52 GENERAL MICROBIOLOGY 



10. Transfer two loopfuls from tube No. 2 to tube No. 

 3 and mix. 



11. Plate tube No. 2 in Petri dish No. 2 (see 9). 



12. Plate tube No. 3. 



13. Label the plates with name of culture, number of 

 dilution and date, and with your own name or desk number. 



14. When the agar has solidified firmly, invert the 

 plates and place in the incubator at 37 C., or at room 

 temperature. 



Note. If the plates are placed right side up, condensation water 

 forms on the cover and drops down upon the surface of the agar, caus- 

 ing the colonies to run together and thus destroying their character- 

 istic appearance. 



II. Procedure for Gelatin Plates. 



1-3. Proceed as in I. " Procedure for Agar Plates." 



4. Liquefy three tubes of gelatin in the water-bath 



and keep at a constant temperature of 30 to 35 C. 

 5-13. Proceed as in I. " Procedure for Agar Plates." 

 14. Place at a constant temperature of 21 C. The 



gelatin may not harden until placed at this temperature. 



Note. Gelatin plates are kept right side up, as the organisms may 

 liquefy the gelatin. The liquefied part would then fall from the medium 

 upon the cover and ruin the plate for study. 



EXERCISE 13. PREPARATION OF PLATE CULTURES, 

 QUANTITATIVE DILUTION METHOD 



In the method given below, " dilution flasks " are pre- 

 pared containing measured amounts of water or salt solu- 

 tion in which a measured amount of the substance under 

 investigation is placed. 



As to whether water or salt solution is used depends upon 

 the nature of the material to be dissolved or placed in 

 suspension. If the substance whose microflora is to be 

 studied contains a certain amount of various salts or other 

 electrolytes in solution, an effort should be made to approx-* 



