PREPARATION OF POTATOES. §3 



its gelatinizing powers, which cannot be regained by 

 subsequent neutralization. 



Another method by which the agar-agar can easily 

 and quickly be melted, is by steam under pressure. If 

 the flask containing the mixture of bouillon and agar 

 be kept in the digester or autoclav, with the steam 

 under a pressure of one-half to one atmosphere, as 

 shown by the gauge, for ten minutes, the agar-agar 

 will be found at the end of this time completely melted, 

 and filtration may then be accomplished with but little 

 difficulty. 



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

 after filtration and before sterilization. The nijtritive 

 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 ex- 

 tracts now on the market may usually be substituted for 

 the meat itself in preparing the bouillon. In this case 

 the preparation known as Liebig's Meat Extract may 

 be employed in the proportion of from two to four 

 grammes to the litre of water. 



Pkeparation of Potatoes. — Potatoes are pre- 

 jjared for use in two ways : 



