68 APPLIED BACTERIOLOGY 



subsequently make from it. It may so happen that the 

 dilution may in some cases have been carried too far, in 

 which event we shall obtain the plate we require from the 

 second tube ; but success in this operation is a matter' of 

 experience and judgment. When inoculating the tubes, 

 care must be taken to hold the plug of cotton-wool between 

 the fingers, best between the third and fourth, using the 

 back of the hand, and thus twist it out of the tube, which 

 must be again carefully returned into the tube after inocu- 

 lation, without being allowed to come into contact with the 

 surface of the hand or bench. During these operations 

 some glass plates, from 8 to 10 centimetres wide and 10 to 

 12 centimetres long (the glass ' quarter-plates ' used by 

 photographers are a convenient size), are carefully cleaned 

 and sterilised in the hot-air oven at 150° C. for an hour. 

 A box made of sheet iron is very convenient for holding the 

 plates during and after sterilisation. In the absence of a 

 hot-air steriliser, the plates can be sterilised in an oven or 

 over a flame by holding the plates in a pair of tongs. 



In order that the liquid gelatine may be distributed 

 evenly over the plates, the apparatus figured below is 

 used. This consists of a glass plate supported by a tripod. 



Fig. 13. — Plate Culture Apparatus. 



By means of a spirit-level the glass plate is adjusted to an 

 exactly horizontal position. The sterilised glass plate is 

 placed in the glass tray shown in Fig. 14, and the gelatine 

 from one of the prepared tubes quickly poured on to it, 



