82 GENERAL MICROBIOLOGY 



Apparatus. Clean cover-glasses; clean concave slides; 

 ordinary slides, clean; tube of sterile nutrient agar or 

 gelatin; paraffin; two sterile Petri dishes: scalpel; plati- 

 num loop. 



Culture. Pure culture of the organism to be studied. 



Method. 1. Liquefy a tube of nutrient agar or gelatin, 

 pour it into a sterile Petri dish to the depth of about 4 mm. 

 and allow it to harden. 



2. With the flame-sterilized scalpel, cut out a block of 

 agar about 8 mm. square. 



3. Raise the agar block on the blade of the scalpel and 

 transfer it, under side down, to the center of a sterile slide. 



4. With a sterile platinum loop, spread a drop of the 

 liquid culture (or suspension of organisms from a solid 

 culture medium) over the upper surface of the agar block 

 as if making a cover-glass film. 



5. Place the slide and block in a sterile Petri dish and 

 incubate for ten minutes at 37 C. to dry slightly. 



6. With sterile forceps, lower a clean, dry, sterile cover- 

 glass carefully on the inoculated surface of the agar (avoid- 

 ing air bubbles), so as to leave a clear margin of cover- 

 glass overlapping the agar block. 



7. Invert the preparation and, with the blade of the 

 scalpel, remove the slide from the agar block. 



8. With the platinum loop, run a drop or two of melted 

 agar around the edges of the block. This solidifies at once 

 and seals the block to the cover-glass. 



9. Sterilize a concave slide. 



10. Invert the cover-glass with the block attached on 

 the concave slide and seal it in place, firmly, with paraffin. 



11. Observe immediately and later from time to time 

 with ocular No. 1 and objective No. 7 or the oil immersion 

 lens. 



