54 



GENERAL BACTERIOLOGY 



Petri dish is placed on the desk; the gelatin tube is taken in the 

 right hand, the cotton plug removed with the left hand ; the mouth 

 of the tube sterilized by flaming it once or twice, and, when the glass 

 is cool, the gelatin is 

 poured into the lower half 

 of the dish while the cover 

 is slightly raised (Fig. 

 15), but not inverted or 



1 -J ,, .11 rrn Pia. 15. Method of pouring plates. 



laid on the table. The 



cover of the dish is then replaced, the test-tube filled with a solu- 

 tion of corrosive sublimate, and the cotton plug returned. The 

 gelatin is spread over the entire bottom of the dish by tipping it 

 from side to side. It is then allowed to harden by placing the dish on 

 the cooling apparatus, or leaving it on a horizontal surface at room 

 temperature. A simple, inexpensive and effective cooling apparatus 



is a piece of soapstone, such as is 

 sold at hardware stores (Fig. 

 16). In winter this can be 

 cooled by hanging it out of 

 doors, at other seasons by im- 

 mersing it in cold water. The 

 three Petri dishes thus prepared 

 should be properly labeled and 

 placed under conditions where the gelatin will remain solid and yet 

 growth will take place. The temperature of the laboratory should 

 not be allowed to exceed 23° C. or gelatin cultures are in danger of 

 melting while under examination. Within a few days colonies will 

 make their appearance, in varying numbers, depending upon the 

 dilution used. 



Inasmuch as the first plate is generally too thickly seeded to be 

 of much service, this gelatin tube is often replaced by a water blank, 

 which is treated exactly as the gelatin tube No. 1, but is not, of course, 

 "plated" but used simply to dilute the material. 



References. A. 130 ; H. 65 ; M. & R. 53 ; McF. 199 ; P. 224. 



Special Directions. 



a. Make three gelatin plate cultures, as directed above, and inoc- 

 ulate with B. subtilis, introducing a minute portion of agar culture 

 (13) into tube No. 1, two loops of No. 1 into No. 2, and three of No. 

 2 into No. 3. Label, and when the gelatin has solidified, place plates 

 in cool chamber (14). 



Fig. 16. Soapstone used for solidifying 

 gelatin in Petri dishes. 



