ISOLATION AND PURE-CULTURE METHODS 27 



very seldom becomes liquefied by the action of growing organisms. 

 Nutrient agar agar consists of some nutrient "base," like sugar 

 beet or prune decoction, bouillon, etc., to which is added, for pur- 

 poses of solidification, 11 to 1 5 grams of the commercial agar agar 

 per liter. The nutrient base, whether plant decoction or bouillon, 

 may be made as already indicated. There are then several methods 

 of procedure for making the agar. (1) When an autoclave is at 

 hand, 12 grams of the agar are merely cut up finely into a liter 

 of the desired decoction and this is placed in the autoclave and 

 steamed at from 1 io° to 1 1 5 C. In thirty minutes the agar will be 



Fig. 6. Gelid/ um corneum Lam., Furnishing Agar Agar 

 (After Erw. F. Smith) 



dissolved. It is then neutralized, or brought to the desired reaction ; 

 and if not clear, the white of an egg may be added, as usual, when 

 a second similar or shorter steaming is necessary. (2) This same 

 method may be used with the steam sterilizer, except that it may 

 require from one to two hours for the complete solution of the 

 agar. Some grades of agar dissolve very slowly by this method, 

 and it is often recommended in such a case to soak the agar pre- 

 viously twelve to eighteen hours in water containing sodium chlo- 

 ride. (3) Many find it more convenient to put the agar into an 

 agate iron cup, add about 200 cc. of distilled water and boil directly 

 over a flame, stirring constantly, until the agar is thoroughly dis- 

 solved. This is then added to the decoction to be used. This last 



