GELATINE PLATE CULTURES 71 



the platinum wire against the side of the tube below the 

 surface of the gelatine. The wire is again sterilised by pass- 

 ing it through the flame, and when cool it is again intro- 

 duced into tube 1, and a loopful of the gelatine transferred 

 to tube 2, and the contents well mixed ; after which a loop- 

 ful from tube 2 is transferred to tube 3. The reason for 

 using three tubes will now be apparent. It is usually 

 impossible to introduce a few organisms into the first tube, 

 so we effect our object by dilution; by the above proce- 

 dure we commonly succeed in so reducing the number of 

 organisms that only a few will develop upon the plate we 

 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 in 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 on p. 72 is 

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

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



