234 APPENDIX. 



whole is sterilised by exposure to 150 C. for an hour in 

 the hot-air steriliser. The nutrient gelatine in a stock- 

 tube is liquefied, and the contents emptied into the glass 

 capsule. The jar is exposed to the air to be examined 

 for a definite time, the cotton wool plug replaced, and 

 the apparatus set aside for the colonies to develop. 



Hesse's Apparatus (Fig. 41). The advantage of 



FIG. 41. HESSE'S APPARATUS. 



this apparatus consists in that a known volume of air can 

 be examined. A glass cylinder, 70 cm. long and 3*5 cm. 

 in diameter, is closed at one end by an india-rubber cap, 

 perforated in the centre. Over this fits another cap, which 

 is not perforated. The opposite end of the cylinder is 

 closed with a caoutchouc stopper, perforated to admit 

 a glass tube plugged with cotton wool. The tube can be 

 connected by means of india-rubber tubing with an 



