102 LABORATORY BACTERIOLOGY 
At the next exercise examine the heated tubes and note 
which are clear and which contain a growth. If the tubes 
heated for 1o minutes or longer have a growth, repeat the 
experiment at 70° C. If this fails to destroy them, repeat at 
80° C., and if necessary apply a still higher temperature. 
Examine the cultures microscopically in all the fertile tubes 
to determine if they are pure. 
Explain the cause for the difference in the thermal death 
point between these two organisms. 
Examine the cultures made from the scrapings of the skin 
in the previous exercise. 
