PLATES 139 



rubbing it against the sides of the tube. The more carefully 

 this is done the more uniform will be the distribution 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 tube No. 2 

 into tube No. 3. This completes the dilution. The loop is 

 now sterilized before laying it aside. 



FIG. 24 



Levelling tripod with glass cooling 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 be lower than 

 39 C., and never higher than 43 C. If it falls below 38 

 C., the agar-agar solidifies, and can only be redissolved at 

 a temperature that would be destructive to the organisms 

 which may have been introduced into the tubes. This is 



