APPENDIX. 127 



J liter and make exactly neutral with caustic soda or 

 potash. To test for the neutral point, place one drop of 



N 



the solution in a test-tube. One drop of HC1 should 



20 



N 

 turn the drop red, while one drop of NaOH should turn 



it blue. Filter the solution and sterilize at 110 C. This 

 solution should be added to the media just before use in 

 the proportion of about J c.c. to 5 c.c. of medium. 



