64 MANUAL OF BACTERIOLOGY. 



of the gelatin must be carefully guarded, and too much boiling 

 is to be avoided. Certain bacteria, it will be found, have the 

 property of causing gelatin to become permanently liquid: 

 this is called liquefaction or peptonization. Gelatin melts at 

 about 25° C. and solidifies at about 10° C. It cannot be used 

 in the incubator, where it would melt at the temperature of 

 38° C. In hot weather it may be necessary to use 150 grams 

 of dry gelatin to the liter. Nutrient gelatin is usually spoken 

 of simply as "gelatin." 



Nutrient Agar-agar. — Agar-agar (French, gelose) is a kind 

 of vegetable gelatin which comes from the southern and eastern 

 coast of Asia. It melts with much greater difficulty than 

 gelatin, and remains solid at much higher temperatures. In 

 this respect it behaves very peculiarly, since it will not melt 

 unless it is heated to about 80° C; but after it is once melted 

 it remains fluid at 40° C, or over. After it solidifies it has 

 to be heated up to about 80° C. before it will melt again. 



The medium is not quite transparent. The finished medium 

 is commonly called "agar." 



Beef-extract 3 grams. 



Peptone 10 grams. 



Sodium chloride 5 grams. 



Agar ID grams. 



Water i liter. 



The dry agar, cut fine, is to be dissolved in water over a 

 flame. It should be boiled for from one-half hour to two 

 hours, skimming off the scum which forms on the surface 

 from time to time. The beef-extract, peptone and sodium 

 chloride are dissolved in a liter of water, boiled and neutrahzed. 

 Add the agar now in solution in a small quantity of water. 

 The reaction of the agar alone is faintly alkaline. Mix thor- 

 oughly; the bulk of the mixture is a httle more than a hter, 

 and should be reduced to a Hter by the subsequent boiling. 

 Cool to about 60° C; stir in the whites of one or two eggs and 



