44 



CULTURE MEDIA. 



four hours. Pour off the water and add the flesh-peptone solution. 

 Boil for several hours until the agar is completely dissolved. Neu- 

 tralize hy adding gradually a solution of carbonate of soda (or render 

 slightly alkaline). Filter. 



The last operation is the most troublesome, and various plans 

 have been proposed to avoid the tedious filtration through filtering 

 paper in a hot- water filter. A method which gives satisfactory re- 

 sults is to place the filter containing the hot agar solution, and the 

 flask which is to receive the filtrate, in a steam sterilizing apparatus, 

 where it is left in an atmosphere of streaming steam until the filtra- 



Fio. 18. 



tion is completed. Or the solution may be put in a tall jar and left 

 in the steam sterilizer for several hours until it is clear as a result of 

 sedimentation. The clear solution is then obtained by decantation. 

 Or by conducting the operation in a tall cylindrical vessel, and al- 

 lowing sedimentation to occur in the steam sterilizer and the agar 

 subsequently to solidify by cooling, the cylinder of jelly may be re- 

 moved from the jar and the part containing the sediment can be cut 

 away. The transparent portion is then melted again and distributed 

 in test tubes for use. 



In the present volume we frequently refer to the nutrient medium 

 madr hy adding one to two per cent of apir-agar to the standard 

 tl< >h-peptone solution as ** nutrient agar '' or simply as " agar." 



