44 SOLID MEDIA 



Karlin ski's filter. To facilitate filtration, Karlinski has devised an apparatus in which 

 the agar is filtered under pressure. A water-jacketed copper vessel heated below by a 

 ring Bunsen is fitted with a copper cylinder, the bottom of which shaped like a funnel 

 and terminating in a delivery tube fitted with a tap is covered with a layer of absorbent 

 wool. The agar is poured on to the wool, and the upper opening is hermetically sealed 

 by means of a cover through which the tube of an india-rubber syringe passes. When 

 the ball of the syringe is squeezed, the air in the apparatus above the agar is compressed 

 and forces the agar through the wool. This ingenious piece of apparatus does not appear 

 essential because if the agar be prepared in either of the ways described no difficulty will 

 be experienced in filtering through Chardin paper. 



Fischer's method. Fischer proposes to overcome the difficulty of filtration as follows. 

 Plug the narrow end of a funnel with an ordinary cork, and pour the agar into the funnel 

 at once on taking it out of the autoclave. Allow to cool. The solid particles which 

 cause the cloudiness settle to the bottom of the funnel. When the agar is set the jelly 

 is turned out whole and the opaque conical part cut off with a knife. Cut up the remainder 

 into small pieces and put into tubes. Plug and sterilize the tubes. The resulting agar 

 is always opaque. 



Malm's Agar. 



Add 2 per cent, of agar to Liebig's or Cibils' broth (p. 34). Proceed as for 

 ordinary agar. 



Peptone-agar (Salomonsen). 



1. Make a broth with 



Water, - - 1000 grams. 



Liebig's extract, - 5 



Peptone, - 30 



Cane sugar, - 5 ,, 



If necessary add a little alkali. 



2. Dissolve 15 grams of agar in the broth, and proceed as above. 



Glycerin-agar. 



Add 2 per cent, of agar to glycerin broth (p. 35), and proceed in the 

 ordinary way. 



Glucose-glycerin-agar. 



Prepare a glucose broth (p. 34), and after neutralization add 5 per cent, 

 neutral glycerin and 2 per cent. agar. Complete the preparation in the 

 usual manner. 



Gelatin-agar. 



By mixing agar and gelatin a medium is obtained the melting point of 

 which lies between that of agar and that of gelatin. In warm climates, in 



the summer, agar-gelatin may be used in place of gelatin. But it must be 



borne in mind that the cultural characteristics of micro-organisms are far 



from being identical on the two media. Gelatin-agar is prepared as 

 follows. 



1. To 1000 grams of peptone-broth, add 



Gelatin, - 80 grams. 

 Agar, - 5 



or 



Gelatin, - 50 grams. 

 Agar, - 8 ., 



Dissolve the gelatin in the broth, neutralize, and then add the agar. 



2. Complete the preparation as in the case of ordinary agar, but at Stage 5 

 do not let the temperature exceed 115 C. 



Iceland moss. 



Some workers use Iceland moss (Lichen crispus) in place of agar, but this sub- 

 stitution cannot be recommended. 



