200 Culture Media and Bacteria Cultivation 



the text-books as one "requiring considerable patience and 

 much waste of filter paper." In reality, it is not difficult 

 if a good heavy filter paper be obtained and no attempt 

 made to filter the solution until the agar-agar is perfectly 

 dissolved. 



It is prepared as follows : To 1000 c.c. of bouillon made as 

 described above, preferably of meat instead of beef -extract, 

 10 to 15 grams of agar-agar are added. The mixture is boiled 

 vigorously for an hour in an open pot over the direct gas 

 flame or in the double boiler with saturated calcium chlorid 

 solution in the outside pot. After being cooled to about 

 60 C., and after the correction of the reaction by titration, 

 an egg beaten up in water is added, and the liquid again 

 boiled until the egg-albumen is entirely coagulated. 



After the second boiling and the replacement of the 

 volatilized water, the agar-agar is filtered through a care- 

 fully folded pharmaceutic filter wet with boiling water. 

 It may expedite matters to pour in about one-half of the 

 solution, keep the remainder hot, and subsequently add 

 it when necessary. 



The formerly much employed hot-water and gas-jet 

 filters are unnecessary. If properly prepared, the whole 

 quantity will filter in from fifteen to thirty minutes. 



Ravenel * prepares agar-agar by making two solutions, 

 one representing the meat-infusion, but twice the usual 

 strength, the other the agar-agar dissolved in one-half the 

 usual quantity of water. The agar-agar is dissolved by 

 exposure to superheated steam in the autoclave, after which 

 the two solutions are poured together and boiled until all 

 of the albumins are precipitated. The coagulation of the 

 albumins of the meat-infusion serves to clarify the agar-agar. 



If agar-agar is to be made with beef -extract, the bouillon 

 should be made first and filtered when cold, to exclude the 

 uratic salts which otherwise precipitate in the agar-agar 

 when cold and form an unsightly cloud. 



The finished agar-agar should be a colorless, nearly trans- 

 parent, firm jelly. It is dispensed in tubes like the gelatin 

 and bouillon, sterilized by steam, either by the intermittent 

 process or in the autoclave, and after the last sterilization, 

 before cooling, each tube is inclined against a slight eleva- 

 tion, so as to permit the jelly to solidify obliquely and afford 

 an extensive flat surface for the culture. 



* " Journal of Applied Microscopy, "June, 1898, vol. i, No. 6, p. 106. 



