CULTURE-MEDIA. 69 



This expedient answers very well, but filtration through paper 

 is apt to give better results. 



If the product appears cloudy after it has been sterilized, 

 it may be that the egg-albumen was incompletely coagulated 

 in the first place or that the reaction has been made too alkaline. 

 In any case it will be desirable to melt it and filter a second 

 time, correcting the reaction with hydrochloric acid if neces- 

 sary. It may be well to stir in another egg to entangle the 

 opaque particles; then to boil a second time and filter. 



The medium is sometimes modified by adding to it other 

 substances, as sugar, glycerin, etc. The solidifying property 

 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 io° 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 east- 

 ern coast of Asia. It melts with much greater difiiculty 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 me- 

 dium is commonly called "agar." 



Beef-extract . .... 3 grams. 



Peptone . 10 grams. 



Sodium chloride . . 5 grams. 



Dry Agar . 15 grams. 



Water . . . . . i liter. 



