CULTURAL CHARACTERS OF BACTERIA 25 



Oleaginous, transparent and yellow ; olive to linseed-oil colored. 



Resinous, transparent and brown ; varnish or resin colored. 



Translucent, faintly transparent. 



Porcelaneous, translucent and white. 



Opalescent, translucent, grayish white by reflected light, smoky 



brown by transmitted light. 



Nacreous, translucent, grayish white, with pearly lustre. 

 Sebaceous, translucent, yellowish or grayish white. 

 Butyrous, translucent, and yellow. 

 Ceraceous, translucent, and wax-colored. 

 Opaque. 



Cretaceous, opaque and white, chalky. 

 Dull, without lustre. 

 Glistening, shining. 

 Fluorescent. 

 Iridescent. 



2. CULTURE MEDIA 

 The Reaction of Media 



The reaction of media is a question of primary importance, 

 since variations in their titre will often produce marked differ- 

 ences in the micro and macro characters of a growth. Hence it 

 is important to work* with media whose reaction is accurately 

 known and uniformly the same. Phenolphthalein has been 

 generally found to be the best indicator. A medium which is 

 alkaline to litmus may be acid to phenolphthalein, showing that 

 there are present in such media substances possessing an acid 

 character which the litmus fails to indicate. "' These substances 

 are weak organic acids and organic compounds, theoretically 

 amphoteric, but in which an acid character predominates. Fur- 

 thermore, the dibasic phosphates (Na 2 HPO 4 ), present in con- 

 siderable quantities in culture media, react alkaline to litmus, 

 but neutral to phenolphthalein. Hence, if the acid phosphates 

 are to be entirely neutralized, the medium must be made more 

 than neutral to litmus. Exact neutralization can therefore only 

 be determined with phenolphthalein. For the titration of media, 



