44 CULTURE MEDIA. 



Agar-gelatin, a medium which has recently come into favor and 

 is said to be very useful, as it resembles gelatin in transparency and 

 has a considerably higher melting point than ten-per-cent gelatin, is 

 made by adding fifty grammes of gelatin and 7. 5 grammes of agar 

 to a litre of flesh-peptone solution. Care should be taken not to cook 

 this longer than is necessary. 



In making all of these agar culture media the main difficulties 

 encountered result from the difficulty of dissolving the agar and the 

 slowness with which the solution passes through filtering paper. 

 These difficulties are best met as follows : Break up the sticks of agar 

 into small fragments and allow them to soak in cold water for twenty- 

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

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

 tralize by 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- 

 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 

 made by adding one to two per cent of agar-agar to the standard 

 flesh-peptone solution as " nutrient agar" or simply as "agar." 



The following method of filtering agar has (1890) been proposed 

 by Karlinsky. It is a modification of the method previously de- 

 scribed by Jakobi and depends upon the use of pressure. 



In Fig. 18, a is a cylindrical vessel of tin, which is closed above by 

 a perforated rubber cork, through which is passed a glass tube, b. 

 This is enclosed in a larger tin cylinder, c, which contains water, 

 which may be kept hot by placing an alcohol lamp under the pro- 

 jecting arm d. The central cylinder has a tube, e, passing through 

 the bottom of the hot- water cylinder, and which is provided with a 

 stopcock for drawing off the filtered solution. Before pouring the 

 hot agar solution into the cylinder a, a cotton filter about ten centi- 



