l6o NUTRIENT MEDIA 



but with a sharp clean-cut (made with a cataract or 

 tenotomy knife) running completely through its axis 

 from the centre of the disc to the top of the dome. 

 During sterilisation the air in the neck of the bottle, 

 expanded by the heat, is driven out through the valvu- 

 lar aperture in the solid portion of the stopper. On 

 removing the bottle from the steam chamber, the 

 liquid contracts as it cools, and the pressure of the 

 external air drives the solid piece of rubber down into 

 the neck of the bottle, and forces together the lips of the 

 slit (Fig. 102,6). Thus sealed, the bottle will preserve its 

 contents sterile for an indefinite period without loss 

 from evaporation. 



TUBING NUTRIENT MEDIA. 



After the final filtration, the nutrient medium is 

 usually " tubed" i. e., filled into sterile tubes in defi- 

 nite measured quantities, usually 10 c.c. This process 

 is sometimes carried out by means of a large separator 

 funnel fitted with a "three-way" tap which communi- 

 cates with a small graduated tube (capacity 20 c.c. 

 and graduated in cubic centimetres) attached to the side. 

 The shape of this piece of apparatus, known asTreskow's 

 funnel, renders it particularly liable to damage. It is 

 better, therefore, to arrange a less expensive piece of 

 apparatus which will serve the purpose equally well 

 (Fig. 103). 



A Geissler's three-way stop-cock has the tube on one 

 side of the tap ground obliquely at its extremity, and 

 the tube on the opposite side cut off within 3 cm. of the 

 tap. The short tube is connected by means of a per- 

 forated rubber cork with a 10 cm. length of stout glass 

 tubing (1.5 cm. bore). The third channel of the three- 

 way tap is connected, by means of rubber tubing, with 

 the nozzle of an ordinary separator funnel. Finally, 

 the receiving cylinder above the three-way tap is gradu- 



