THE PREPAKATION OF CULTURE MEDIA 31 



gelatine is now carefully withdrawn by means of sterile 

 forceps from the box or oven (the door of which is im- 

 mediately reclosed), and the future upper surface of 

 the plate is held downwards during its transfer to the 

 levelled plate, and only turned up when the bell jar is 

 momentarily raised to admit it. In this way the falling 

 of air-organisms on the culture plate is avoided. 



All is now readj?" for the gelatine-tubes, which should 

 have been previously melted in a beaker of hot water 

 and then cooled down to 30° C, at which temperature 

 the gelatine remains liquid. 



The cotton-wool stopper is first singed in a bunsen- 

 flame to get rid of any chance organisms which may 

 have fallen upon it, and is then removed very care- 

 fully, not pulled straight out, but by gently twisting ; 

 the mouth of the tube is then passed quickly through 

 the flame to destroy any organisms which may be 

 present, and the contents are poured on to the sterilised 

 glass plate, the bell jar being again lifted for a moment 

 and held over the plate during the operation. After it 

 has congealed, the gelatine-plate is quickly removed to 

 a damp chamber, where it is placed on a glass bench 

 upon which another glass bench can be placed with its 

 gelatine-plate until the chamber is filled. 



The damp chamber consists of an ordinary dinner 

 plate covered with a common glass bell jar which fits 

 into the depression of the plate. The air in this cham- 

 ber is rendered moist by just covering the bottom of 

 the plate with a little sterilised distilled water. 



The damp chamber with its contents is allowed to 

 remain for an hour or two in a cold room and is then 

 placed in a cupboard maintained at a uniform tem- 

 perature of from 18-22° C. The plates prepared as 

 above would remain sterile excepting in so far as they 

 might be accidentally contaminated by aerial microbes 



