70 MANUAL OF BACTERIOLOGY. 



The dry agar, cut fine, is to be dissolved in water over a 

 flame or in the autoclave. It should be boiled for from one- 

 half hour to two hours, skimming off the scum which forms 

 on the surface from time to time. The beef-extract, peptone 

 and sodium chloride are dissolved in a liter of water, boiled and 

 neutralized. Add the agar now in solution in a small quantity 

 of water. The reaction of the agar alone is faintly alkaline. 

 Mix thoroughly; the bulk of the mixture is a little more than a 

 liter, and should be reduced to a liter by the subsequent boiling. 

 Cool to about 60 C.; stir in the whites of one or two eggs and 

 boil thoroughly. Avoid breaking the coagulum of egg which 

 is designed to entangle the solid particles that make the medium 

 cloudy; stir at the bottom, however, to prevent burning. 

 Filter while hot, using filter-paper or absorbent cotton covered 

 with cheese-cloth. The hot-water funnel originally devised for 

 the filtration of agar is not necessary. If filtration is slow, the 

 funnel and flask may be placed inside of the steam sterilizer 

 and kept heated during filtration. The medium is collected in 

 suitable flasks or tubes plugged with cotton, and sterilized once 

 in the autoclave or in the ordinary steam sterilizer for fifteen 

 minutes on each of three consecutive days. As agar is fre- 

 quently used for smear-cultures where a slanted medium is 

 desired, some of the tubes may be allowed to cool in a slantiug 

 position. It is not well to keep on hand many tubes which 

 have been slanted, as the medium dries more rapidly. Agar is 

 seldom liquefied by bacteria, though a few bacteria possess the 

 power of doing this. Its solidifying qualities are impaired 

 somewhat if the reaction be acid. 



The remarks on pages 66 to 67 with regard to the use of 

 beef and the titration method for the preparation of bouillon 

 apply equally to agar-agar. 



Glycerin-agar is used extensively. It is agar, made as 

 above directed, to which 6 per cent, of glycerin is added before 



