110 BACTERIOLOGY. 



If after filtration a fine flocculent precipitate is seen, 

 look to the reaction of the medium. If it is quite 

 alkaline, boil, neutralize, and filter again. If the reac- 

 tion is neutral or only very slightly acid, dissolve and 

 again clarify with egg-albumin by the method given. 



The most important feature of all the media, aside 

 from the correct proportion of the ingredients, is their 

 reaction. It must be neutral or very slightly 

 alkaline to litmus. (See remarks on Neutral- 

 ization of Media.) Only a' few organisms 

 develop well on media of an acid reaction. 



PREPARATION OF POTATOES. l With an 

 ordinary cork borer punch out from sound 

 potatoes cylindrical bits that will slip easily 

 into the test tubes to be used. Cut away all 

 particles of the skin. Then cut on each cylin- 

 der a slanting surface extending from about 

 the middle diagonally to the end. Leave the 

 cylinders in running water over night to pre- 

 vent them from becoming discolored when 

 they are sterilized. 



One potato cylinder thus prepared is then to 

 be placed in each of the already cleaned, 

 plugged and sterilized test tubes, after which 

 ^ ie y are ster ili ze d by Cither the intermittent 

 method with streaming steam or by steam 

 under pressure in the autoclave. In the latter event 

 one atmosphere of pressure should be continued for 

 twenty minutes. (See Fig. 18.) 



For some purposes potatoes may be advantageously 



i By Bolton's method : Med. News, 1887, vol. 1, No. 12, p. 318. 



