PREPARATION OF NUTRIENT MEDIA. 45 



cottoa-wool stopper^ as in. tliat case the cotton-wool 

 would adhere to the tube, and thus cause difficulties. 

 In order to prevent this, it is best to pour the gelatine 

 first into a bottle which is shaped like an. Erlen- 

 mayer's flask, only a piece of thin glass tubing, about 

 5 cm. long, is fused in at right angles just under its 

 mouth. This tubing is now introduced into the test- 

 tube, which is held in a slanting position, and the 

 flask is tipped up until the gelatine flows into the test- 

 tube. When this is about one-third full, the flask is 

 again tipped up somewhat, so that the gelatine may 

 not run out, and the tubing is drawn out, great care 

 being taken that it should not touch the sides of the 

 tube. However, if the hand is steady, a little practice 

 soon enables the operator to pour the gelatine into the 

 test-tubes from an ordinary Brlenmayer's flask. 



The test-tubes may be sterilised, before they are 

 filled, in the hot-air steriliser (see p. 39) ; but this is 

 not absolutely necessary, and a great deal of time and 

 trouble may be spared by omitting to do so, as they 

 must all be sterilised anyhow after they have been 

 filled. When the gelatine has been poured in, they 

 are closed up with their cotton- wool plugs and put into 

 a wire basket, which is placed in the steam steriliser, 

 in which they undergo the so-called " fractional steri- 

 lisation.^'' 



The steam steriliser is a tall water-bath, which can 

 be closed with a lid. The vessels (test-tubes, etc.) 



