PREPARATION OF POTATOES. 117 



after filtration and before sterilization. The nutritive 

 properties of the media for certain organisms, particu- 

 larly the tubercle bacillus, are increased by the addition 

 of glycerin in the proportion of 5 to 7 per cent. 



If after filtration a fine flocculent precipitate is seen, 

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

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

 reaction 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. They must be neutral or very slightly alkaline 

 to litmus. (See remarks on Neutralization of Media.) 

 Only a few organisms develop well on media of an acid 

 reaction. In all of the media mentioned above the meat- 

 extracts now on the market may usually be substituted 

 for the meat itself in preparing the bouillon. They 

 may be employed in the proportion of from two to four 

 grammes to the litre of water. 



PREPARATION OF POTATOES. Potatoes are prepared 

 for use in two ways : 



1. They are taken as they come to market old 

 potatoes being usually recommended and carefully 

 scrubbed under a water-tap with a stiff brush until 

 all adherent dirt has been removed ; " the eyes " and 

 all discolored or decayed parts are carefully removed 

 with a pointed knife. They are then placed in a solu- 

 tion of corrosive sublimate of the strength of 1 : 1000, 

 where they are allowed to remain for twenty minutos ; 

 at the end of this time, without rinsing off the sublimate, 

 they are placed in a covered tin bucket with a perforated 

 bottom and sterilized in the steam sterilizer for forty- 



