112 CLASSIFICATION AND DESCRIPTION OF BACTERIA 
To secure success with this method both the iodine and starch solutions 
roust be very dilute The same method has been applied to potato, 
slants. In this procedure the slants arc thoroughly mashed and after 
dilution with water aic treated as above. 
Allen (1918) has described another convenient method which has 
had rather extended use. This involves the use of a 0.2 per cent thymol 
starch agar which is poured into a sterile Petri dish and allowed to 
harden. Stieaks are made on this and after incubation the dish may be 
flooded with iodine solution. A clear halo about the growth indicates 
diastasic action on starch. In order to satisfy the requirements of the 
Descriptive Chart Allen has stated that a clear zone of more than 2 mm. 
width should be regarded as strong while a feeble action 1s denoted by 
a width of less than 2mm. This differentiation called for on Descrip- 
tive Chart is rather ambiguous since to distinguish between feeble 
and strong diastasic action is probably unnecessary. 
Edson and Carpenter (1912) to determine the presence or absence 
of diastasic action added a 2 per cent thymol starch paste to a 10-day- 
old broth culture. After incubation for about eight hours, the culture 
was tested for reducing sugars by means of Fehling’s solution. 
Temperature Relations. The determination of this characteristic 
has been usually limited to 37° C. and 20° C. Other temperatures may 
be used as desired. The organism should be inoculated into a medium 
in which it will grow well. This culture should then be incubated at 
different temperatures. The possibility of other factors inhibiting 
growth should be carefully guarded against. It maz be necessary to 
study this characteristic before starting the other culture work in 
order to determine the optimum temperature. 
Cohn’s and Uschinsky’s Media. After inoculation they should 
be incubated at the optimum temperature. The observations which 
should be made are mentioned on the Chart. These media may also 
be used as a base to which other compounds, such as fats, proteins, etc., 
may be added. 
Loeffier’s Blood Serum. ‘Treat in the same way as described for 
‘“ Agar Stroke.”’ Make the necessary records on the Chart. 
Milk. Freshly skimmed milk should be sterilized either in test 
tubes or Erlenmeyer flasks. The various observations should be 
recorded on the Chart. The proteolysis of casein may be determined 
either by the appearance of the tube or by means of the Hastings (1904) 
milk agar plate. For preparing this milk agar plate, sterile skimmed 
milk is added to melted plain agar which has been cooled to 50° C. 
and poured into a sterile Petri dish to harden. The strain which is 
