76 MANUAL OF BACTERIOLOGY. 



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 prop- 

 erty 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 fluid. 

 Gelatin melts at about 25 C. and solidifies at about 10 C. 

 It cannot be used in the incubator, where it would liquefy 

 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 diffi- 

 culty than gelatin. 



The medium is not quite transparent. The finished me- 

 dium is commonly called " agar.' 1 



Beef-extract 3 grams. 



Peptone 10 grams. 



Sodium chloride 5 grams. 



Agar 10 grams. 



Water I liter. 



The dry agar, cut tine, 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 neu- 

 tralized. Add the agar now in solution in a small quantity 

 of water. The reaction of the agar alone is faintly alka- 

 line. Mix thoroughly; the bulk of the mixture is a little 

 more than a liter, and should be reduced to a liter after the 

 subsequent boiling. Cool to about 60 C. ; stir in the whites 



