PREPARATION OF NUTRIENT MEDIA. 67 



bursting the filter at the point, the broth when 

 poured out from the flask should be directed 

 against the side of the filter with a glass rod. 

 During filtration the funnel should be covered over 

 with a circular plate of glass, and the process of 

 filtration must be repeated, if necessary, until a 

 pale, straw-coloured, perfectly transparent filtrate 

 results. 



The sterilised test-tubes are filled for about 

 a third of their depth by pouring in the gelatine 



Fig. 26. METHOD OF MAKING A FOLDED FILTER. 



carefully and steadily, or by employing a small 

 sterilised glass funnel. The object of this care 

 is to prevent the mixture touching the part of 

 the tube with which the plug comes into contact ; 

 otherwise, when the gelatine sets, the cotton wool 

 adheres to the tube, and becomes a source of 

 embarrassment in subsequent procedures. As the 

 tubes are filled they are placed in the test-tube 

 basket, and must then be sterilised. They are 

 either lowered into the steam steriliser, when the 



