TECHNIQUE OF MAKING PLATES. 



115 



of the organisms and the better the results. The loop 

 is then again sterilized^ and three of its loopfuls are 

 passed, without touching the sides of the tube, from ''the 

 original" into tube No. 2, where they are carefully 

 mixed. Again the loop is sterilized, and again three 

 dips are made from tul)e 2 into tube 3. This completes 

 the dilution. The loop is now sterilized before laying 

 it aside. 



Fig. 24. 



Leyelling-tripod with glass chamber for plates. 



During this manipulation, which must be done 

 quickly if agar-agar be employed, the temperature of 

 the water in the bath in which the tubes stand should 

 never get lower than 39° C, and never higher than 

 43° C. If it falls too low, below 38° C, the agar-agar 

 gelatinizes, and can only be redissolved by a tempera- 

 ture that would be destructive to the organisms which 

 may have been introduced into the tubes. This is not 

 of so much moment with gelatin, as it may readily be 

 redissolved at a temperature not detrimental to the 



