FREPARATION OF NUTRIENT MEDIA. 35 



funnels, which were formerly so much recommended, 

 may be used. It is however much better to place the 

 whole filtering apparatus in the steam steriliser, keep- 

 ing the temperature at between 60° and 80° Centi- 

 grade, in order to prevent the solidification of the 

 gelatine. This can be done by putting the funnel 

 containing the liquid nutrient gelatine into a suffi- 

 ciently large Florence flask, and placing both in the 

 steam steriliser. In order to prevent the steam, which 

 condenses on the walls of the funnel, from becoming 

 mixed with the gelatine, a thick ring of cotton wool is 

 placed in the mouth of the flask round the stem of the 

 funnel. The gelatine does not always take the same 

 time to pass through the filter ; sometimes the whole 

 litre will be filtered in a quarter of an hour, sometimes 

 one must wait for hours. As before, the filtrate is 

 occasionally turbid ; in this case the liquid is cooled to 

 40° C, and the white of an egg is well stirred into it ; 

 the mixture is then boiled up and refiltered. 



In order to prepare nutrient agar-agar, twenty 

 grammes of agar-agar are added instead of the gelatine. 

 Agar-agar is a vegetable substance, procured from 

 marine Algse ; it is generally obtained commercially in 

 the form of long threads, which must be pounded up 

 as finely as possible, in order to render them easily 

 soluble. If the threads are only cut up into little 

 bits, it takes much longer to dissolve them completely. 

 The composition of nutrient agar-agar is as follows : 



