140 A MANUAL OF BACTERIOLOGY 



prevent shrinking of the medium. The colonies preserve their orig- 

 inal form and appearance. The dishes can be cleaned in the ordinary 

 way when their usefulness is over. 



Gelatin plates can be preserved by this method if they are first 

 subjected to the vapor of formaldehyde to harden the gelatin and to 

 destroy the enzymes which cause liquefication. 



Another method for preserving Petri-dish cultures is by use of a 

 sealing mixture. When the agar has become somewhat dried, invert 

 the dish and pour melted paraffin into the crevice between the dish 

 and the margin of the lid. 



