes are always 

 present in the solution, and these are sure to find 

 their way into the culture tubes, and to create diffi- 

 culties in various ways. An unpractised student is 

 often unable to distinguish such impurities from the 

 bacteria present in the medium. They may also com- 

 pletely hide from him colonies of bacteria, which have 

 either entered accidentally, or which have remained in 

 the medium in consequence of imperfect sterilisation, 

 and finally they may considerably alter the appearance ' 

 of an artificially prepared culture. 



As such very great care is necessary to prepare 

 nutrient agar-agar, it is best only to use it in such 

 cases, later on to be enumerated, for which nutrient 

 gelatine is not suitable as a nutrient medium. Tlie 

 same is true of blood serum, which can be procured, 

 already sterilised and fit for use, from several different 

 firms. This is advisable, as the preparation of it is 

 so very difficult, that the beginner almost always 

 makes a failure of it. Fluid blood serum is obtained 



