APPENDIX. 127 



£ 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 



c 20 



N 

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



it blue. Filter the solution and sterilize at no° C. This 

 solution should be added to the media just before use in 

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



