PREPARATION OF CULTURE MEDIA 13 



of normal NaOH needed to neutralize i liter of medium. 

 To this add 5 c.c. normal HCl. 



The medium is then 0.5 per cent, acid to phenol- 

 phthalein (+ 0.5 per cent. Fuller's standard), or fairly 

 strongly alkaline to litmus. 



The same result may be obtained by deducting 5 c.c. 

 from the amount of normal NaOH to be added per liter. 

 If, therefore, in the above example 12 c.c. — 5 c.c. =7 c.c. 

 normal NaOH is added, the resulting reaction of the me- 

 dium is 0.5 per cent, acid without the addition of HCl. 



A normal solution is the equivalent weight in grams 

 (gram-molecule) of a chemical in 1000 c.c. of distilled 

 water. In the case of monovalent elements combining 

 to form the chemical, the molecular weight of the latter is 

 taken; in the case of bivalent ones, the molecular 

 weight is divided by 2 ; etc. 



7. Allow the liquid to cool to 60°; test by placing 

 the thermometer in it. 



8. While cooling, dissolve the whites of two eggs in 

 75 c.c. of water, and stir well into the liquid while still at 

 60°. The egg albumen is added for the purpose of clari- 

 fying the liquid. If added before the temperature has 

 fallen to 60°, it would partially coagulate, and thus not 

 serve the purpose. On the other hand, if allowed to 

 cool to about 40°, the agar would solidify. Upon gradu- 

 ally heating the medium the albumen coagulates, me- 

 chanically incloses suspended particles, carries them to 

 the top, and forms a superficial film. 



9. Heat again, without stirring, on a piece of asbestos 

 over the flame. A film will gradually form and harden 

 with a dry surface. 



10. Now adjust the weight by calculating the total 



