PREPARATION" OP POTATOES. 67 



If glycerin is to be added to the agar-agar, it is done 

 after filtration and before sterilization. The nutritive 

 properties of the media for certain organisms, particu- 

 larly the tubercle bacillus, is improved 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 alka- 

 line, neutralize, boil, and filter again. If the reaction 

 is neutral or only very slightly acid, dissolve and clarify 

 again with egg albumin by the method given. 



The most important point in all the media, aside 

 from the correct proportion of the ingredients, is their 

 reaction. They must be neutral or very slightly alka- 

 line. But few organisms develop well on media of an 

 acid reaction. In all of the above media the meat extracts 

 now on the market may occasionally be substituted for 

 the meat itself in preparing the bouillon. In this case 

 the preparation known -as Liebig's Meat Extract maybe 

 employed in the proportion of from three to five 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 the market^^old 

 potatoes being usually recommended, and carefully 

 scrubbed under the 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 to be placed in a solu- 

 tion of corrosive sublimate of the strength of 1:1000 

 and allowed to remain there for twenty minutes ; at the 

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

 are placed into a covered tin bucket with a perforated 

 bottom and sterilized in the steam sterilizer for forty- 



