NUTRIENT AGAR- AGAR 43 



90° C. Hence this jelly is well adapted for use as a culture- 

 medium for those micro-organisms which must be grown 

 at the higher temperatures in the incubating chamber. 

 Agar-agar appears commercially in the form of transparent 

 strips, or four-cornered pieces, or as a white powder, and 

 swells ap in water. 



Preparation of peptone bouillon agar. — To make nutrient 

 agar {peptone broth agar), 500 grm. of meat free from fat 

 are taken, minced up, and mixed with a litre of water, and 

 after standing for twenty-four hours in a cool place the 

 liquid is filtered through a cloth and squeezed out from the 

 mass of meat. The albuminoid bodies are precipitated by 

 boUing the meat infusion, and are removed from the liquid 

 by filtration, the result being ordinary nutrient bouillon. 

 This is rendered feebly alkaline with sodium bicarbonate, 

 and mixed with 10 grm. peptone, 5 grm. common salt, and 

 20 grm. agar-agar cut up small. The agar swells up in the 

 broth, and is then boiled over a sand-bath, in the steam 

 steriliser, or even over the naked flame, until only small 

 flakes and slight turbidities are observed, the fluid lost by 

 evaporation being then made up by the addition of water. 

 It is seldom necessary to neutrahse the fluid a second time 

 with sodium bicarbonate, as the agar has of itself a 

 neutral reaction, so that it only remains to filter the solu- 

 tion, though this is in some cases difficult to do suc- 

 cessfully. The filtration is carried on through a double 

 layer of filter-paper, by means of the hot-water funnel, or 

 in the steam steriliser, and is a very slow process. After 

 addition of white of egg it sometimes happens that when 

 the agar mass has been boiled the small particles are 

 gathered into lumps by the albumen, and the filtration is 

 much easier in consequence. 



Some recommend that the hot agar should be allowed 

 to cool gradually in a tall cylindical vessel placed in the 



