MAGNESIUM-GYPSUM BLOCKS 105 



4. Make the mixture 10 per cent, acid with strong 

 hydrochloric acid. 



5. Tube and sterilize as before, and proceed as with the 

 undialyzed media. 



6. In case the silicic acid does not solidify, the dialyzed 

 solution may be concentrated. 



Medium 31 



Washed Agar for the Nitrifying Bacteria 



1. Heat ordinary agar with distilled water until in solu- 

 tion. 



2. Pour into Erlenmeyer flasks and allow to sohdify. 



3. After standing for one or two weeks with several 

 changes of water, all the soluble organic substances will 

 have been removed. 



4. Add the inorganic substances and sterilize. It is well 

 to use precipitated calcium carbonate and hydrogen am- 

 monium sodium phosphate (NH4NaHP04 + 4H2O). 



Medium 32 



Magnesium-gypsum Blocks 



1. Add I per cent, of magnesium carbonate (MgCOs) to 

 dried gypsum (CaS04 + H2O) and mix thoroughly. 



2. To this mass add water until the mixture has the 

 consistency of sour cream. 



3. Pour upon plate glass and cut into circular blocks for 

 Petri dishes. It is best to cut with a Petri dish of a size 

 smaller than the dish for which it is intended. If the 

 glass plate or Petri dish is previously washed in soapy 

 water, the gypsum block is readily removed. 



4. When hard, remove blocks from glass, place bottom 



