114 A MANUAL OF BACTERIOLOGY 



3. Add to the different flasks amounts of zinc sulphate to 

 make the following concentrations of the toxic agent: 1.0, 0.25, 

 0.12, 0.06, 0.03, 0.015, 0.007, 0.0035, 0.00 per cent. 



4. Sterilize in the Arnold sterilizer and inoculate with Asper- 

 gillus. 



5. Watch the cultures and make records of the amount of 

 mycelium and spore development. 



Exercise 159. Demonstration of the Presence of Arsenic by Means of 

 Penicillium brevicaule 



1. Mix the materials to be examined with bread crumbs 

 (preferably brown or graham bread) and neutralize if necessary. 



2. Sterilize in steam and inoculate when cool with Peni- 

 cillium brevicaule. Close the flasks with tight-fitting plugs. 

 Incubate at 37 C. 



3. Twenty-four to forty-eight hours after the mass becomes 

 thoroughly overgrown with Penicillium, open the flasks and ex- 

 amine for odor. An odor of garlic is indicative of the presence 



of arsenic. 



Exercise 160. The Amylolytic Power of Molds 



1. Measure off 50 cc. of liquid wort into a flask of 200 cc. capac- 

 ity and add 1 g. of starch. Sterilize in the Arnold sterilizer. 



2. Inoculate with Aspergillus oryzae. After growth is evi- 

 dent, test the solution for reducing sugars with Fehling's solution. 



Exercise 161. The Peptonizing Action of Molds 



1. Melt several tubes of beef -peptone gelatin and pour into 

 sterile Petri dishes. 



2. When the gelatin has hardened, inoculate the center of 

 each plate with spores of Penicillium. 



3. Keep the plates at 20 C. and note the progress of lique- 

 faction as the colony spreads over the gelatin. 



4. Another method consists in growing the Penicillium in a 

 tube of beef broth until spores are formed, then filtering off the 

 medium. Add several drops of toluol and pour the liquid into 

 a tube containing sterile gelatin. Follow the liquefaction of the 

 solid gelatin from day to day by marks on the wall of the tube. 



